PERLDATA(1) Perl Programmers Reference Guide PERLDATA(1)
NAME
perldata - Perl data types
DESCRIPTION
Variable names
Perl has three built-in data types: scalars, arrays of scalars, and
associative arrays of scalars, known as "hashes". A scalar is a single
string (of any size, limited only by the available memory), number, or a
reference to something (which will be discussed in perlref). Normal
arrays are ordered lists of scalars indexed by number, starting with 0.
Hashes are unordered collections of scalar values indexed by their
associated string key.
Values are usually referred to by name, or through a named reference.
The first character of the name tells you to what sort of data structure
it refers. The rest of the name tells you the particular value to which
it refers. Usually this name is a single identifier, that is, a string
beginning with a letter or underscore, and containing letters,
underscores, and digits. In some cases, it may be a chain of
identifiers, separated by "::" (or by the deprecated "'"); all but the
last are interpreted as names of packages, to locate the namespace in
which to look up the final identifier (see "Packages" in perlmod for
details). For a more in-depth discussion on identifiers, see
"Identifier parsing". It's possible to substitute for a simple
identifier, an expression that produces a reference to the value at
runtime. This is described in more detail below and in perlref.
Perl also has its own built-in variables whose names don't follow these
rules. They have strange names so they don't accidentally collide with
one of your normal variables. Strings that match parenthesized parts of
a regular expression are saved under names containing only digits after
the "$" (see perlop and perlre). In addition, several special variables
that provide windows into the inner working of Perl have names
containing punctuation characters. These are documented in perlvar.
Scalar values are always named with '$', even when referring to a scalar
that is part of an array or a hash. The '$' symbol works semantically
like the English word "the" in that it indicates a single value is
expected.
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$days{'Feb'} # the 'Feb' value from hash %days
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are denoted by '@',
which works much as the word "these" or "those" does in English, in that
it indicates multiple values are expected.
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as ($days[3],$days[4],$days[5])
@days{'a','c'} # same as ($days{'a'},$days{'c'})
Entire hashes are denoted by '%':
%days # (key1, val1, key2, val2 ...)
In addition, subroutines are named with an initial '&', though this is
optional when unambiguous, just as the word "do" is often redundant in
English. Symbol table entries can be named with an initial '*', but you
don't really care about that yet (if ever :-).
Every variable type has its own namespace, as do several non-variable
identifiers. This means that you can, without fear of conflict, use the
same name for a scalar variable, an array, or a hash--or, for that
matter, for a filehandle, a directory handle, a subroutine name, a
format name, or a label. This means that $foo and @foo are two
different variables. It also means that $foo[1] is a part of @foo, not
a part of $foo. This may seem a bit weird, but that's okay, because it
is weird.
Because variable references always start with '$', '@', or '%', the
"reserved" words aren't in fact reserved with respect to variable names.
They are reserved with respect to labels and filehandles, however, which
don't have an initial special character. You can't have a filehandle
named "log", for instance. Hint: you could say "open(LOG,'logfile')"
rather than "open(log,'logfile')". Using uppercase filehandles also
improves readability and protects you from conflict with future reserved
words. Case is significant--"FOO", "Foo", and "foo" are all different
names. Names that start with a letter or underscore may also contain
digits and underscores.
It is possible to replace such an alphanumeric name with an expression
that returns a reference to the appropriate type. For a description of
this, see perlref.
Names that start with a digit may contain only more digits. Names that
do not start with a letter, underscore, digit or a caret are limited to
one character, e.g., $% or $$. (Most of these one character names have
a predefined significance to Perl. For instance, $$ is the current
process id. And all such names are reserved for Perl's possible use.)
Identifier parsing
Up until Perl 5.18, the actual rules of what a valid identifier was were
a bit fuzzy. However, in general, anything defined here should work on
previous versions of Perl, while the opposite -- edge cases that work in
previous versions, but aren't defined here -- probably won't work on
newer versions. As an important side note, please note that the
following only applies to bareword identifiers as found in Perl source
code, not identifiers introduced through symbolic references, which have
much fewer restrictions. If working under the effect of the "use utf8;"
pragma, the following rules apply:
/ (?[ ( \p{Word} & \p{XID_Start} ) + [_] ])
(?[ ( \p{Word} & \p{XID_Continue} ) ]) * /x
That is, a "start" character followed by any number of "continue"
characters. Perl requires every character in an identifier to also
match "\w" (this prevents some problematic cases); and Perl additionally
accepts identifier names beginning with an underscore.
If not under "use utf8", the source is treated as ASCII + 128 extra
generic characters, and identifiers should match
/ (?aa) (?!\d) \w+ /x
That is, any word character in the ASCII range, as long as the first
character is not a digit.
There are two package separators in Perl: A double colon ("::") and a
single quote ("'"). Use of "'" as the package separator is deprecated
and will be removed in Perl 5.40. Normal identifiers can start or end
with a double colon, and can contain several parts delimited by double
colons. Single quotes have similar rules, but with the exception that
they are not legal at the end of an identifier: That is, "$'foo" and
"$foo'bar" are legal, but "$foo'bar'" is not.
Additionally, if the identifier is preceded by a sigil -- that is, if
the identifier is part of a variable name -- it may optionally be
enclosed in braces.
While you can mix double colons with singles quotes, the quotes must
come after the colons: "$::::'foo" and "$foo::'bar" are legal, but
"$::'::foo" and "$foo'::bar" are not.
