PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)
NAME
perlfaq4 - Data Manipulation
VERSION
version 5.20240218
DESCRIPTION
This section of the FAQ answers questions related to manipulating
numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
Data: Numbers
Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers
I should be getting (eg, 19.95)?
For the long explanation, see David Goldberg's "What Every Computer
Scientist Should Know About Floating-Point Arithmetic"
(<http://web.cse.msu.edu/~cse320/Documents/FloatingPoint.pdf>).
Internally, your computer represents floating-point numbers in binary.
Digital (as in powers of two) computers cannot store all numbers
exactly. Some real numbers lose precision in the process. This is a
problem with how computers store numbers and affects all computer
languages, not just Perl.
perlnumber shows the gory details of number representations and
conversions.
To limit the number of decimal places in your numbers, you can use the
"printf" or "sprintf" function. See "Floating-point Arithmetic" in
perlop for more details.
printf "%.2f", 10/3;
my $number = sprintf "%.2f", 10/3;
Why is int() broken?
Your int() is most probably working just fine. It's the numbers that
aren't quite what you think.
First, see the answer to "Why am I getting long decimals (eg,
19.9499999999999) instead of the numbers I should be getting (eg,
19.95)?".
For example, this
print int(0.6/0.2-2), "\n";
will in most computers print 0, not 1, because even such simple numbers
as 0.6 and 0.2 cannot be presented exactly by floating-point numbers.
What you think in the above as 'three' is really more like
2.9999999999999995559.
Why isn't my octal data interpreted correctly?
(contributed by brian d foy)
You're probably trying to convert a string to a number, which Perl only
converts as a decimal number. When Perl converts a string to a number,
it ignores leading spaces and zeroes, then assumes the rest of the
digits are in base 10:
my $string = '0644';
print $string + 0; # prints 644
print $string + 44; # prints 688, certainly not octal!
This problem usually involves one of the Perl built-ins that has the
same name a Unix command that uses octal numbers as arguments on the
command line. In this example, "chmod" on the command line knows that
its first argument is octal because that's what it does:
%prompt> chmod 644 file
If you want to use the same literal digits (644) in Perl, you have to
tell Perl to treat them as octal numbers either by prefixing the digits
with a 0 or using "oct":
chmod( 0644, $filename ); # right, has leading zero
chmod( oct(644), $filename ); # also correct
The problem comes in when you take your numbers from something that Perl
thinks is a string, such as a command line argument in @ARGV:
chmod( $ARGV[0], $filename ); # wrong, even if "0644"
chmod( oct($ARGV[0]), $filename ); # correct, treat string as octal
You can always check the value you're using by printing it in octal
notation to ensure it matches what you think it should be. Print it in
octal and decimal format:
printf "0%o %d", $number, $number;
Does Perl have a round() function? What about ceil() and floor()? Trig
functions?
Remember that int() merely truncates toward 0. For rounding to a certain
number of digits, sprintf() or printf() is usually the easiest route.
printf("%.3f", 3.1415926535); # prints 3.142
The POSIX module (part of the standard Perl distribution) implements
ceil(), floor(), and a number of other mathematical and trigonometric
functions.
use POSIX;
my $ceil = ceil(3.5); # 4
my $floor = floor(3.5); # 3
In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
module. With 5.004, the Math::Trig module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it uses
the Math::Complex module and some functions can break out from the real
axis into the complex plane, for example the inverse sine of 2.
Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely. In these cases,
it probably pays not to trust whichever system of rounding is being used
by Perl, but instead to implement the rounding function you need
yourself.
To see why, notice how you'll still have an issue on half-way-point
alternation:
for (my $i = -5; $i <= 5; $i += 0.5) { printf "%.0f ",$i }
-5 -4 -4 -4 -3 -2 -2 -2 -1 -0 0 0 1 2 2 2 3 4 4 4 5
Don't blame Perl. It's the same as in C. IEEE says we have to do this.
Perl numbers whose absolute values are integers under 2**31 (on 32-bit
machines) will work pretty much like mathematical integers. Other
numbers are not guaranteed.
How do I convert between numeric representations/bases/radixes?
As always with Perl there is more than one way to do it. Below are a few
examples of approaches to making common conversions between number
representations. This is intended to be representational rather than
exhaustive.
Some of the examples later in perlfaq4 use the Bit::Vector module from
CPAN. The reason you might choose Bit::Vector over the perl built-in
functions is that it works with numbers of ANY size, that it is
optimized for speed on some operations, and for at least some
programmers the notation might be familiar.
How do I convert hexadecimal into decimal
Using perl's built in conversion of "0x" notation:
my $dec = 0xDEADBEEF;
Using the "hex" function:
my $dec = hex("DEADBEEF");
Using "pack":
my $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
Using the CPAN module "Bit::Vector":
use Bit::Vector;
my $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
my $dec = $vec->to_Dec();
How do I convert from decimal to hexadecimal
Using "sprintf":
my $hex = sprintf("%X", 3735928559); # upper case A-F
my $hex = sprintf("%x", 3735928559); # lower case a-f
Using "unpack":
my $hex = unpack("H*", pack("N", 3735928559));
Using Bit::Vector:
use Bit::Vector;
my $vec = Bit::Vector->new_Dec(32, -559038737);
my $hex = $vec->to_Hex();
And Bit::Vector supports odd bit counts:
use Bit::Vector;
my $vec = Bit::Vector->new_Dec(33, 3735928559);
$vec->Resize(32); # suppress leading 0 if unwanted
my $hex = $vec->to_Hex();
How do I convert from octal to decimal
Using Perl's built in conversion of numbers with leading zeros:
my $dec = 033653337357; # note the leading 0!
Using the "oct" function:
my $dec = oct("33653337357");
Using Bit::Vector:
use Bit::Vector;
my $vec = Bit::Vector->new(32);
$vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
my $dec = $vec->to_Dec();
How do I convert from decimal to octal
Using "sprintf":
my $oct = sprintf("%o", 3735928559);
Using Bit::Vector:
use Bit::Vector;
my $vec = Bit::Vector->new_Dec(32, -559038737);
my $oct = reverse join('', $vec->Chunk_List_Read(3));
How do I convert from binary to decimal
Perl 5.6 lets you write binary numbers directly with the "0b"
notation:
my $number = 0b10110110;
Using "oct":
my $input = "10110110";
my $decimal = oct( "0b$input" );
Using "pack" and "ord":
my $decimal = ord(pack('B8', '10110110'));
Using "pack" and "unpack" for larger strings:
my $int = unpack("N", pack("B32",
substr("0" x 32 . "11110101011011011111011101111", -32)));
my $dec = sprintf("%d", $int);
# substr() is used to left-pad a 32-character string with zeros.
Using Bit::Vector:
my $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
my $dec = $vec->to_Dec();
How do I convert from decimal to binary
Using "sprintf" (perl 5.6+):
my $bin = sprintf("%b", 3735928559);
Using "unpack":
my $bin = unpack("B*", pack("N", 3735928559));
Using Bit::Vector:
use Bit::Vector;
my $vec = Bit::Vector->new_Dec(32, -559038737);
my $bin = $vec->to_Bin();
The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
are left as an exercise to the inclined reader.
Why doesn't & work the way I want it to?
Perl's "&" bitwise operator works on both numbers and strings, sometimes
producing surprising results when you expected a number but received a
string. You probably expected perl to automatically convert the operands
to numbers like the mathematical operators would. Instead, perl treats
string operands as bitvectors.
Consider the bitwise difference between the number 3 and the bitvector
represented by "3". A number has the bit pattern for its magnitude. The
number 3 is 0b11 (a 2 and a 1). The bitvector has the bit pattern that
is the ordinal value for each octet, and that value is unrelated to any
numeric value that the digit represents. The character "3" is the
bitvector 0b0011_0011.
These operations have different results even though you might think they
look like the same "number":
11 & 3; # 0b0000_1011 & 0b0000_0011
# -> 0b0000_0011 (number 3)
"11" & "3"; # 0b0011_0001_0011_0001 & 0b0011_0011
# -> 0b0011_0001 (ASCII char "1")
Note that if any operand has a numeric value, perl uses numeric
semantics (although you should not count on this):
my($i, $j) = ( 11, 3 ); # $i & $j # 11 & 3 -> 3
my($i, $j) = ("11", 3 ); # $i & $j # 11 & 3 -> 3
my($i, $j) = ("11", "3"); # $i & $j # "11" & "3" -> 1
Remember that a perl scalar can have both string and numeric values at
the same time. A value that starts as a string and has never encountered
a numeric operation has no numeric value yet. Perl does this to save
time and work so it doesn't have to decide a numeric value for a scalar
it might never use as a number. In that case, string semantics wins.
But, if there is a numeric value already, numeric semantics win. Force
perl to compute the numeric value by adding 0:
my($i, $j) = ("11", "3"); $j += 0 # $i & $j # "11" & 3 -> 3
However, this is not a documented feature, or as perlop says, it "is not
well defined". One way to fix ensure numeric semantics is to explicitly
convert both of values to numbers:
(0+$i) & (0+$j)
To fix this annoyance, Perl v5.22 separated the string and number
behavior. The "bitwise" feature introduced four new operators that would
work with only string semantics: "&.", "|.", "^.", and "~.". The
original operators, "&", "|", "^", and "~", would then apply only
numeric semantics.
Enable this feature explicitly with feature:
use feature qw(bitwise);
Or, as of v5.28, require the minimum version of perl with "use":
use v5.28; # bitwise feature for free
How do I multiply matrices?
Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
or the PDL extension (also available from CPAN).
How do I perform an operation on a series of integers?
To call a function on each element in an array, and collect the results,
use:
my @results = map { my_func($_) } @array;
For example:
my @triple = map { 3 * $_ } @single;
To call a function on each element of an array, but ignore the results:
foreach my $iterator (@array) {
some_func($iterator);
}
To call a function on each integer in a (small) range, you can use:
my @results = map { some_func($_) } (5 .. 25);
but you should be aware that in this form, the ".." operator creates a
list of all integers in the range, which can take a lot of memory for
large ranges. However, the problem does not occur when using ".." within
a "for" loop, because in that case the range operator is optimized to
iterate over the range, without creating the entire list. So
my @results = ();
for my $i (5 .. 500_005) {
push(@results, some_func($i));
}
or even
push(@results, some_func($_)) for 5 .. 500_005;
will not create an intermediate list of 500,000 integers.
How can I output Roman numerals?
Get the <http://www.cpan.org/modules/by-module/Roman> module.
Why aren't my random numbers random?
If you're using a version of Perl before 5.004, you must call "srand"
once at the start of your program to seed the random number generator.
BEGIN { srand() if $] < 5.004 }
5.004 and later automatically call "srand" at the beginning. Don't call
"srand" more than once--you make your numbers less random, rather than
more.
Computers are good at being predictable and bad at being random (despite
appearances caused by bugs in your programs :-). The random article in
the "Far More Than You Ever Wanted To Know" collection in
<http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz>, courtesy of Tom
Phoenix, talks more about this. John von Neumann said, "Anyone who
attempts to generate random numbers by deterministic means is, of
course, living in a state of sin."
Perl relies on the underlying system for the implementation of "rand"
and "srand"; on some systems, the generated numbers are not random
enough (especially on Windows : see
<http://www.perlmonks.org/?node_id=803632>). Several CPAN modules in
the "Math" namespace implement better pseudorandom generators; see for
example Math::Random::MT ("Mersenne Twister", fast), or
Math::TrulyRandom (uses the imperfections in the system's timer to
generate random numbers, which is rather slow). More algorithms for
random numbers are described in "Numerical Recipes in C" at
<http://www.nr.com/>
How do I get a random number between X and Y?
To get a random number between two values, you can use the rand() built-
in to get a random number between 0 and 1. From there, you shift that
into the range that you want.
rand($x) returns a number such that "0 <= rand($x) < $x". Thus what you
want to have perl figure out is a random number in the range from 0 to
the difference between your X and Y.
That is, to get a number between 10 and 15, inclusive, you want a random
number between 0 and 5 that you can then add to 10.
my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )
Hence you derive the following simple function to abstract that. It
selects a random integer between the two given integers (inclusive). For
example: "random_int_between(50,120)".
sub random_int_between {
my($min, $max) = @_;
# Assumes that the two arguments are integers themselves!
return $min if $min == $max;
($min, $max) = ($max, $min) if $min > $max;
return $min + int rand(1 + $max - $min);
}
Data: Dates
How do I find the day or week of the year?
The day of the year is in the list returned by the "localtime" function.
Without an argument "localtime" uses the current time.
my $day_of_year = (localtime)[7];
The POSIX module can also format a date as the day of the year or week
of the year.
use POSIX qw/strftime/;
my $day_of_year = strftime "%j", localtime;
my $week_of_year = strftime "%W", localtime;
To get the day of year for any date, use POSIX's "mktime" to get a time
in epoch seconds for the argument to "localtime".
use POSIX qw/mktime strftime/;
my $week_of_year = strftime "%j",
localtime( mktime( 0, 0, 0, 18, 11, 87 ) );
You can also use Time::Piece, which comes with Perl and provides a
"localtime" that returns an object:
use Time::Piece;
my $day_of_year = localtime->yday;
my $week_of_year = localtime->week;
The Date::Calc module provides two functions to calculate these, too:
use Date::Calc;
my $day_of_year = Day_of_Year( 1987, 12, 18 );
my $week_of_year = Week_of_Year( 1987, 12, 18 );
How do I find the current century or millennium?
Use the following simple functions:
sub get_century {
return int((((localtime(shift || time))[5] + 1999))/100);
}
sub get_millennium {
return 1+int((((localtime(shift || time))[5] + 1899))/1000);
}
On some systems, the POSIX module's strftime() function has been
extended in a non-standard way to use a %C format, which they sometimes
claim is the "century". It isn't, because on most such systems, this is
only the first two digits of the four-digit year, and thus cannot be
used to determine reliably the current century or millennium.
How can I compare two dates and find the difference?
(contributed by brian d foy)
You could just store all your dates as a number and then subtract. Life
isn't always that simple though.
The Time::Piece module, which comes with Perl, replaces localtime with a
version that returns an object. It also overloads the comparison
operators so you can compare them directly:
use Time::Piece;
my $date1 = localtime( $some_time );
my $date2 = localtime( $some_other_time );
if( $date1 < $date2 ) {
print "The date was in the past\n";
}
You can also get differences with a subtraction, which returns a
Time::Seconds object:
my $date_diff = $date1 - $date2;
print "The difference is ", $date_diff->days, " days\n";
If you want to work with formatted dates, the Date::Manip, Date::Calc,
or DateTime modules can help you.
How can I take a string and turn it into epoch seconds?
If it's a regular enough string that it always has the same format, you
can split it up and pass the parts to "timelocal" in the standard
Time::Local module. Otherwise, you should look into the Date::Calc,
Date::Parse, and Date::Manip modules from CPAN.
How can I find the Julian Day?
(contributed by brian d foy and Dave Cross)
You can use the Time::Piece module, part of the Standard Library, which
can convert a date/time to a Julian Day:
$ perl -MTime::Piece -le 'print localtime->julian_day'
2455607.7959375
Or the modified Julian Day:
$ perl -MTime::Piece -le 'print localtime->mjd'
55607.2961226851
Or even the day of the year (which is what some people think of as a
Julian day):
$ perl -MTime::Piece -le 'print localtime->yday'
45
You can also do the same things with the DateTime module:
$ perl -MDateTime -le'print DateTime->today->jd'
2453401.5
$ perl -MDateTime -le'print DateTime->today->mjd'
53401
$ perl -MDateTime -le'print DateTime->today->doy'
31
You can use the Time::JulianDay module available on CPAN. Ensure that
you really want to find a Julian day, though, as many people have
different ideas about Julian days (see
<http://www.hermetic.ch/cal_stud/jdn.htm> for instance):
$ perl -MTime::JulianDay -le 'print local_julian_day( time )'
55608
How do I find yesterday's date?
(contributed by brian d foy)
To do it correctly, you can use one of the "Date" modules since they
work with calendars instead of times. The DateTime module makes it
simple, and give you the same time of day, only the day before, despite
daylight saving time changes:
use DateTime;
my $yesterday = DateTime->now->subtract( days => 1 );
print "Yesterday was $yesterday\n";
You can also use the Date::Calc module using its "Today_and_Now"
function.
use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
print "@date_time\n";
Most people try to use the time rather than the calendar to figure out
dates, but that assumes that days are twenty-four hours each. For most
people, there are two days a year when they aren't: the switch to and
from summer time throws this off. For example, the rest of the
suggestions will be wrong sometimes:
Starting with Perl 5.10, Time::Piece and Time::Seconds are part of the
standard distribution, so you might think that you could do something
like this:
use Time::Piece;
use Time::Seconds;
my $yesterday = localtime() - ONE_DAY; # WRONG
print "Yesterday was $yesterday\n";
The Time::Piece module exports a new "localtime" that returns an object,
and Time::Seconds exports the "ONE_DAY" constant that is a set number of
seconds. This means that it always gives the time 24 hours ago, which is
not always yesterday. This can cause problems around the end of daylight
saving time when there's one day that is 25 hours long.
You have the same problem with Time::Local, which will give the wrong
answer for those same special cases:
# contributed by Gunnar Hjalmarsson
use Time::Local;
my $today = timelocal 0, 0, 12, ( localtime )[3..5];
my ($d, $m, $y) = ( localtime $today-86400 )[3..5]; # WRONG
printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;
Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant?
(contributed by brian d foy)
Perl itself never had a Y2K problem, although that never stopped people
from creating Y2K problems on their own. See the documentation for
"localtime" for its proper use.
Starting with Perl 5.12, "localtime" and "gmtime" can handle dates past
03:14:08 January 19, 2038, when a 32-bit based time would overflow. You
still might get a warning on a 32-bit "perl":
% perl5.12 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )'
Integer overflow in hexadecimal number at -e line 1.
Wed Nov 1 19:42:39 5576711
On a 64-bit "perl", you can get even larger dates for those really long
running projects:
% perl5.12 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )'
Thu Nov 2 00:42:39 5576711
You're still out of luck if you need to keep track of decaying protons
though.
Data: Strings
How do I validate input?
(contributed by brian d foy)
There are many ways to ensure that values are what you expect or want to
accept. Besides the specific examples that we cover in the perlfaq, you
can also look at the modules with "Assert" and "Validate" in their
names, along with other modules such as Regexp::Common.
Some modules have validation for particular types of input, such as
Business::ISBN, Business::CreditCard, Email::Valid, and
Data::Validate::IP.
How do I unescape a string?
It depends just what you mean by "escape". URL escapes are dealt with in
perlfaq9. Shell escapes with the backslash ("\") character are removed
with
s/\\(.)/$1/g;
This won't expand "\n" or "\t" or any other special escapes.
