List::SomeUtils(3pm) User Contributed Perl Documentation List::SomeUtils(3pm)
NAME
List::SomeUtils - Provide the stuff missing in List::Util
VERSION
version 0.58
SYNOPSIS
# import specific functions
use List::SomeUtils qw( any uniq );
if ( any {/foo/} uniq @has_duplicates ) {
# do stuff
}
# import everything
use List::SomeUtils ':all';
DESCRIPTION
List::SomeUtils provides some trivial but commonly needed functionality
on lists which is not going to go into List::Util.
All of the below functions are implementable in only a couple of lines
of Perl code. Using the functions from this module however should give
slightly better performance as everything is implemented in C. The
pure-Perl implementation of these functions only serves as a fallback
in case the C portions of this module couldn't be compiled on this
machine.
WHY DOES THIS MODULE EXIST?
You might wonder why this module exists when we already have
List::MoreUtils. In fact, this module is (nearly) the same code as is
found in LMU with no significant changes. However, the LMU distribution
depends on several modules for configuration (to run the Makefile.PL)
that some folks in the Perl community don't think are appropriate for a
module high upstream in the CPAN river.
I (Dave Rolsky) don't have a strong opinion on this, but I do like the
functions provided by LMU, and I'm tired of getting patches and PRs to
remove LMU from my code.
This distribution exists to let me use the functionality I like without
having to get into tiring arguments about issues I don't really care
about.
EXPORTS
Default behavior
Nothing by default. To import all of this module's symbols use the
":all" tag. Otherwise functions can be imported by name as usual:
use List::SomeUtils ':all';
use List::SomeUtils qw{ any firstidx };
Because historical changes to the API might make upgrading
List::SomeUtils difficult for some projects, the legacy API is
available via special import tags.
FUNCTIONS
Junctions
Treatment of an empty list
There are two schools of thought for how to evaluate a junction on an
empty list:
• Reduction to an identity (boolean)
• Result is undefined (three-valued)
In the first case, the result of the junction applied to the empty list
is determined by a mathematical reduction to an identity depending on
whether the underlying comparison is "or" or "and". Conceptually:
"any are true" "all are true"
-------------- --------------
2 elements: A || B || 0 A && B && 1
1 element: A || 0 A && 1
0 elements: 0 1
In the second case, three-value logic is desired, in which a junction
applied to an empty list returns "undef" rather than true or false
Junctions with a "_u" suffix implement three-valued logic. Those
without are boolean.
all BLOCK LIST
all_u BLOCK LIST
Returns a true value if all items in LIST meet the criterion given
through BLOCK. Sets $_ for each item in LIST in turn:
print "All values are non-negative"
if all { $_ >= 0 } ($x, $y, $z);
For an empty LIST, "all" returns true (i.e. no values failed the
condition) and "all_u" returns "undef".
Thus, "all_u(@list)" is equivalent to "@list ? all(@list) : undef".
Note: because Perl treats "undef" as false, you must check the return
value of "all_u" with "defined" or you will get the opposite result of
what you expect.
any BLOCK LIST
any_u BLOCK LIST
Returns a true value if any item in LIST meets the criterion given
through BLOCK. Sets $_ for each item in LIST in turn:
print "At least one non-negative value"
if any { $_ >= 0 } ($x, $y, $z);
For an empty LIST, "any" returns false and "any_u" returns "undef".
Thus, "any_u(@list)" is equivalent to "@list ? any(@list) : undef".
none BLOCK LIST
none_u BLOCK LIST
Logically the negation of "any". Returns a true value if no item in
LIST meets the criterion given through BLOCK. Sets $_ for each item in
LIST in turn:
print "No non-negative values"
if none { $_ >= 0 } ($x, $y, $z);
For an empty LIST, "none" returns true (i.e. no values failed the
condition) and "none_u" returns "undef".
Thus, "none_u(@list)" is equivalent to "@list ? none(@list) : undef".
Note: because Perl treats "undef" as false, you must check the return
value of "none_u" with "defined" or you will get the opposite result of
what you expect.
notall BLOCK LIST
notall_u BLOCK LIST
Logically the negation of "all". Returns a true value if not all items
in LIST meet the criterion given through BLOCK. Sets $_ for each item
in LIST in turn:
print "Not all values are non-negative"
if notall { $_ >= 0 } ($x, $y, $z);
For an empty LIST, "notall" returns false and "notall_u" returns
"undef".
Thus, "notall_u(@list)" is equivalent to "@list ? notall(@list) :
undef".
one BLOCK LIST
one_u BLOCK LIST
Returns a true value if precisely one item in LIST meets the criterion
given through BLOCK. Sets $_ for each item in LIST in turn:
print "Precisely one value defined"
if one { defined($_) } @list;
Returns false otherwise.
For an empty LIST, "one" returns false and "one_u" returns "undef".
The expression "one BLOCK LIST" is almost equivalent to "1 == true
BLOCK LIST", except for short-cutting. Evaluation of BLOCK will
immediately stop at the second true value.
Transformation
apply BLOCK LIST
Makes a copy of the list and then passes each element from the copy to
the BLOCK. Any changes or assignments to $_ in the BLOCK will only
affect the elements of the new list. However, if $_ is a reference then
changes to the referenced value will be seen in both the original and
new list.