Put together, a grammar to match a basic identifier becomes
/
(?(DEFINE)
(?<variable>
(?&sigil)
(?:
(?&normal_identifier)
| \{ \s* (?&normal_identifier) \s* \}
)
)
(?<normal_identifier>
(?: :: )* '?
(?&basic_identifier)
(?: (?= (?: :: )+ '? | (?: :: )* ' ) (?&normal_identifier) )?
(?: :: )*
)
(?<basic_identifier>
# is use utf8 on?
(?(?{ (caller(0))[8] & $utf8::hint_bits })
(?&Perl_XIDS) (?&Perl_XIDC)*
| (?aa) (?!\d) \w+
)
)
(?<sigil> [&*\$\@\%])
(?<Perl_XIDS> (?[ ( \p{Word} & \p{XID_Start} ) + [_] ]) )
(?<Perl_XIDC> (?[ \p{Word} & \p{XID_Continue} ]) )
)
/x
Meanwhile, special identifiers don't follow the above rules; For the
most part, all of the identifiers in this category have a special
meaning given by Perl. Because they have special parsing rules, these
generally can't be fully-qualified. They come in six forms (but don't
use forms 5 and 6):
1. A sigil, followed solely by digits matching "\p{POSIX_Digit}", like
$0, $1, or $10000.
2. A sigil followed by a single character matching the
"\p{POSIX_Punct}" property, like $! or "%+", except the character
"{" doesn't work.
3. A sigil, followed by a caret and any one of the characters
"[][A-Z^_?\]", like $^V or $^].
4. Similar to the above, a sigil, followed by bareword text in braces,
where the first character is a caret. The next character is any one
of the characters "[][A-Z^_?\]", followed by ASCII word characters.
An example is "${^GLOBAL_PHASE}".
5. A sigil, followed by any single character in the range
"[\xA1-\xAC\xAE-\xFF]" when not under "use utf8". (Under
"use utf8", the normal identifier rules given earlier in this
section apply.) Use of non-graphic characters (the C1 controls, the
NO-BREAK SPACE, and the SOFT HYPHEN) has been disallowed since
v5.26.0. The use of the other characters is unwise, as these are
all reserved to have special meaning to Perl, and none of them
currently do have special meaning, though this could change without
notice.
Note that an implication of this form is that there are identifiers
only legal under "use utf8", and vice-versa, for example the
identifier "$état" is legal under "use utf8", but is otherwise
considered to be the single character variable $é followed by the
bareword "tat", the combination of which is a syntax error.
6. This is a combination of the previous two forms. It is valid only
when not under "use utf8" (normal identifier rules apply when under
"use utf8"). The form is a sigil, followed by text in braces, where
the first character is any one of the characters in the range
"[\x80-\xFF]" followed by ASCII word characters up to the trailing
brace.
The same caveats as the previous form apply: The non-graphic
characters are no longer allowed with "use utf8", it is unwise to
use this form at all, and utf8ness makes a big difference.
Prior to Perl v5.24, non-graphical ASCII control characters were also
allowed in some situations; this had been deprecated since v5.20.
Context
The interpretation of operations and values in Perl sometimes depends on
the requirements of the context around the operation or value. There
are two major contexts: list and scalar. Certain operations return list
values in contexts wanting a list, and scalar values otherwise. If this
is true of an operation it will be mentioned in the documentation for
that operation. In other words, Perl overloads certain operations based
on whether the expected return value is singular or plural. Some words
in English work this way, like "fish" and "sheep".
In a reciprocal fashion, an operation provides either a scalar or a list
context to each of its arguments. For example, if you say
int( <STDIN> )
the integer operation provides scalar context for the <> operator, which
responds by reading one line from STDIN and passing it back to the
integer operation, which will then find the integer value of that line
and return that. If, on the other hand, you say
sort( <STDIN> )
then the sort operation provides list context for <>, which will proceed
to read every line available up to the end of file, and pass that list
of lines back to the sort routine, which will then sort those lines and
return them as a list to whatever the context of the sort was.
Assignment is a little bit special in that it uses its left argument to
determine the context for the right argument. Assignment to a scalar
evaluates the right-hand side in scalar context, while assignment to an
array or hash evaluates the righthand side in list context. Assignment
to a list (or slice, which is just a list anyway) also evaluates the
right-hand side in list context.
When you use the "use warnings" pragma or Perl's -w command-line option,
you may see warnings about useless uses of constants or functions in
"void context". Void context just means the value has been discarded,
such as a statement containing only ""fred";" or "getpwuid(0);". It
still counts as scalar context for functions that care whether or not
they're being called in list context.
User-defined subroutines may choose to care whether they are being
called in a void, scalar, or list context. Most subroutines do not need
to bother, though. That's because both scalars and lists are
automatically interpolated into lists. See "wantarray" in perlfunc for
how you would dynamically discern your function's calling context.
Scalar values
All data in Perl is a scalar, an array of scalars, or a hash of scalars.
A scalar may contain one single value in any of three different flavors:
a number, a string, or a reference. In general, conversion from one
form to another is transparent. Although a scalar may not directly hold
multiple values, it may contain a reference to an array or hash which in
turn contains multiple values.