How do I remove consecutive pairs of characters?
(contributed by brian d foy)
You can use the substitution operator to find pairs of characters (or
runs of characters) and replace them with a single instance. In this
substitution, we find a character in "(.)". The memory parentheses store
the matched character in the back-reference "\g1" and we use that to
require that the same thing immediately follow it. We replace that part
of the string with the character in $1.
s/(.)\g1/$1/g;
We can also use the transliteration operator, "tr///". In this example,
the search list side of our "tr///" contains nothing, but the "c" option
complements that so it contains everything. The replacement list also
contains nothing, so the transliteration is almost a no-op since it
won't do any replacements (or more exactly, replace the character with
itself). However, the "s" option squashes duplicated and consecutive
characters in the string so a character does not show up next to itself
my $str = 'Haarlem'; # in the Netherlands
$str =~ tr///cs; # Now Harlem, like in New York
How do I expand function calls in a string?
(contributed by brian d foy)
This is documented in perlref, and although it's not the easiest thing
to read, it does work. In each of these examples, we call the function
inside the braces used to dereference a reference. If we have more than
one return value, we can construct and dereference an anonymous array.
In this case, we call the function in list context.
print "The time values are @{ [localtime] }.\n";
If we want to call the function in scalar context, we have to do a bit
more work. We can really have any code we like inside the braces, so we
simply have to end with the scalar reference, although how you do that
is up to you, and you can use code inside the braces. Note that the use
of parens creates a list context, so we need "scalar" to force the
scalar context on the function:
print "The time is ${\(scalar localtime)}.\n"
print "The time is ${ my $x = localtime; \$x }.\n";
If your function already returns a reference, you don't need to create
the reference yourself.
sub timestamp { my $t = localtime; \$t }
print "The time is ${ timestamp() }.\n";
The "Interpolation" module can also do a lot of magic for you. You can
specify a variable name, in this case "E", to set up a tied hash that
does the interpolation for you. It has several other methods to do this
as well.
use Interpolation E => 'eval';
print "The time values are $E{localtime()}.\n";
In most cases, it is probably easier to simply use string concatenation,
which also forces scalar context.
print "The time is " . localtime() . ".\n";
How do I find matching/nesting anything?
To find something between two single characters, a pattern like
"/x([^x]*)x/" will get the intervening bits in $1. For multiple ones,
then something more like "/alpha(.*?)omega/" would be needed. For nested
patterns and/or balanced expressions, see the so-called (?PARNO)
construct (available since perl 5.10). The CPAN module Regexp::Common
can help to build such regular expressions (see in particular
Regexp::Common::balanced and Regexp::Common::delimited).
More complex cases will require to write a parser, probably using a
parsing module from CPAN, like Regexp::Grammars, Parse::RecDescent,
Parse::Yapp, Text::Balanced, or Marpa::R2.
How do I reverse a string?
Use reverse() in scalar context, as documented in "reverse" in perlfunc.
my $reversed = reverse $string;
How do I expand tabs in a string?
You can do it yourself:
1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
Or you can just use the Text::Tabs module (part of the standard Perl
distribution).
use Text::Tabs;
my @expanded_lines = expand(@lines_with_tabs);
How do I reformat a paragraph?
Use Text::Wrap (part of the standard Perl distribution):
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
The paragraphs you give to Text::Wrap should not contain embedded
newlines. Text::Wrap doesn't justify the lines (flush-right).
Or use the CPAN module Text::Autoformat. Formatting files can be easily
done by making a shell alias, like so:
alias fmt="perl -i -MText::Autoformat -n0777 \
-e 'print autoformat $_, {all=>1}' $*"
See the documentation for Text::Autoformat to appreciate its many
capabilities.
How can I access or change N characters of a string?
You can access the first characters of a string with substr(). To get
the first character, for example, start at position 0 and grab the
string of length 1.
my $string = "Just another Perl Hacker";
my $first_char = substr( $string, 0, 1 ); # 'J'
To change part of a string, you can use the optional fourth argument
which is the replacement string.
substr( $string, 13, 4, "Perl 5.8.0" );
You can also use substr() as an lvalue.
substr( $string, 13, 4 ) = "Perl 5.8.0";
How do I change the Nth occurrence of something?
You have to keep track of N yourself. For example, let's say you want to
change the fifth occurrence of "whoever" or "whomever" into "whosoever"
or "whomsoever", case insensitively. These all assume that $_ contains
the string to be altered.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}ige;
In the more general case, you can use the "/g" modifier in a "while"
loop, keeping count of matches.
$WANT = 3;
$count = 0;
$_ = "One fish two fish red fish blue fish";
while (/(\w+)\s+fish\b/gi) {
if (++$count == $WANT) {
print "The third fish is a $1 one.\n";
}
}
That prints out: "The third fish is a red one." You can also use a
repetition count and repeated pattern like this:
/(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
How can I count the number of occurrences of a substring within a string?
There are a number of ways, with varying efficiency. If you want a count
of a certain single character (X) within a string, you can use the
"tr///" function like so:
my $string = "ThisXlineXhasXsomeXx'sXinXit";
my $count = ($string =~ tr/X//);
print "There are $count X characters in the string";
This is fine if you are just looking for a single character. However, if
you are trying to count multiple character substrings within a larger
string, "tr///" won't work. What you can do is wrap a while() loop
around a global pattern match. For example, let's count negative
integers:
my $string = "-9 55 48 -2 23 -76 4 14 -44";
my $count = 0;
while ($string =~ /-\d+/g) { $count++ }
print "There are $count negative numbers in the string";
Another version uses a global match in list context, then assigns the
result to a scalar, producing a count of the number of matches.
my $count = () = $string =~ /-\d+/g;
How do I capitalize all the words on one line?
(contributed by brian d foy)
Damian Conway's Text::Autoformat handles all of the thinking for you:
use Text::Autoformat;
my $s = "Going to the desert with a camel";
printf "%-10s %s\n", "original:", $s;
for my $style (qw( sentence title highlight )) {
printf "%-10s %s\n",
"$style:",
autoformat($s, { case => $style }) =~ s/\R+\z//r;
}
Each style has a different idea of what if should do with small words:
original: Going to the desert with a camel
sentence: Going to the desert with a camel
title: Going To The Desert With A Camel
highlight: Going to the Desert with a Camel
Trying this yourself in a simple regex has more than a few pitfalls.
Perl "words" are groups of "\w+", but that's not what you want to
capitalize. Some of those characters aren't even letters. What if you
used a word boundary, "\b"?
my $string = "A camel's journey";
$string =~ s/\b(\w)/\U$1/g; # A Camel'S Journey
How is Perl supposed to know not to capitalize that "s"? You could work
harder to look for preceding whitespace or the beginning of the string:
$string =~ s/( (?:^|\s) \w ) /\U$1/xg; # A Camel's Journey
Or maybe you match the entire group, lowercase it all ("L"), then
uppercase the first letter ("\u"):
$string =~ s/([\w']+)/\u\L$1/g;
Each of these have their annoying edge cases. Perl v5.22 added fancier
Unicode word and sentence boundaries to fix this sort of problem. The
"\b{wb}" word boundary ignores some things that it doesn't think start
or end "human" words:
use v5.22;
my $string = "A camel's journey";
$string =~ s/\b{wb}(\w)/\U$1/g; # A Camel's Journey
The details are in Unicode Technical Report #29
(<https://unicode.org/reports/tr29/>), and you should make sure that its
idea of a word boundary corresponds to yours.
Now, what if you don't want to capitalize that "and" or "the"? Just use
Text::Autoformat and get on with the next problem. :)
How can I split a [character]-delimited string except when inside
[character]?
Several modules can handle this sort of parsing--Text::Balanced,
Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.
Take the example case of trying to split a string that is comma-
separated into its different fields. You can't use "split(/,/)" because
you shouldn't split if the comma is inside quotes. For example, take a
data line like this:
SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
Due to the restriction of the quotes, this is a fairly complex problem.
Thankfully, we have Jeffrey Friedl, author of Mastering Regular
Expressions, to handle these for us. He suggests (assuming your string
is contained in $text):
my @new = ();
push(@new, $+) while $text =~ m{
"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';
If you want to represent quotation marks inside a quotation-mark-
delimited field, escape them with backslashes (eg, "like \"this\"".
Alternatively, the Text::ParseWords module (part of the standard Perl
distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
For parsing or generating CSV, though, using Text::CSV rather than
implementing it yourself is highly recommended; you'll save yourself odd
bugs popping up later by just using code which has already been tried
and tested in production for years.
How do I strip blank space from the beginning/end of a string?
(contributed by brian d foy)
A substitution can do this for you. For a single line, you want to
replace all the leading or trailing whitespace with nothing. You can do
that with a pair of substitutions:
s/^\s+//;
s/\s+$//;
You can also write that as a single substitution, although it turns out
the combined statement is slower than the separate ones. That might not
matter to you, though:
s/^\s+|\s+$//g;
In this regular expression, the alternation matches either at the
beginning or the end of the string since the anchors have a lower
precedence than the alternation. With the "/g" flag, the substitution
makes all possible matches, so it gets both. Remember, the trailing
newline matches the "\s+", and the "$" anchor can match to the absolute
end of the string, so the newline disappears too. Just add the newline
to the output, which has the added benefit of preserving "blank"
(consisting entirely of whitespace) lines which the "^\s+" would remove
all by itself:
while( <> ) {
s/^\s+|\s+$//g;
print "$_\n";
}
For a multi-line string, you can apply the regular expression to each
logical line in the string by adding the "/m" flag (for "multi-line").