Scalars aren't necessarily one thing or another. There's no place to
declare a scalar variable to be of type "string", type "number", type
"reference", or anything else. Because of the automatic conversion of
scalars, operations that return scalars don't need to care (and in fact,
cannot care) whether their caller is looking for a string, a number, or
a reference. Perl is a contextually polymorphic language whose scalars
can be strings, numbers, or references (which includes objects).
Although strings and numbers are considered pretty much the same thing
for nearly all purposes, references are strongly-typed, uncastable
pointers with builtin reference-counting and destructor invocation.
A scalar value is interpreted as FALSE in the Boolean sense if it is
undefined, the null string or the number 0 (or its string equivalent,
"0"), and TRUE if it is anything else. The Boolean context is just a
special kind of scalar context where no conversion to a string or a
number is ever performed. Negation of a true value by "!" or "not"
returns a special false value. When evaluated as a string it is treated
as "", but as a number, it is treated as 0. Most Perl operators that
return true or false behave this way.
There are actually two varieties of null strings (sometimes referred to
as "empty" strings), a defined one and an undefined one. The defined
version is just a string of length zero, such as "". The undefined
version is the value that indicates that there is no real value for
something, such as when there was an error, or at end of file, or when
you refer to an uninitialized variable or element of an array or hash.
Although in early versions of Perl, an undefined scalar could become
defined when first used in a place expecting a defined value, this no
longer happens except for rare cases of autovivification as explained in
perlref. You can use the defined() operator to determine whether a
scalar value is defined (this has no meaning on arrays or hashes), and
the undef() operator to produce an undefined value.
To find out whether a given string is a valid non-zero number, it's
sometimes enough to test it against both numeric 0 and also lexical "0"
(although this will cause noises if warnings are on). That's because
strings that aren't numbers count as 0, just as they do in awk:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
}
That method may be best because otherwise you won't treat IEEE notations
like "NaN" or "Infinity" properly. At other times, you might prefer to
determine whether string data can be used numerically by calling the
POSIX::strtod() function or by inspecting your string with a regular
expression (as documented in perlre).
warn "has nondigits" if /\D/;
warn "not a natural number" unless /^\d+$/; # rejects -3
warn "not an integer" unless /^-?\d+$/; # rejects +3
warn "not an integer" unless /^[+-]?\d+$/;
warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
warn "not a C float"
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
The length of an array is a scalar value. You may find the length of
array @days by evaluating $#days, as in csh. However, this isn't the
length of the array; it's the subscript of the last element, which is a
different value since there is ordinarily a 0th element. Assigning to
$#days actually changes the length of the array. Shortening an array
this way destroys intervening values. Lengthening an array that was
previously shortened does not recover values that were in those
elements.
You can also gain some minuscule measure of efficiency by pre-extending
an array that is going to get big. You can also extend an array by
assigning to an element that is off the end of the array. You can
truncate an array down to nothing by assigning the null list () to it.
The following are equivalent:
@whatever = ();
$#whatever = -1;
If you evaluate an array in scalar context, it returns the length of the
array. (Note that this is not true of lists, which return the last
value, like the C comma operator, nor of built-in functions, which
return whatever they feel like returning.) The following is always
true:
scalar(@whatever) == $#whatever + 1;
Some programmers choose to use an explicit conversion so as to leave
nothing to doubt:
$element_count = scalar(@whatever);
If you evaluate a hash in scalar context, it returns a false value if
the hash is empty. If there are any key/value pairs, it returns a true
value. A more precise definition is version dependent.
Prior to Perl 5.25 the value returned was a string consisting of the
number of used buckets and the number of allocated buckets, separated by
a slash. This is pretty much useful only to find out whether Perl's
internal hashing algorithm is performing poorly on your data set. For
example, you stick 10,000 things in a hash, but evaluating %HASH in
scalar context reveals "1/16", which means only one out of sixteen
buckets has been touched, and presumably contains all 10,000 of your
items. This isn't supposed to happen.
As of Perl 5.25 the return was changed to be the count of keys in the
hash. If you need access to the old behavior you can use
Hash::Util::bucket_ratio() instead.
If a tied hash is evaluated in scalar context, the "SCALAR" method is
called (with a fallback to "FIRSTKEY").
You can preallocate space for a hash by assigning to the keys()
function. This rounds up the allocated buckets to the next power of
two:
keys(%users) = 1000; # allocate 1024 buckets
Scalar value constructors
Numeric literals are specified in any of the following floating point or
integer formats:
12345
12345.67
.23E-10 # a very small number
3.14_15_92 # a very important number
4_294_967_296 # underscore for legibility
0xff # hex
0xdead_beef # more hex
0377 # octal (only numbers, begins with 0)
0o12_345 # alternative octal (introduced in Perl 5.33.5)
0b011011 # binary
0x1.999ap-4 # hexadecimal floating point (the 'p' is required)
You are allowed to use underscores (underbars) in numeric literals
between digits for legibility (but not multiple underscores in a row:
"23__500" is not legal; "23_500" is). You could, for example, group
binary digits by threes (as for a Unix-style mode argument such as
0b110_100_100) or by fours (to represent nibbles, as in 0b1010_0110) or
in other groups.
String literals are usually delimited by either single or double quotes.
They work much like quotes in the standard Unix shells: double-quoted
string literals are subject to backslash and variable substitution;
single-quoted strings are not (except for "\'" and "\\"). The usual
C-style backslash rules apply for making characters such as newline,
tab, etc., as well as some more exotic forms. See "Quote and Quote-like
Operators" in perlop for a list.