With the "/m" flag, the "$" matches before an embedded newline, so it
doesn't remove it. This pattern still removes the newline at the end of
the string:
$string =~ s/^\s+|\s+$//gm;
Remember that lines consisting entirely of whitespace will disappear,
since the first part of the alternation can match the entire string and
replace it with nothing. If you need to keep embedded blank lines, you
have to do a little more work. Instead of matching any whitespace (since
that includes a newline), just match the other whitespace:
$string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
How do I pad a string with blanks or pad a number with zeroes?
In the following examples, $pad_len is the length to which you wish to
pad the string, $text or $num contains the string to be padded, and
$pad_char contains the padding character. You can use a single character
string constant instead of the $pad_char variable if you know what it is
in advance. And in the same way you can use an integer in place of
$pad_len if you know the pad length in advance.
The simplest method uses the "sprintf" function. It can pad on the left
or right with blanks and on the left with zeroes and it will not
truncate the result. The "pack" function can only pad strings on the
right with blanks and it will truncate the result to a maximum length of
$pad_len.
# Left padding a string with blanks (no truncation):
my $padded = sprintf("%${pad_len}s", $text);
my $padded = sprintf("%*s", $pad_len, $text); # same thing
# Right padding a string with blanks (no truncation):
my $padded = sprintf("%-${pad_len}s", $text);
my $padded = sprintf("%-*s", $pad_len, $text); # same thing
# Left padding a number with 0 (no truncation):
my $padded = sprintf("%0${pad_len}d", $num);
my $padded = sprintf("%0*d", $pad_len, $num); # same thing
# Right padding a string with blanks using pack (will truncate):
my $padded = pack("A$pad_len",$text);
If you need to pad with a character other than blank or zero you can use
one of the following methods. They all generate a pad string with the
"x" operator and combine that with $text. These methods do not truncate
$text.
Left and right padding with any character, creating a new string:
my $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
my $padded = $text . $pad_char x ( $pad_len - length( $text ) );
Left and right padding with any character, modifying $text directly:
substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
$text .= $pad_char x ( $pad_len - length( $text ) );
How do I extract selected columns from a string?
(contributed by brian d foy)
If you know the columns that contain the data, you can use "substr" to
extract a single column.
my $column = substr( $line, $start_column, $length );
You can use "split" if the columns are separated by whitespace or some
other delimiter, as long as whitespace or the delimiter cannot appear as
part of the data.
my $line = ' fred barney betty ';
my @columns = split /\s+/, $line;
# ( '', 'fred', 'barney', 'betty' );
my $line = 'fred||barney||betty';
my @columns = split /\|/, $line;
# ( 'fred', '', 'barney', '', 'betty' );
If you want to work with comma-separated values, don't do this since
that format is a bit more complicated. Use one of the modules that
handle that format, such as Text::CSV, Text::CSV_XS, or Text::CSV_PP.
If you want to break apart an entire line of fixed columns, you can use
"unpack" with the A (ASCII) format. By using a number after the format
specifier, you can denote the column width. See the "pack" and "unpack"
entries in perlfunc for more details.
my @fields = unpack( $line, "A8 A8 A8 A16 A4" );
Note that spaces in the format argument to "unpack" do not denote
literal spaces. If you have space separated data, you may want "split"
instead.
How do I find the soundex value of a string?
(contributed by brian d foy)
You can use the "Text::Soundex" module. If you want to do fuzzy or close
matching, you might also try the String::Approx, and Text::Metaphone,
and Text::DoubleMetaphone modules.
How can I expand variables in text strings?
(contributed by brian d foy)
If you can avoid it, don't, or if you can use a templating system, such
as Text::Template or Template Toolkit, do that instead. You might even
be able to get the job done with "sprintf" or "printf":
my $string = sprintf 'Say hello to %s and %s', $foo, $bar;
However, for the one-off simple case where I don't want to pull out a
full templating system, I'll use a string that has two Perl scalar
variables in it. In this example, I want to expand $foo and $bar to
their variable's values:
my $foo = 'Fred';
my $bar = 'Barney';
$string = 'Say hello to $foo and $bar';
One way I can do this involves the substitution operator and a double
"/e" flag. The first "/e" evaluates $1 on the replacement side and turns
it into $foo. The second /e starts with $foo and replaces it with its
value. $foo, then, turns into 'Fred', and that's finally what's left in
the string:
$string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'
The "/e" will also silently ignore violations of strict, replacing
undefined variable names with the empty string. Since I'm using the "/e"
flag (twice even!), I have all of the same security problems I have with
"eval" in its string form. If there's something odd in $foo, perhaps
something like "@{[ system "rm -rf /" ]}", then I could get myself in
trouble.
To get around the security problem, I could also pull the values from a
hash instead of evaluating variable names. Using a single "/e", I can
check the hash to ensure the value exists, and if it doesn't, I can
replace the missing value with a marker, in this case "???" to signal
that I missed something:
my $string = 'This has $foo and $bar';
my %Replacements = (
foo => 'Fred',
);
# $string =~ s/\$(\w+)/$Replacements{$1}/g;
$string =~ s/\$(\w+)/
exists $Replacements{$1} ? $Replacements{$1} : '???'
/eg;
print $string;
Does Perl have anything like Ruby's #{} or Python's f string?
Unlike the others, Perl allows you to embed a variable naked in a double
quoted string, e.g. "variable $variable". When there isn't whitespace or
other non-word characters following the variable name, you can add
braces (e.g. "foo ${foo}bar") to ensure correct parsing.
An array can also be embedded directly in a string, and will be expanded
by default with spaces between the elements. The default LIST_SEPARATOR
can be changed by assigning a different string to the special variable
$", such as "local $" = ', ';".
Perl also supports references within a string providing the equivalent
of the features in the other two languages.
"${\ ... }" embedded within a string will work for most simple
statements such as an object->method call. More complex code can be
wrapped in a do block "${\ do{...} }".
When you want a list to be expanded per $", use "@{[ ... ]}".
use Time::Piece;
use Time::Seconds;
my $scalar = 'STRING';
my @array = ( 'zorro', 'a', 1, 'B', 3 );
# Print the current date and time and then Tommorrow
my $t = Time::Piece->new;
say "Now is: ${\ $t->cdate() }";
say "Tomorrow: ${\ do{ my $T=Time::Piece->new + ONE_DAY ; $T->fullday }}";
# some variables in strings
say "This is some scalar I have $scalar, this is an array @array.";
say "You can also write it like this ${scalar} @{array}.";
# Change the $LIST_SEPARATOR
local $" = ':';
say "Set \$\" to delimit with ':' and sort the Array @{[ sort @array ]}";
You may also want to look at the module Quote::Code, and templating
tools such as Template::Toolkit and Mojo::Template.
See also: "How can I expand variables in text strings?" and "How do I
expand function calls in a string?" in this FAQ.
What's wrong with always quoting "$vars"?
The problem is that those double-quotes force stringification--coercing
numbers and references into strings--even when you don't want them to be
strings. Think of it this way: double-quote expansion is used to produce
new strings. If you already have a string, why do you need more?
If you get used to writing odd things like these:
print "$var"; # BAD
my $new = "$old"; # BAD
somefunc("$var"); # BAD
You'll be in trouble. Those should (in 99.8% of the cases) be the
simpler and more direct:
print $var;
my $new = $old;
somefunc($var);
Otherwise, besides slowing you down, you're going to break code when the
thing in the scalar is actually neither a string nor a number, but a
reference:
func(\@array);
sub func {
my $aref = shift;
my $oref = "$aref"; # WRONG
}
You can also get into subtle problems on those few operations in Perl
that actually do care about the difference between a string and a
number, such as the magical "++" autoincrement operator or the syscall()
function.
Stringification also destroys arrays.
my @lines = `command`;
print "@lines"; # WRONG - extra blanks
print @lines; # right
Why don't my <<HERE documents work?
Here documents are found in perlop. Check for these three things:
There must be no space after the << part.
There (probably) should be a semicolon at the end of the opening token
You can't (easily) have any space in front of the tag.
There needs to be at least a line separator after the end token.
If you want to indent the text in the here document, you can do this:
# all in one
(my $VAR = <<HERE_TARGET) =~ s/^\s+//gm;
your text
goes here
HERE_TARGET
But the HERE_TARGET must still be flush against the margin. If you want
that indented also, you'll have to quote in the indentation.
(my $quote = <<' FINIS') =~ s/^\s+//gm;
...we will have peace, when you and all your works have
perished--and the works of your dark master to whom you
would deliver us. You are a liar, Saruman, and a corrupter
of men's hearts. --Theoden in /usr/src/perl/taint.c
FINIS
$quote =~ s/\s+--/\n--/;
A nice general-purpose fixer-upper function for indented here documents
follows. It expects to be called with a here document as its argument.
It looks to see whether each line begins with a common substring, and if
so, strips that substring off. Otherwise, it takes the amount of leading
whitespace found on the first line and removes that much off each
subsequent line.
sub fix {
local $_ = shift;
my ($white, $leader); # common whitespace and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\g1\g2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
($white, $leader) = (/^(\s+)/, '');
}
s/^\s*?$leader(?:$white)?//gm;
return $_;
}
This works with leading special strings, dynamically determined:
my $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
@@@ int
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
@@@ while ( op = (*op->op_ppaddr)() );
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
MAIN_INTERPRETER_LOOP
Or with a fixed amount of leading whitespace, with remaining indentation
correctly preserved:
my $poem = fix<<EVER_ON_AND_ON;
Now far ahead the Road has gone,
And I must follow, if I can,
Pursuing it with eager feet,
Until it joins some larger way
Where many paths and errands meet.
And whither then? I cannot say.
--Bilbo in /usr/src/perl/pp_ctl.c
EVER_ON_AND_ON
Beginning with Perl version 5.26, a much simpler and cleaner way to
write indented here documents has been added to the language: the tilde
(~) modifier. See "Indented Here-docs" in perlop for details.
Data: Arrays
What is the difference between a list and an array?
(contributed by brian d foy)
A list is a fixed collection of scalars. An array is a variable that
holds a variable collection of scalars. An array can supply its
collection for list operations, so list operations also work on arrays:
# slices
( 'dog', 'cat', 'bird' )[2,3];
@animals[2,3];
# iteration
foreach ( qw( dog cat bird ) ) { ... }
foreach ( @animals ) { ... }
my @three = grep { length == 3 } qw( dog cat bird );
my @three = grep { length == 3 } @animals;
# supply an argument list
wash_animals( qw( dog cat bird ) );
wash_animals( @animals );
Array operations, which change the scalars, rearrange them, or add or
subtract some scalars, only work on arrays. These can't work on a list,
which is fixed. Array operations include "shift", "unshift", "push",
"pop", and "splice".
An array can also change its length:
$#animals = 1; # truncate to two elements
$#animals = 10000; # pre-extend to 10,001 elements
You can change an array element, but you can't change a list element:
$animals[0] = 'Rottweiler';
qw( dog cat bird )[0] = 'Rottweiler'; # syntax error!
foreach ( @animals ) {
s/^d/fr/; # works fine
}
foreach ( qw( dog cat bird ) ) {
s/^d/fr/; # Error! Modification of read only value!
}
However, if the list element is itself a variable, it appears that you
can change a list element. However, the list element is the variable,
not the data. You're not changing the list element, but something the
list element refers to. The list element itself doesn't change: it's
still the same variable.
You also have to be careful about context. You can assign an array to a
scalar to get the number of elements in the array. This only works for
arrays, though:
my $count = @animals; # only works with arrays
If you try to do the same thing with what you think is a list, you get a
quite different result. Although it looks like you have a list on the
righthand side, Perl actually sees a bunch of scalars separated by a
comma:
my $scalar = ( 'dog', 'cat', 'bird' ); # $scalar gets bird
Since you're assigning to a scalar, the righthand side is in scalar
context. The comma operator (yes, it's an operator!) in scalar context
evaluates its lefthand side, throws away the result, and evaluates its
righthand side and returns the result. In effect, that list-lookalike
assigns to $scalar its rightmost value. Many people mess this up because
they choose a list-lookalike whose last element is also the count they
expect:
my $scalar = ( 1, 2, 3 ); # $scalar gets 3, accidentally
What is the difference between $array[1] and @array[1]?
(contributed by brian d foy)
The difference is the sigil, that special character in front of the
array name. The "$" sigil means "exactly one item", while the "@" sigil
means "zero or more items". The "$" gets you a single scalar, while the
"@" gets you a list.
The confusion arises because people incorrectly assume that the sigil
denotes the variable type.
The $array[1] is a single-element access to the array. It's going to
return the item in index 1 (or undef if there is no item there). If you
intend to get exactly one element from the array, this is the form you
should use.
The @array[1] is an array slice, although it has only one index. You
can pull out multiple elements simultaneously by specifying additional
indices as a list, like @array[1,4,3,0].
Using a slice on the lefthand side of the assignment supplies list
context to the righthand side. This can lead to unexpected results. For
instance, if you want to read a single line from a filehandle, assigning
to a scalar value is fine:
$array[1] = <STDIN>;
However, in list context, the line input operator returns all of the
lines as a list. The first line goes into @array[1] and the rest of the
lines mysteriously disappear:
@array[1] = <STDIN>; # most likely not what you want
Either the "use warnings" pragma or the -w flag will warn you when you
use an array slice with a single index.
How can I remove duplicate elements from a list or array?
(contributed by brian d foy)
Use a hash. When you think the words "unique" or "duplicated", think
"hash keys".
If you don't care about the order of the elements, you could just create
the hash then extract the keys. It's not important how you create that
hash: just that you use "keys" to get the unique elements.
my %hash = map { $_, 1 } @array;
# or a hash slice: @hash{ @array } = ();
# or a foreach: $hash{$_} = 1 foreach ( @array );
my @unique = keys %hash;
If you want to use a module, try the "uniq" function from List::Util. In
list context it returns the unique elements, preserving their order in
the list. In scalar context, it returns the number of unique elements.
use List::Util qw(uniq);
my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 8 ); # 1,2,3,4,5,6,8
my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 8 ); # 7
You can also go through each element and skip the ones you've seen
before. Use a hash to keep track. The first time the loop sees an
element, that element has no key in %seen. The "next" statement creates
the key and immediately uses its value, which is "undef", so the loop
continues to the "push" and increments the value for that key. The next
time the loop sees that same element, its key exists in the hash and the
value for that key is true (since it's not 0 or "undef"), so the next
skips that iteration and the loop goes to the next element.
my @unique = ();
my %seen = ();
foreach my $elem ( @array ) {
next if $seen{ $elem }++;
push @unique, $elem;
}
You can write this more briefly using a grep, which does the same thing.
my %seen = ();
my @unique = grep { ! $seen{ $_ }++ } @array;
How can I tell whether a certain element is contained in a list or array?
(portions of this answer contributed by Anno Siegel and brian d foy)
Hearing the word "in" is an indication that you probably should have
used a hash, not a list or array, to store your data. Hashes are
designed to answer this question quickly and efficiently. Arrays aren't.
That being said, there are several ways to approach this. If you are
going to make this query many times over arbitrary string values, the
fastest way is probably to invert the original array and maintain a hash
whose keys are the first array's values:
my @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
my %is_blue = ();
for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$some_color}. It might have been a
good idea to keep the blues all in a hash in the first place.
If the values are all small integers, you could use a simple indexed
array. This kind of an array will take up less space:
my @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
my @is_tiny_prime = ();
for (@primes) { $is_tiny_prime[$_] = 1 }
# or simply @istiny_prime[@primes] = (1) x @primes;
Now you check whether $is_tiny_prime[$some_number].
If the values in question are integers instead of strings, you can save
quite a lot of space by using bit strings instead:
my @articles = ( 1..10, 150..2000, 2017 );
undef $read;
for (@articles) { vec($read,$_,1) = 1 }
Now check whether "vec($read,$n,1)" is true for some $n.
These methods guarantee fast individual tests but require a re-
organization of the original list or array. They only pay off if you
have to test multiple values against the same array.
If you are testing only once, the standard module List::Util exports the
function "any" for this purpose. It works by stopping once it finds the
element. It's written in C for speed, and its Perl equivalent looks like
this subroutine:
sub any (&@) {
my $code = shift;
foreach (@_) {
return 1 if $code->();
}
return 0;
}
If speed is of little concern, the common idiom uses grep in scalar
context (which returns the number of items that passed its condition) to
traverse the entire list. This does have the benefit of telling you how
many matches it found, though.
my $is_there = grep $_ eq $whatever, @array;
If you want to actually extract the matching elements, simply use grep
in list context.
my @matches = grep $_ eq $whatever, @array;
How do I compute the difference of two arrays? How do I compute the
intersection of two arrays?
Use a hash. Here's code to do both and more. It assumes that each
element is unique in a given array:
my (@union, @intersection, @difference);
my %count = ();
foreach my $element (@array1, @array2) { $count{$element}++ }
foreach my $element (keys %count) {
push @union, $element;
push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
}
Note that this is the symmetric difference, that is, all elements in
either A or in B but not in both. Think of it as an xor operation.
How do I test whether two arrays or hashes are equal?
The following code works for single-level arrays. It uses a stringwise
comparison, and does not distinguish defined versus undefined empty
strings. Modify if you have other needs.
$are_equal = compare_arrays(\@frogs, \@toads);
sub compare_arrays {
my ($first, $second) = @_;
no warnings; # silence spurious -w undef complaints
return 0 unless @$first == @$second;
for (my $i = 0; $i < @$first; $i++) {
return 0 if $first->[$i] ne $second->[$i];
}
return 1;
}
For multilevel structures, you may wish to use an approach more like
this one. It uses the CPAN module FreezeThaw:
use FreezeThaw qw(cmpStr);
my @a = my @b = ( "this", "that", [ "more", "stuff" ] );
printf "a and b contain %s arrays\n",
cmpStr(\@a, \@b) == 0
? "the same"
: "different";
This approach also works for comparing hashes. Here we'll demonstrate
two different answers:
use FreezeThaw qw(cmpStr cmpStrHard);
my %a = my %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
$a{EXTRA} = \%b;
$b{EXTRA} = \%a;
printf "a and b contain %s hashes\n",
cmpStr(\%a, \%b) == 0 ? "the same" : "different";
printf "a and b contain %s hashes\n",
cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
The first reports that both those the hashes contain the same data,
while the second reports that they do not. Which you prefer is left as
an exercise to the reader.
How do I find the first array element for which a condition is true?