Hexadecimal, octal, or binary, representations in string literals (e.g.
'0xff') are not automatically converted to their integer representation.
The hex() and oct() functions make these conversions for you. See "hex"
in perlfunc and "oct" in perlfunc for more details.
Hexadecimal floating point can start just like a hexadecimal literal,
and it can be followed by an optional fractional hexadecimal part, but
it must be followed by "p", an optional sign, and a power of two. The
format is useful for accurately presenting floating point values,
avoiding conversions to or from decimal floating point, and therefore
avoiding possible loss in precision. Notice that while most current
platforms use the 64-bit IEEE 754 floating point, not all do. Another
potential source of (low-order) differences are the floating point
rounding modes, which can differ between CPUs, operating systems, and
compilers, and which Perl doesn't control.
You can also embed newlines directly in your strings, i.e., they can end
on a different line than they begin. This is nice, but if you forget
your trailing quote, the error will not be reported until Perl finds
another line containing the quote character, which may be much further
on in the script. Variable substitution inside strings is limited to
scalar variables, arrays, and array or hash slices. (In other words,
names beginning with $ or @, followed by an optional bracketed
expression as a subscript.) The following code segment prints out "The
price is $100."
$Price = '$100'; # not interpolated
print "The price is $Price.\n"; # interpolated
There is no double interpolation in Perl, so the $100 is left as is.
By default floating point numbers substituted inside strings use the dot
(".") as the decimal separator. If "use locale" is in effect, and
POSIX::setlocale() has been called, the character used for the decimal
separator is affected by the LC_NUMERIC locale. See perllocale and
POSIX.
Demarcated variable names using braces
As in some shells, you can enclose the variable name in braces as a
demarcator to disambiguate it from following alphanumerics and
underscores or other text. You must also do this when interpolating a
variable into a string to separate the variable name from a following
double-colon or an apostrophe since these would be otherwise treated as
a package separator:
$who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
print "We use ${who}speak when ${who}'s here.\n";
Without the braces, Perl would have looked for a $whospeak, a $who::0,
and a "$who's" variable. The last two would be the $0 and the $s
variables in the (presumably) non-existent package "who".
In fact, a simple identifier within such curly braces is forced to be a
string, and likewise within a hash subscript. Neither need quoting. Our
earlier example, $days{'Feb'} can be written as $days{Feb} and the
quotes will be assumed automatically. But anything more complicated in
the subscript will be interpreted as an expression. This means for
example that "$version{2.0}++" is equivalent to "$version{2}++", not to
"$version{'2.0'}++".
There is a similar problem with interpolation with text that looks like
array or hash access notation. Placing a simple variable like $who
immediately in front of text like "[1]" or "{foo}" would cause the
variable to be interpolated as accessing an element of @who or a value
stored in %who:
$who = "Larry Wall";
print "$who[1] is the father of Perl.\n";
would attempt to access index 1 of an array named @who. Again, using
braces will prevent this from happening:
$who = "Larry Wall";
print "${who}[1] is the father of Perl.\n";
will be treated the same as
$who = "Larry Wall";
print $who . "[1] is the father of Perl.\n";
This notation also applies to more complex variable descriptions, such
as array or hash access with subscripts. For instance
@name = qw(Larry Curly Moe);
print "Also ${name[0]}[1] was a member\n";
Without the braces the above example would be parsed as a two level
array subscript in the @name array, and under "use strict" would likely
produce a fatal exception, as it would be parsed like this:
print "Also " . $name[0][1] . " was a member\n";
and not as the intended:
print "Also " . $name[0] . "[1] was a member\n";
A similar result may be derived by using a backslash on the first
character of the subscript or package notation that is not part of the
variable you want to access. Thus the above example could also be
written:
@name = qw(Larry Curly Moe);
print "Also $name[0]\[1] was a member\n";
however for some special variables (multi character caret variables) the
demarcated form using curly braces is the only way you can reference the
variable at all, and the only way you can access a subscript of the
variable via interpolation.
Consider the magic array "@{^CAPTURE}" which is populated by the regex
engine with the contents of all of the capture buffers in a pattern (see
perlvar and perlre). The only way you can access one of these members
inside of a string is via the braced (demarcated) form:
"abc"=~/(.)(.)(.)/
and print "Second buffer is ${^CAPTURE[1]}";
is equivalent to
"abc"=~/(.)(.)(.)/
and print "Second buffer is " . ${^CAPTURE}[1];
Saying "@^CAPTURE" is a syntax error, so it must be referenced as
"@{^CAPTURE}", and to access one of its elements in normal code you
would write " ${^CAPTURE}[1] ". However when interpolating in a string
"${^CAPTURE}[1]" would be equivalent to "${^CAPTURE} . "[1]"", which
does not even refer to the same variable! Thus the subscripts must also
be placed inside of the braces: "${^CAPTURE[1]}".
The demarcated form using curly braces can be used with all the
different types of variable access, including array and hash slices. For
instance code like the following:
@name = qw(Larry Curly Moe);
local $" = " and ";
print "My favorites were @{name[1,2]}.\n";
would output
My favorites were Curly and Moe.
Special floating point: infinity (Inf) and not-a-number (NaN)
Floating point values include the special values "Inf" and "NaN", for
infinity and not-a-number. The infinity can be also negative.