To find the first array element which satisfies a condition, you can use
the first() function in the List::Util module, which comes with Perl
5.8. This example finds the first element that contains "Perl".
use List::Util qw(first);
my $element = first { /Perl/ } @array;
If you cannot use List::Util, you can make your own loop to do the same
thing. Once you find the element, you stop the loop with last.
my $found;
foreach ( @array ) {
if( /Perl/ ) { $found = $_; last }
}
If you want the array index, use the firstidx() function from
"List::MoreUtils":
use List::MoreUtils qw(firstidx);
my $index = firstidx { /Perl/ } @array;
Or write it yourself, iterating through the indices and checking the
array element at each index until you find one that satisfies the
condition:
my( $found, $index ) = ( undef, -1 );
for( $i = 0; $i < @array; $i++ ) {
if( $array[$i] =~ /Perl/ ) {
$found = $array[$i];
$index = $i;
last;
}
}
How do I handle linked lists?
(contributed by brian d foy)
Perl's arrays do not have a fixed size, so you don't need linked lists
if you just want to add or remove items. You can use array operations
such as "push", "pop", "shift", "unshift", or "splice" to do that.
Sometimes, however, linked lists can be useful in situations where you
want to "shard" an array so you have many small arrays instead of a
single big array. You can keep arrays longer than Perl's largest array
index, lock smaller arrays separately in threaded programs, reallocate
less memory, or quickly insert elements in the middle of the chain.
Steve Lembark goes through the details in his YAPC::NA 2009 talk "Perly
Linked Lists" ( <http://www.slideshare.net/lembark/perly-linked-lists>
), although you can just use his LinkedList::Single module.
How do I handle circular lists?
(contributed by brian d foy)
If you want to cycle through an array endlessly, you can increment the
index modulo the number of elements in the array:
my @array = qw( a b c );
my $i = 0;
while( 1 ) {
print $array[ $i++ % @array ], "\n";
last if $i > 20;
}
You can also use Tie::Cycle to use a scalar that always has the next
element of the circular array:
use Tie::Cycle;
tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
print $cycle; # FFFFFF
print $cycle; # 000000
print $cycle; # FFFF00
The Array::Iterator::Circular creates an iterator object for circular
arrays:
use Array::Iterator::Circular;
my $color_iterator = Array::Iterator::Circular->new(
qw(red green blue orange)
);
foreach ( 1 .. 20 ) {
print $color_iterator->next, "\n";
}
How do I shuffle an array randomly?
If you either have Perl 5.8.0 or later installed, or if you have Scalar-
List-Utils 1.03 or later installed, you can say:
use List::Util 'shuffle';
@shuffled = shuffle(@list);
If not, you can use a Fisher-Yates shuffle.
sub fisher_yates_shuffle {
my $deck = shift; # $deck is a reference to an array
return unless @$deck; # must not be empty!
my $i = @$deck;
while (--$i) {
my $j = int rand ($i+1);
@$deck[$i,$j] = @$deck[$j,$i];
}
}
# shuffle my mpeg collection
#
my @mpeg = <audio/*/*.mp3>;
fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
print @mpeg;
Note that the above implementation shuffles an array in place, unlike
the List::Util::shuffle() which takes a list and returns a new shuffled
list.
You've probably seen shuffling algorithms that work using splice,
randomly picking another element to swap the current element with
srand;
@new = ();
@old = 1 .. 10; # just a demo
while (@old) {
push(@new, splice(@old, rand @old, 1));
}
This is bad because splice is already O(N), and since you do it N times,
you just invented a quadratic algorithm; that is, O(N**2). This does
not scale, although Perl is so efficient that you probably won't notice
this until you have rather largish arrays.
How do I process/modify each element of an array?
Use "for"/"foreach":
for (@lines) {
s/foo/bar/; # change that word
tr/XZ/ZX/; # swap those letters
}
Here's another; let's compute spherical volumes:
my @volumes = @radii;
for (@volumes) { # @volumes has changed parts
$_ **= 3;
$_ *= (4/3) * 3.14159; # this will be constant folded
}
which can also be done with map() which is made to transform one list
into another:
my @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
If you want to do the same thing to modify the values of the hash, you
can use the "values" function. As of Perl 5.6 the values are not copied,
so if you modify $orbit (in this case), you modify the value.
for my $orbit ( values %orbits ) {
($orbit **= 3) *= (4/3) * 3.14159;
}
Prior to perl 5.6 "values" returned copies of the values, so older perl
code often contains constructions such as @orbits{keys %orbits} instead
of "values %orbits" where the hash is to be modified.
How do I select a random element from an array?
Use the rand() function (see "rand" in perlfunc):
my $index = rand @array;
my $element = $array[$index];
Or, simply:
my $element = $array[ rand @array ];
How do I permute N elements of a list?
Use the List::Permutor module on CPAN. If the list is actually an array,
try the Algorithm::Permute module (also on CPAN). It's written in XS
code and is very efficient:
use Algorithm::Permute;
my @array = 'a'..'d';
my $p_iterator = Algorithm::Permute->new ( \@array );
while (my @perm = $p_iterator->next) {
print "next permutation: (@perm)\n";
}
For even faster execution, you could do:
use Algorithm::Permute;
my @array = 'a'..'d';
Algorithm::Permute::permute {
print "next permutation: (@array)\n";
} @array;
Here's a little program that generates all permutations of all the words
on each line of input. The algorithm embodied in the permute() function
is discussed in Volume 4 (still unpublished) of Knuth's The Art of
Computer Programming and will work on any list:
#!/usr/bin/perl -n
# Fischer-Krause ordered permutation generator
sub permute (&@) {
my $code = shift;
my @idx = 0..$#_;
while ( $code->(@_[@idx]) ) {
my $p = $#idx;
--$p while $idx[$p-1] > $idx[$p];
my $q = $p or return;
push @idx, reverse splice @idx, $p;
++$q while $idx[$p-1] > $idx[$q];
@idx[$p-1,$q]=@idx[$q,$p-1];
}
}
permute { print "@_\n" } split;
The Algorithm::Loops module also provides the "NextPermute" and
"NextPermuteNum" functions which efficiently find all unique
permutations of an array, even if it contains duplicate values,
modifying it in-place: if its elements are in reverse-sorted order then
the array is reversed, making it sorted, and it returns false; otherwise
the next permutation is returned.
"NextPermute" uses string order and "NextPermuteNum" numeric order, so
you can enumerate all the permutations of 0..9 like this:
use Algorithm::Loops qw(NextPermuteNum);
my @list= 0..9;
do { print "@list\n" } while NextPermuteNum @list;
How do I sort an array by (anything)?
Supply a comparison function to sort() (described in "sort" in
perlfunc):
@list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which would sort
"(1, 2, 10)" into "(1, 10, 2)". "<=>", used above, is the numerical
comparison operator.
If you have a complicated function needed to pull out the part you want
to sort on, then don't do it inside the sort function. Pull it out
first, because the sort BLOCK can be called many times for the same
element. Here's an example of how to pull out the first word after the
first number on each item, and then sort those words case-insensitively.
my @idx;
for (@data) {
my $item;
($item) = /\d+\s*(\S+)/;
push @idx, uc($item);
}
my @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
which could also be written this way, using a trick that's come to be
known as the Schwartzian Transform:
my @sorted = map { $_->[0] }
sort { $a->[1] cmp $b->[1] }
map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
If you need to sort on several fields, the following paradigm is useful.
my @sorted = sort {
field1($a) <=> field1($b) ||
field2($a) cmp field2($b) ||
field3($a) cmp field3($b)
} @data;
This can be conveniently combined with precalculation of keys as given
above.
See the sort article in the "Far More Than You Ever Wanted To Know"
collection in <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz> for more
about this approach.
See also the question later in perlfaq4 on sorting hashes.
How do I manipulate arrays of bits?
Use pack() and unpack(), or else vec() and the bitwise operations.
For example, you don't have to store individual bits in an array (which
would mean that you're wasting a lot of space). To convert an array of
bits to a string, use vec() to set the right bits. This sets $vec to
have bit N set only if $ints[N] was set:
my @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
my $vec = '';
foreach( 0 .. $#ints ) {
vec($vec,$_,1) = 1 if $ints[$_];
}
The string $vec only takes up as many bits as it needs. For instance, if
you had 16 entries in @ints, $vec only needs two bytes to store them
(not counting the scalar variable overhead).
Here's how, given a vector in $vec, you can get those bits into your
@ints array:
sub bitvec_to_list {
my $vec = shift;
my @ints;
# Find null-byte density then select best algorithm
if ($vec =~ tr/\0// / length $vec > 0.95) {
use integer;
my $i;
# This method is faster with mostly null-bytes
while($vec =~ /[^\0]/g ) {
$i = -9 + 8 * pos $vec;
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
}
}
else {
# This method is a fast general algorithm
use integer;
my $bits = unpack "b*", $vec;
push @ints, 0 if $bits =~ s/^(\d)// && $1;
push @ints, pos $bits while($bits =~ /1/g);
}
return \@ints;
}
This method gets faster the more sparse the bit vector is. (Courtesy of
Tim Bunce and Winfried Koenig.)
You can make the while loop a lot shorter with this suggestion from
Benjamin Goldberg:
while($vec =~ /[^\0]+/g ) {
push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
}
Or use the CPAN module Bit::Vector:
my $vector = Bit::Vector->new($num_of_bits);
$vector->Index_List_Store(@ints);
my @ints = $vector->Index_List_Read();
Bit::Vector provides efficient methods for bit vector, sets of small
integers and "big int" math.
Here's a more extensive illustration using vec():
# vec demo
my $vector = "\xff\x0f\xef\xfe";
print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
unpack("N", $vector), "\n";
my $is_set = vec($vector, 23, 1);
print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
pvec($vector);
set_vec(1,1,1);
set_vec(3,1,1);
set_vec(23,1,1);
set_vec(3,1,3);
set_vec(3,2,3);
set_vec(3,4,3);
set_vec(3,4,7);
set_vec(3,8,3);
set_vec(3,8,7);
set_vec(0,32,17);
set_vec(1,32,17);
sub set_vec {
my ($offset, $width, $value) = @_;
my $vector = '';
vec($vector, $offset, $width) = $value;
print "offset=$offset width=$width value=$value\n";
pvec($vector);
}
sub pvec {
my $vector = shift;
my $bits = unpack("b*", $vector);
my $i = 0;
my $BASE = 8;
print "vector length in bytes: ", length($vector), "\n";
@bytes = unpack("A8" x length($vector), $bits);
print "bits are: @bytes\n\n";
}
Why does defined() return true on empty arrays and hashes?
The short story is that you should probably only use defined on scalars
or functions, not on aggregates (arrays and hashes). See "defined" in
perlfunc in the 5.004 release or later of Perl for more detail.
Data: Hashes (Associative Arrays)
How do I process an entire hash?
(contributed by brian d foy)
There are a couple of ways that you can process an entire hash. You can
get a list of keys, then go through each key, or grab a one key-value
pair at a time.
To go through all of the keys, use the "keys" function. This extracts
all of the keys of the hash and gives them back to you as a list. You
can then get the value through the particular key you're processing:
foreach my $key ( keys %hash ) {
my $value = $hash{$key}
...
}
Once you have the list of keys, you can process that list before you
process the hash elements. For instance, you can sort the keys so you
can process them in lexical order:
foreach my $key ( sort keys %hash ) {
my $value = $hash{$key}
...
}
Or, you might want to only process some of the items. If you only want
to deal with the keys that start with "text:", you can select just those
using "grep":
foreach my $key ( grep /^text:/, keys %hash ) {
my $value = $hash{$key}
...
}
If the hash is very large, you might not want to create a long list of
keys. To save some memory, you can grab one key-value pair at a time
using each(), which returns a pair you haven't seen yet:
while( my( $key, $value ) = each( %hash ) ) {
...
}
The "each" operator returns the pairs in apparently random order, so if
ordering matters to you, you'll have to stick with the "keys" method.
The each() operator can be a bit tricky though. You can't add or delete
keys of the hash while you're using it without possibly skipping or re-
processing some pairs after Perl internally rehashes all of the
elements. Additionally, a hash has only one iterator, so if you mix
"keys", "values", or "each" on the same hash, you risk resetting the
iterator and messing up your processing. See the "each" entry in
perlfunc for more details.
How do I merge two hashes?
(contributed by brian d foy)
Before you decide to merge two hashes, you have to decide what to do if
both hashes contain keys that are the same and if you want to leave the
original hashes as they were.
If you want to preserve the original hashes, copy one hash (%hash1) to a
new hash (%new_hash), then add the keys from the other hash (%hash2 to
the new hash. Checking that the key already exists in %new_hash gives
you a chance to decide what to do with the duplicates:
my %new_hash = %hash1; # make a copy; leave %hash1 alone
foreach my $key2 ( keys %hash2 ) {
if( exists $new_hash{$key2} ) {
warn "Key [$key2] is in both hashes!";
# handle the duplicate (perhaps only warning)
...
next;
}
else {
$new_hash{$key2} = $hash2{$key2};
}
}
If you don't want to create a new hash, you can still use this looping
technique; just change the %new_hash to %hash1.
foreach my $key2 ( keys %hash2 ) {
if( exists $hash1{$key2} ) {
warn "Key [$key2] is in both hashes!";
# handle the duplicate (perhaps only warning)
...
next;
}
else {
$hash1{$key2} = $hash2{$key2};
}
}
If you don't care that one hash overwrites keys and values from the
other, you could just use a hash slice to add one hash to another. In
this case, values from %hash2 replace values from %hash1 when they have
keys in common:
@hash1{ keys %hash2 } = values %hash2;
What happens if I add or remove keys from a hash while iterating over it?
(contributed by brian d foy)
The easy answer is "Don't do that!"
If you iterate through the hash with each(), you can delete the key most
recently returned without worrying about it. If you delete or add other
keys, the iterator may skip or double up on them since perl may
rearrange the hash table. See the entry for each() in perlfunc.
How do I look up a hash element by value?
Create a reverse hash:
my %by_value = reverse %by_key;
my $key = $by_value{$value};
That's not particularly efficient. It would be more space-efficient to
use:
while (my ($key, $value) = each %by_key) {
$by_value{$value} = $key;
}
If your hash could have repeated values, the methods above will only
find one of the associated keys. This may or may not worry you. If it
does worry you, you can always reverse the hash into a hash of arrays
instead:
while (my ($key, $value) = each %by_key) {
push @{$key_list_by_value{$value}}, $key;
}
How can I know how many entries are in a hash?
(contributed by brian d foy)
This is very similar to "How do I process an entire hash?", also in
perlfaq4, but a bit simpler in the common cases.
You can use the keys() built-in function in scalar context to find out
how many entries you have in a hash:
my $key_count = keys %hash; # must be scalar context!
If you want to find out how many entries have a defined value, that's a
bit different. You have to check each value. A "grep" is handy:
my $defined_value_count = grep { defined } values %hash;
You can use that same structure to count the entries any way that you
like. If you want the count of the keys with vowels in them, you just
test for that instead:
my $vowel_count = grep { /[aeiou]/ } keys %hash;
The "grep" in scalar context returns the count. If you want the list of
matching items, just use it in list context instead:
my @defined_values = grep { defined } values %hash;
The keys() function also resets the iterator, which means that you may
see strange results if you use this between uses of other hash operators
such as each().
How do I sort a hash (optionally by value instead of key)?
(contributed by brian d foy)
To sort a hash, start with the keys. In this example, we give the list
of keys to the sort function which then compares them as strings. The
output list has the keys in string order. Once we have the keys, we can
go through them to create a report which lists the keys in string order:
my @keys = sort { $a cmp $b } keys %hash;
foreach my $key ( @keys ) {
printf "%-20s %6d\n", $key, $hash{$key};
}
We could get more fancy in the sort() block though. Instead of comparing
the keys, we can compute a value with them and use that value as the
comparison.
For instance, to make our report order case-insensitive, we use "fc" to
safely lowercase the keys before comparing them:
use v5.16;
my @keys = sort { fc($a) cmp fc($b) } keys %hash;
Earlier versions of this answer used "lc", but that could give
unexpected results with some Unicode strings. See "fc" in perlfunc for
the details. The Unicode::UCD module does the same thing for earlier
perls.
Note: if the computation is expensive or the hash has many elements, you
may want to look at the Schwartzian Transform to cache the computation
results.
If we want to sort by the hash value instead, we use the hash key to
look it up. We still get out a list of keys, but this time they are
ordered by their value:
my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
From there we can get more complex. If the hash values are the same, we
can provide a secondary sort on the hash key:
use v5.16;
my @keys = sort {
$hash{$a} <=> $hash{$b}
or
fc($a) cmp fc($b)
} keys %hash;
How can I always keep my hash sorted?
You can look into using the "DB_File" module and tie() using the
$DB_BTREE hash bindings as documented in "In Memory Databases" in
DB_File. The Tie::IxHash module from CPAN might also be instructive.
Although this does keep your hash sorted, you might not like the
slowdown you suffer from the tie interface. Are you sure you need to do
this? :)
What's the difference between "delete" and "undef" with hashes?
Hashes contain pairs of scalars: the first is the key, the second is the
value. The key will be coerced to a string, although the value can be
any kind of scalar: string, number, or reference. If a key $key is
present in %hash, exists($hash{$key}) will return true. The value for a
given key can be "undef", in which case $hash{$key} will be "undef"
while "exists $hash{$key}" will return true. This corresponds to ($key,
"undef") being in the hash.
Pictures help... Here's the %hash table:
keys values
+------+------+
| a | 3 |
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
And these conditions hold
$hash{'a'} is true
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is true
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is true
If you now say
undef $hash{'a'}
your table now reads:
keys values
+------+------+
| a | undef|
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is FALSE
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is FALSE
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
delete $hash{'a'}
your table now reads:
keys values
+------+------+
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is false
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is false
exists $hash{'a'} is FALSE (Perl 5 only)
grep ($_ eq 'a', keys %hash) is FALSE
See, the whole entry is gone!
Why don't my tied hashes make the defined/exists distinction?
This depends on the tied hash's implementation of EXISTS(). For
example, there isn't the concept of undef with hashes that are tied to
DBM* files. It also means that exists() and defined() do the same thing
with a DBM* file, and what they end up doing is not what they do with
ordinary hashes.
How do I reset an each() operation part-way through?
(contributed by brian d foy)
You can use the "keys" or "values" functions to reset "each". To simply
reset the iterator used by "each" without doing anything else, use one
of them in void context:
keys %hash; # resets iterator, nothing else.
values %hash; # resets iterator, nothing else.
See the documentation for "each" in perlfunc.
How can I get the unique keys from two hashes?