The infinity is the result of certain math operations that overflow the
floating point range, like 9**9**9. The not-a-number is the result when
the result is undefined or unrepresentable. Though note that you cannot
get "NaN" from some common "undefined" or "out-of-range" operations like
dividing by zero, or square root of a negative number, since Perl
generates fatal errors for those.
The infinity and not-a-number have their own special arithmetic rules.
The general rule is that they are "contagious": "Inf" plus one is "Inf",
and "NaN" plus one is "NaN". Where things get interesting is when you
combine infinities and not-a-numbers: "Inf" minus "Inf" and "Inf"
divided by "Inf" are "NaN" (while "Inf" plus "Inf" is "Inf" and "Inf"
times "Inf" is "Inf"). "NaN" is also curious in that it does not equal
any number, including itself: "NaN" != "NaN".
Perl doesn't understand "Inf" and "NaN" as numeric literals, but you can
have them as strings, and Perl will convert them as needed: "Inf" + 1.
(You can, however, import them from the POSIX extension; "use POSIX
qw(Inf NaN);" and then use them as literals.)
Note that on input (string to number) Perl accepts "Inf" and "NaN" in
many forms. Case is ignored, and the Win32-specific forms like
"1.#INF" are understood, but on output the values are normalized to
"Inf" and "NaN".
Version Strings
A literal of the form "v1.20.300.4000" is parsed as a string composed of
characters with the specified ordinals. This form, known as v-strings,
provides an alternative, more readable way to construct strings, rather
than use the somewhat less readable interpolation form
"\x{1}\x{14}\x{12c}\x{fa0}". This is useful for representing Unicode
strings, and for comparing version "numbers" using the string comparison
operators, "cmp", "gt", "lt" etc. If there are two or more dots in the
literal, the leading "v" may be omitted.
print v9786; # prints SMILEY, "\x{263a}"
print v102.111.111; # prints "foo"
print 102.111.111; # same
Such literals are accepted by both "require" and "use" for doing a
version check. Note that using the v-strings for IPv4 addresses is not
portable unless you also use the inet_aton()/inet_ntoa() routines of the
Socket package.
Note that since Perl 5.8.1 the single-number v-strings (like "v65") are
not v-strings before the "=>" operator (which is usually used to
separate a hash key from a hash value); instead they are interpreted as
literal strings ('v65'). They were v-strings from Perl 5.6.0 to Perl
5.8.0, but that caused more confusion and breakage than good. Multi-
number v-strings like "v65.66" and 65.66.67 continue to be v-strings
always.
Special Literals
The special literals __FILE__, __LINE__, and __PACKAGE__ represent the
current filename, line number, and package name at that point in your
program. __SUB__ gives a reference to the current subroutine. They may
be used only as separate tokens; they will not be interpolated into
strings. If there is no current package (due to an empty "package;"
directive), __PACKAGE__ is the undefined value. (But the empty
"package;" is no longer supported, as of version 5.10.) Outside of a
subroutine, __SUB__ is the undefined value. __SUB__ is only available
in 5.16 or higher, and only with a "use v5.16" or "use feature
"current_sub"" declaration.
The two control characters ^D and ^Z, and the tokens __END__ and
__DATA__ may be used to indicate the logical end of the script before
the actual end of file. Any following text is ignored by the
interpreter unless read by the program as described below.
Text after __DATA__ may be read via the filehandle "PACKNAME::DATA",
where "PACKNAME" is the package that was current when the __DATA__ token
was encountered. The filehandle is left open pointing to the line after
__DATA__. The program should "close DATA" when it is done reading from
it. (Leaving it open leaks filehandles if the module is reloaded for
any reason, so it's a safer practice to close it.) For compatibility
with older scripts written before __DATA__ was introduced, __END__
behaves like __DATA__ in the top level script (but not in files loaded
with "require" or "do") and leaves the remaining contents of the file
accessible via "main::DATA".
while (my $line = <DATA>) { print $line; }
close DATA;
__DATA__
Hello world.
The "DATA" file handle by default has whatever PerlIO layers were in
place when Perl read the file to parse the source. Normally that means
that the file is being read bytewise, as if it were encoded in Latin-1,
but there are two major ways for it to be otherwise. Firstly, if the
"__END__"/"__DATA__" token is in the scope of a "use utf8" pragma then
the "DATA" handle will be in UTF-8 mode. And secondly, if the source is
being read from perl's standard input then the "DATA" file handle is
actually aliased to the "STDIN" file handle, and may be in UTF-8 mode
because of the "PERL_UNICODE" environment variable or perl's command-
line switches.
See SelfLoader for more description of __DATA__, and an example of its
use. Note that you cannot read from the DATA filehandle in a BEGIN
block: the BEGIN block is executed as soon as it is seen (during
compilation), at which point the corresponding __DATA__ (or __END__)
token has not yet been seen.
Barewords
A word that has no other interpretation in the grammar will be treated
as if it were a quoted string. These are known as "barewords". As with
filehandles and labels, a bareword that consists entirely of lowercase
letters risks conflict with future reserved words, and if you use the
"use warnings" pragma or the -w switch, Perl will warn you about any
such words. Perl limits barewords (like identifiers) to about 250
characters. Future versions of Perl are likely to eliminate these
arbitrary limitations.
Some people may wish to outlaw barewords entirely. If you say
use strict 'subs';
then any bareword that would NOT be interpreted as a subroutine call
produces a compile-time error instead. The restriction lasts to the end
of the enclosing block. An inner block may countermand this by saying
"no strict 'subs'".
Array Interpolation
Arrays and slices are interpolated into double-quoted strings by joining
the elements with the delimiter specified in the $" variable
($LIST_SEPARATOR if "use English;" is specified), space by default. The
following are equivalent:
$temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish substitution)
there is an unfortunate ambiguity: Is "/$foo[bar]/" to be interpreted
as "/${foo}[bar]/" (where "[bar]" is a character class for the regular
expression) or as "/${foo[bar]}/" (where "[bar]" is the subscript to
array @foo)? If @foo doesn't otherwise exist, then it's obviously a
character class. If @foo exists, Perl takes a good guess about "[bar]",
and is almost always right. If it does guess wrong, or if you're just
plain paranoid, you can force the correct interpretation with curly
braces as above.
If you're looking for the information on how to use here-documents,
which used to be here, that's been moved to "Quote and Quote-like
Operators" in perlop.
List value constructors
List values are denoted by separating individual values by commas (and
enclosing the list in parentheses where precedence requires it):
(LIST)
In a context not requiring a list value, the value of what appears to be
a list literal is simply the value of the final element, as with the C
comma operator. For example,
@foo = ('cc', '-E', $bar);
assigns the entire list value to array @foo, but
$foo = ('cc', '-E', $bar);
assigns the value of variable $bar to the scalar variable $foo. Note
that the value of an actual array in scalar context is the length of the
array; the following assigns the value 3 to $foo:
@foo = ('cc', '-E', $bar);
$foo = @foo; # $foo gets 3
You may have an optional comma before the closing parenthesis of a list
literal, so that you can say:
@foo = (
1,
2,
3,
);
To use a here-document to assign an array, one line per element, you
might use an approach like this:
@sauces = <<End_Lines =~ m/(\S.*\S)/g;
normal tomato
spicy tomato
green chile
pesto
white wine
End_Lines
LISTs do automatic interpolation of sublists. That is, when a LIST is
evaluated, each element of the list is evaluated in list context, and
the resulting list value is interpolated into LIST just as if each
individual element were a member of LIST. Thus arrays and hashes lose
their identity in a LIST--the list
(@foo,@bar,&SomeSub,%glarch)
contains all the elements of @foo followed by all the elements of @bar,
followed by all the elements returned by the subroutine named SomeSub
called in list context, followed by the key/value pairs of %glarch. To
make a list reference that does NOT interpolate, see perlref.
The null list is represented by (). Interpolating it in a list has no
effect. Thus ((),(),()) is equivalent to (). Similarly, interpolating
an array with no elements is the same as if no array had been
interpolated at that point.
This interpolation combines with the facts that the opening and closing
parentheses are optional (except when necessary for precedence) and
lists may end with an optional comma to mean that multiple commas within
lists are legal syntax. The list "1,,3" is a concatenation of two
lists, "1," and 3, the first of which ends with that optional comma.
"1,,3" is "(1,),(3)" is "1,3" (And similarly for "1,,,3" is "(1,),(,),3"
is "1,3" and so on.) Not that we'd advise you to use this obfuscation.
A list value may also be subscripted like a normal array. You must put
the list in parentheses to avoid ambiguity. For example:
# Stat returns list value.
$time = (stat($file))[8];
# SYNTAX ERROR HERE.
$time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
# Find a hex digit.
$hexdigit = ('a','b','c','d','e','f')[$digit-10];
# A "reverse comma operator".
return (pop(@foo),pop(@foo))[0];
Lists may be assigned to only when each element of the list is itself
legal to assign to:
($x, $y, $z) = (1, 2, 3);
($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
An exception to this is that you may assign to "undef" in a list. This
is useful for throwing away some of the return values of a function:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
As of Perl 5.22, you can also use "(undef)x2" instead of "undef, undef".
(You can also do "($x) x 2", which is less useful, because it assigns to
the same variable twice, clobbering the first value assigned.)
When you assign a list of scalars to an array, all previous values in
that array are wiped out and the number of elements in the array will
now be equal to the number of elements in the right-hand list -- the
list from which assignment was made. The array will automatically
resize itself to precisely accommodate each element in the right-hand
list.
use warnings;
my (@xyz, $x, $y, $z);
@xyz = (1, 2, 3);
print "@xyz\n"; # 1 2 3
@xyz = ('al', 'be', 'ga', 'de');
print "@xyz\n"; # al be ga de
@xyz = (101, 102);
print "@xyz\n"; # 101 102
When, however, you assign a list of scalars to another list of scalars,
the results differ according to whether the left-hand list -- the list
being assigned to -- has the same, more or fewer elements than the
right-hand list.
($x, $y, $z) = (1, 2, 3);
print "$x $y $z\n"; # 1 2 3
($x, $y, $z) = ('al', 'be', 'ga', 'de');
print "$x $y $z\n"; # al be ga
($x, $y, $z) = (101, 102);
print "$x $y $z\n"; # 101 102
# Use of uninitialized value $z in concatenation (.)
# or string at [program] line [line number].
If the number of scalars in the left-hand list is less than that in the
right-hand list, the "extra" scalars in the right-hand list will simply
not be assigned.