First you extract the keys from the hashes into lists, then solve the
"removing duplicates" problem described above. For example:
my %seen = ();
for my $element (keys(%foo), keys(%bar)) {
$seen{$element}++;
}
my @uniq = keys %seen;
Or more succinctly:
my @uniq = keys %{{%foo,%bar}};
Or if you really want to save space:
my %seen = ();
while (defined ($key = each %foo)) {
$seen{$key}++;
}
while (defined ($key = each %bar)) {
$seen{$key}++;
}
my @uniq = keys %seen;
How can I store a multidimensional array in a DBM file?
Either stringify the structure yourself (no fun), or else get the MLDBM
(which uses Data::Dumper) module from CPAN and layer it on top of either
DB_File or GDBM_File. You might also try DBM::Deep, but it can be a bit
slow.
How can I make my hash remember the order I put elements into it?
Use the Tie::IxHash from CPAN.
use Tie::IxHash;
tie my %myhash, 'Tie::IxHash';
for (my $i=0; $i<20; $i++) {
$myhash{$i} = 2*$i;
}
my @keys = keys %myhash;
# @keys = (0,1,2,3,...)
Why does passing a subroutine an undefined element in a hash create it?
(contributed by brian d foy)
Are you using a really old version of Perl?
Normally, accessing a hash key's value for a nonexistent key will not
create the key.
my %hash = ();
my $value = $hash{ 'foo' };
print "This won't print\n" if exists $hash{ 'foo' };
Passing $hash{ 'foo' } to a subroutine used to be a special case,
though. Since you could assign directly to $_[0], Perl had to be ready
to make that assignment so it created the hash key ahead of time:
my_sub( $hash{ 'foo' } );
print "This will print before 5.004\n" if exists $hash{ 'foo' };
sub my_sub {
# $_[0] = 'bar'; # create hash key in case you do this
1;
}
Since Perl 5.004, however, this situation is a special case and Perl
creates the hash key only when you make the assignment:
my_sub( $hash{ 'foo' } );
print "This will print, even after 5.004\n" if exists $hash{ 'foo' };
sub my_sub {
$_[0] = 'bar';
}
However, if you want the old behavior (and think carefully about that
because it's a weird side effect), you can pass a hash slice instead.
Perl 5.004 didn't make this a special case:
my_sub( @hash{ qw/foo/ } );
How can I make the Perl equivalent of a C structure/C++ class/hash or array
of hashes or arrays?
Usually a hash ref, perhaps like this:
$record = {
NAME => "Jason",
EMPNO => 132,
TITLE => "deputy peon",
AGE => 23,
SALARY => 37_000,
PALS => [ "Norbert", "Rhys", "Phineas"],
};
References are documented in perlref and perlreftut. Examples of
complex data structures are given in perldsc and perllol. Examples of
structures and object-oriented classes are in perlootut.
How can I use a reference as a hash key?
(contributed by brian d foy and Ben Morrow)
Hash keys are strings, so you can't really use a reference as the key.
When you try to do that, perl turns the reference into its stringified
form (for instance, HASH(0xDEADBEEF)). From there you can't get back the
reference from the stringified form, at least without doing some extra
work on your own.
Remember that the entry in the hash will still be there even if the
referenced variable goes out of scope, and that it is entirely possible
for Perl to subsequently allocate a different variable at the same
address. This will mean a new variable might accidentally be associated
with the value for an old.
If you have Perl 5.10 or later, and you just want to store a value
against the reference for lookup later, you can use the core
Hash::Util::Fieldhash module. This will also handle renaming the keys if
you use multiple threads (which causes all variables to be reallocated
at new addresses, changing their stringification), and garbage-
collecting the entries when the referenced variable goes out of scope.
If you actually need to be able to get a real reference back from each
hash entry, you can use the Tie::RefHash module, which does the required
work for you.
How can I check if a key exists in a multilevel hash?
(contributed by brian d foy)
The trick to this problem is avoiding accidental autovivification. If
you want to check three keys deep, you might naïvely try this:
my %hash;
if( exists $hash{key1}{key2}{key3} ) {
...;
}
Even though you started with a completely empty hash, after that call to
"exists" you've created the structure you needed to check for "key3":
%hash = (
'key1' => {
'key2' => {}
}
);
That's autovivification. You can get around this in a few ways. The
easiest way is to just turn it off. The lexical "autovivification"
pragma is available on CPAN. Now you don't add to the hash:
{
no autovivification;
my %hash;
if( exists $hash{key1}{key2}{key3} ) {
...;
}
}
The Data::Diver module on CPAN can do it for you too. Its "Dive"
subroutine can tell you not only if the keys exist but also get the
value:
use Data::Diver qw(Dive);
my @exists = Dive( \%hash, qw(key1 key2 key3) );
if( ! @exists ) {
...; # keys do not exist
}
elsif( ! defined $exists[0] ) {
...; # keys exist but value is undef
}
You can easily do this yourself too by checking each level of the hash
before you move onto the next level. This is essentially what
Data::Diver does for you:
if( check_hash( \%hash, qw(key1 key2 key3) ) ) {
...;
}
sub check_hash {
my( $hash, @keys ) = @_;
return unless @keys;
foreach my $key ( @keys ) {
return unless eval { exists $hash->{$key} };
$hash = $hash->{$key};
}
return 1;
}
How can I prevent addition of unwanted keys into a hash?
Since version 5.8.0, hashes can be restricted to a fixed number of given
keys. Methods for creating and dealing with restricted hashes are
exported by the Hash::Util module.
Data: Misc
How do I handle binary data correctly?
Perl is binary-clean, so it can handle binary data just fine. On
Windows or DOS, however, you have to use "binmode" for binary files to
avoid conversions for line endings. In general, you should use "binmode"
any time you want to work with binary data.
Also see "binmode" in perlfunc or perlopentut.
If you're concerned about 8-bit textual data then see perllocale. If
you want to deal with multibyte characters, however, there are some
gotchas. See the section on Regular Expressions.
How do I determine whether a scalar is a number/whole/integer/float?
Assuming that you don't care about IEEE notations like "NaN" or
"Infinity", you probably just want to use a regular expression (see also
perlretut and perlre):
use 5.010;
if ( /\D/ )
{ say "\thas nondigits"; }
if ( /^\d+\z/ )
{ say "\tis a whole number"; }
if ( /^-?\d+\z/ )
{ say "\tis an integer"; }
if ( /^[+-]?\d+\z/ )
{ say "\tis a +/- integer"; }
if ( /^-?(?:\d+\.?|\.\d)\d*\z/ )
{ say "\tis a real number"; }
if ( /^[+-]?(?=\.?\d)\d*\.?\d*(?:e[+-]?\d+)?\z/i )
{ say "\tis a C float" }
There are also some commonly used modules for the task. Scalar::Util
(distributed with 5.8) provides access to perl's internal function
"looks_like_number" for determining whether a variable looks like a
number. Data::Types exports functions that validate data types using
both the above and other regular expressions. Thirdly, there is
Regexp::Common which has regular expressions to match various types of
numbers. Those three modules are available from the CPAN.
If you're on a POSIX system, Perl supports the "POSIX::strtod" function
for converting strings to doubles (and also "POSIX::strtol" for longs).
Its semantics are somewhat cumbersome, so here's a "getnum" wrapper
function for more convenient access. This function takes a string and
returns the number it found, or "undef" for input that isn't a C float.
The "is_numeric" function is a front end to "getnum" if you just want to
say, "Is this a float?"
sub getnum {
use POSIX qw(strtod);
my $str = shift;
$str =~ s/^\s+//;
$str =~ s/\s+$//;
$! = 0;
my($num, $unparsed) = strtod($str);
if (($str eq '') || ($unparsed != 0) || $!) {
return undef;
}
else {
return $num;
}
}
sub is_numeric { defined getnum($_[0]) }
Or you could check out the String::Scanf module on the CPAN instead.
How do I keep persistent data across program calls?
For some specific applications, you can use one of the DBM modules. See
AnyDBM_File. More generically, you should consult the FreezeThaw or
Storable modules from CPAN. Starting from Perl 5.8, Storable is part of
the standard distribution. Here's one example using Storable's "store"
and "retrieve" functions:
use Storable;
store(\%hash, "filename");
# later on...
$href = retrieve("filename"); # by ref
%hash = %{ retrieve("filename") }; # direct to hash
How do I print out or copy a recursive data structure?
The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
for printing out data structures. The Storable module on CPAN (or the
5.8 release of Perl), provides a function called "dclone" that
recursively copies its argument.
use Storable qw(dclone);
$r2 = dclone($r1);
Where $r1 can be a reference to any kind of data structure you'd like.
It will be deeply copied. Because "dclone" takes and returns references,
you'd have to add extra punctuation if you had a hash of arrays that you
wanted to copy.
%newhash = %{ dclone(\%oldhash) };
How do I define methods for every class/object?
(contributed by Ben Morrow)
You can use the "UNIVERSAL" class (see UNIVERSAL). However, please be
very careful to consider the consequences of doing this: adding methods
to every object is very likely to have unintended consequences. If
possible, it would be better to have all your object inherit from some
common base class, or to use an object system like Moose that supports
roles.
How do I verify a credit card checksum?
Get the Business::CreditCard module from CPAN.
How do I pack arrays of doubles or floats for XS code?
The arrays.h/arrays.c code in the PGPLOT module on CPAN does just this.
If you're doing a lot of float or double processing, consider using the
PDL module from CPAN instead--it makes number-crunching easy.
See <https://metacpan.org/release/PGPLOT> for the code.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other
authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file are
hereby placed into the public domain. You are permitted and encouraged
to use this code in your own programs for fun or for profit as you see
fit. A simple comment in the code giving credit would be courteous but
is not required.
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