If the number of scalars in the left-hand list is greater than that in
the left-hand list, the "missing" scalars will become undefined.
($x, $y, $z) = (101, 102);
for my $el ($x, $y, $z) {
(defined $el) ? print "$el " : print "<undef>";
}
print "\n";
# 101 102 <undef>
List assignment in scalar context returns the number of elements
produced by the expression on the right side of the assignment:
$x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
$x = (($foo,$bar) = f()); # set $x to f()'s return count
This is handy when you want to do a list assignment in a Boolean
context, because most list functions return a null list when finished,
which when assigned produces a 0, which is interpreted as FALSE.
It's also the source of a useful idiom for executing a function or
performing an operation in list context and then counting the number of
return values, by assigning to an empty list and then using that
assignment in scalar context. For example, this code:
$count = () = $string =~ /\d+/g;
will place into $count the number of digit groups found in $string.
This happens because the pattern match is in list context (since it is
being assigned to the empty list), and will therefore return a list of
all matching parts of the string. The list assignment in scalar context
will translate that into the number of elements (here, the number of
times the pattern matched) and assign that to $count. Note that simply
using
$count = $string =~ /\d+/g;
would not have worked, since a pattern match in scalar context will only
return true or false, rather than a count of matches.
The final element of a list assignment may be an array or a hash:
($x, $y, @rest) = split;
my($x, $y, %rest) = @_;
You can actually put an array or hash anywhere in the list, but the
first one in the list will soak up all the values, and anything after it
will become undefined. This may be useful in a my() or local().
A hash can be initialized using a literal list holding pairs of items to
be interpreted as a key and a value:
# same as map assignment above
%map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
While literal lists and named arrays are often interchangeable, that's
not the case for hashes. Just because you can subscript a list value
like a normal array does not mean that you can subscript a list value as
a hash. Likewise, hashes included as parts of other lists (including
parameters lists and return lists from functions) always flatten out
into key/value pairs. That's why it's good to use references sometimes.
It is often more readable to use the "=>" operator between key/value
pairs. The "=>" operator is mostly just a more visually distinctive
synonym for a comma, but it also arranges for its left-hand operand to
be interpreted as a string if it's a bareword that would be a legal
simple identifier. "=>" doesn't quote compound identifiers, that
contain double colons. This makes it nice for initializing hashes:
%map = (
red => 0x00f,
blue => 0x0f0,
green => 0xf00,
);
or for initializing hash references to be used as records:
$rec = {
witch => 'Mable the Merciless',
cat => 'Fluffy the Ferocious',
date => '10/31/1776',
};
or for using call-by-named-parameter to complicated functions:
$field = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
labels => \%labels
);
Note that just because a hash is initialized in that order doesn't mean
that it comes out in that order. See "sort" in perlfunc for examples of
how to arrange for an output ordering.
If a key appears more than once in the initializer list of a hash, the
last occurrence wins:
%circle = (
center => [5, 10],
center => [27, 9],
radius => 100,
color => [0xDF, 0xFF, 0x00],
radius => 54,
);
# same as
%circle = (
center => [27, 9],
color => [0xDF, 0xFF, 0x00],
radius => 54,
);
This can be used to provide overridable configuration defaults:
# values in %args take priority over %config_defaults
%config = (%config_defaults, %args);
Subscripts
An array can be accessed one scalar at a time by specifying a dollar
sign ("$"), then the name of the array (without the leading "@"), then
the subscript inside square brackets. For example:
@myarray = (5, 50, 500, 5000);
print "The Third Element is", $myarray[2], "\n";
The array indices start with 0. A negative subscript retrieves its
value from the end. In our example, $myarray[-1] would have been 5000,
and $myarray[-2] would have been 500.
Hash subscripts are similar, only instead of square brackets curly
brackets are used. For example:
%scientists =
(
"Newton" => "Isaac",
"Einstein" => "Albert",
"Darwin" => "Charles",
"Feynman" => "Richard",
);
print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
You can also subscript a list to get a single element from it:
$dir = (getpwnam("daemon"))[7];
Multi-dimensional array emulation
Multidimensional arrays may be emulated by subscripting a hash with a
list. The elements of the list are joined with the subscript separator
(see "$;" in perlvar).
$foo{$x,$y,$z}
is equivalent to
$foo{join($;, $x, $y, $z)}
The default subscript separator is "\034", the same as SUBSEP in awk.
Slices
A slice accesses several elements of a list, an array, or a hash
simultaneously using a list of subscripts. It's more convenient than
writing out the individual elements as a list of separate scalar values.
($him, $her) = @folks[0,-1]; # array slice
@them = @folks[0 .. 3]; # array slice
($who, $home) = @ENV{"USER", "HOME"}; # hash slice
($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also assign to an
array or hash slice.
@days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
= (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
($colors{'red'}, $colors{'blue'}, $colors{'green'})
= (0xff0000, 0x0000ff, 0x00ff00);
($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
Since changing a slice changes the original array or hash that it's
slicing, a "foreach" construct will alter some--or even all--of the
values of the array or hash.
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
foreach (@hash{qw[key1 key2]}) {
s/^\s+//; # trim leading whitespace
s/\s+$//; # trim trailing whitespace
s/\b(\w)(\w*)\b/\u$1\L$2/g; # "titlecase" words
}
As a special exception, when you slice a list (but not an array or a
hash), if the list evaluates to empty, then taking a slice of that empty
list will always yield the empty list in turn. Thus:
@a = ()[0,1]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
@c = (sub{}->())[0,1]; # @c has no elements
@d = ('a','b')[0,1]; # @d has two elements
@e = (@d)[0,1,8,9]; # @e has four elements
@f = (@d)[8,9]; # @f has two elements
This makes it easy to write loops that terminate when a null list is
returned:
while ( ($home, $user) = (getpwent)[7,0] ) {
printf "%-8s %s\n", $user, $home;
}
As noted earlier in this document, the scalar sense of list assignment
is the number of elements on the right-hand side of the assignment. The
null list contains no elements, so when the password file is exhausted,
the result is 0, not 2.
Slices in scalar context return the last item of the slice.
@a = qw/first second third/;
%h = (first => 'A', second => 'B');
$t = @a[0, 1]; # $t is now 'second'
$u = @h{'first', 'second'}; # $u is now 'B'
If you're confused about why you use an '@' there on a hash slice
instead of a '%', think of it like this. The type of bracket (square or
curly) governs whether it's an array or a hash being looked at. On the
other hand, the leading symbol ('$' or '@') on the array or hash
indicates whether you are getting back a singular value (a scalar) or a
plural one (a list).
Key/Value Hash Slices
Starting in Perl 5.20, a hash slice operation with the % symbol is a
variant of slice operation returning a list of key/value pairs rather
than just values:
%h = (blonk => 2, foo => 3, squink => 5, bar => 8);
%subset = %h{'foo', 'bar'}; # key/value hash slice
# %subset is now (foo => 3, bar => 8)
%removed = delete %h{'foo', 'bar'};
# %removed is now (foo => 3, bar => 8)
# %h is now (blonk => 2, squink => 5)
However, the result of such a slice cannot be localized or assigned to.
These are otherwise very much consistent with hash slices using the @
symbol.
Index/Value Array Slices
Similar to key/value hash slices (and also introduced in Perl 5.20), the
% array slice syntax returns a list of index/value pairs:
@a = "a".."z";
@list = %a[3,4,6];
# @list is now (3, "d", 4, "e", 6, "g")
@removed = delete %a[3,4,6]
# @removed is now (3, "d", 4, "e", 6, "g")
# @list[3,4,6] are now undef
Note that calling "delete" on array values is strongly discouraged.
Typeglobs and Filehandles
Perl uses an internal type called a typeglob to hold an entire symbol
table entry. The type prefix of a typeglob is a "*", because it
represents all types. This used to be the preferred way to pass arrays
and hashes by reference into a function, but now that we have real
references, this is seldom needed.
The main use of typeglobs in modern Perl is create symbol table aliases.
This assignment:
*this = *that;
makes $this an alias for $that, @this an alias for @that, %this an alias
for %that, &this an alias for &that, etc. Much safer is to use a
reference. This:
local *Here::blue = \$There::green;
temporarily makes $Here::blue an alias for $There::green, but doesn't
make @Here::blue an alias for @There::green, or %Here::blue an alias for
%There::green, etc. See "Symbol Tables" in perlmod for more examples of
this. Strange though this may seem, this is the basis for the whole
module import/export system.
Another use for typeglobs is to pass filehandles into a function or to
create new filehandles. If you need to use a typeglob to save away a
filehandle, do it this way:
$fh = *STDOUT;
or perhaps as a real reference, like this:
$fh = \*STDOUT;
See perlsub for examples of using these as indirect filehandles in
functions.
Typeglobs are also a way to create a local filehandle using the local()
operator. These last until their block is exited, but may be passed
back. For example:
sub newopen {
my $path = shift;
local *FH; # not my!
open (FH, $path) or return undef;
return *FH;
}
$fh = newopen('/etc/passwd');
Now that we have the *foo{THING} notation, typeglobs aren't used as much
for filehandle manipulations, although they're still needed to pass
brand new file and directory handles into or out of functions. That's
because *HANDLE{IO} only works if HANDLE has already been used as a
handle. In other words, *FH must be used to create new symbol table
entries; *foo{THING} cannot. When in doubt, use *FH.
All functions that are capable of creating filehandles (open(),
opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
automatically create an anonymous filehandle if the handle passed to
them is an uninitialized scalar variable. This allows the constructs
such as "open(my $fh, ...)" and "open(local $fh,...)" to be used to
create filehandles that will conveniently be closed automatically when
the scope ends, provided there are no other references to them. This
largely eliminates the need for typeglobs when opening filehandles that
must be passed around, as in the following example:
sub myopen {
open my $fh, "@_"
or die "Can't open '@_': $!";
return $fh;
}
{
my $f = myopen("</etc/motd");
print <$f>;
# $f implicitly closed here
}
Note that if an initialized scalar variable is used instead the result
is different: "my $fh='zzz'; open($fh, ...)" is equivalent to "open(
*{'zzz'}, ...)". "use strict 'refs'" forbids such practice.
Another way to create anonymous filehandles is with the Symbol module or
with the IO::Handle module and its ilk. These modules have the
advantage of not hiding different types of the same name during the
local(). See the bottom of "open" in perlfunc for an example.
SEE ALSO
See perlvar for a description of Perl's built-in variables and a
discussion of legal variable names. See perlref, perlsub, and "Symbol
Tables" in perlmod for more discussion on typeglobs and the *foo{THING}
syntax.
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