This function is similar to "map" but will not modify the elements of
the input list:
my @list = (1 .. 4);
my @mult = apply { $_ *= 2 } @list;
print "\@list = @list\n";
print "\@mult = @mult\n";
__END__
@list = 1 2 3 4
@mult = 2 4 6 8
Think of it as syntactic sugar for
for (my @mult = @list) { $_ *= 2 }
Note that you must alter $_ directly inside BLOCK in order for changes
to make effect. New value returned from the BLOCK are ignored:
# @new is identical to @list.
my @new = apply { $_ * 2 } @list;
# @new is different from @list
my @new = apply { $_ =* 2 } @list;
insert_after BLOCK VALUE LIST
Inserts VALUE after the first item in LIST for which the criterion in
BLOCK is true. Sets $_ for each item in LIST in turn.
my @list = qw/This is a list/;
insert_after { $_ eq "a" } "longer" => @list;
print "@list";
__END__
This is a longer list
insert_after_string STRING VALUE LIST
Inserts VALUE after the first item in LIST which is equal to STRING.
my @list = qw/This is a list/;
insert_after_string "a", "longer" => @list;
print "@list";
__END__
This is a longer list
pairwise BLOCK ARRAY1 ARRAY2
Evaluates BLOCK for each pair of elements in ARRAY1 and ARRAY2 and
returns a new list consisting of BLOCK's return values. The two
elements are set to $a and $b. Note that those two are aliases to the
original value so changing them will modify the input arrays.
@a = (1 .. 5);
@b = (11 .. 15);
@x = pairwise { $a + $b } @a, @b; # returns 12, 14, 16, 18, 20
# mesh with pairwise
@a = qw/a b c/;
@b = qw/1 2 3/;
@x = pairwise { ($a, $b) } @a, @b; # returns a, 1, b, 2, c, 3
mesh ARRAY1 ARRAY2 [ ARRAY3 ... ]
zip ARRAY1 ARRAY2 [ ARRAY3 ... ]
Returns a list consisting of the first elements of each array, then the
second, then the third, etc, until all arrays are exhausted.
Examples:
@x = qw/a b c d/;
@y = qw/1 2 3 4/;
@z = mesh @x, @y; # returns a, 1, b, 2, c, 3, d, 4
@a = ('x');
@b = ('1', '2');
@c = qw/zip zap zot/;
@d = mesh @a, @b, @c; # x, 1, zip, undef, 2, zap, undef, undef, zot
"zip" is an alias for "mesh".
uniq LIST
distinct LIST
Returns a new list by stripping duplicate values in LIST by comparing
the values as hash keys, except that undef is considered separate from
''. The order of elements in the returned list is the same as in LIST.
In scalar context, returns the number of unique elements in LIST.
my @x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 1 2 3 5 4
my $x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 5
# returns "Mike", "Michael", "Richard", "Rick"
my @n = distinct "Mike", "Michael", "Richard", "Rick", "Michael", "Rick"
# returns '', undef, 'S1', A5'
my @s = distinct '', undef, 'S1', 'A5'
# returns '', undef, 'S1', A5'
my @w = uniq undef, '', 'S1', 'A5'
"distinct" is an alias for "uniq".
RT#49800 can be used to give feedback about this behavior.
singleton
Returns a new list by stripping values in LIST occurring more than once
by comparing the values as hash keys, except that undef is considered
separate from ''. The order of elements in the returned list is the
same as in LIST. In scalar context, returns the number of elements
occurring only once in LIST.
my @x = singleton 1,1,2,2,3,4,5 # returns 3 4 5
Partitioning
after BLOCK LIST
Returns a list of the values of LIST after (and not including) the
point where BLOCK returns a true value. Sets $_ for each element in
LIST in turn.
@x = after { $_ % 5 == 0 } (1..9); # returns 6, 7, 8, 9
after_incl BLOCK LIST
Same as "after" but also includes the element for which BLOCK is true.
before BLOCK LIST
Returns a list of values of LIST up to (and not including) the point
where BLOCK returns a true value. Sets $_ for each element in LIST in
turn.
before_incl BLOCK LIST
Same as "before" but also includes the element for which BLOCK is true.
part BLOCK LIST
Partitions LIST based on the return value of BLOCK which denotes into
which partition the current value is put.
Returns a list of the partitions thusly created. Each partition created
is a reference to an array.
my $i = 0;
my @part = part { $i++ % 2 } 1 .. 8; # returns [1, 3, 5, 7], [2, 4, 6, 8]
You can have a sparse list of partitions as well where non-set
partitions will be undef:
my @part = part { 2 } 1 .. 10; # returns undef, undef, [ 1 .. 10 ]
Be careful with negative values, though:
my @part = part { -1 } 1 .. 10;
__END__
Modification of non-creatable array value attempted, subscript -1 ...
Negative values are only ok when they refer to a partition previously
created:
my @idx = ( 0, 1, -1 );
my $i = 0;
my @part = part { $idx[$i++ % 3] } 1 .. 8; # [1, 4, 7], [2, 3, 5, 6, 8]
Iteration
each_array ARRAY1 ARRAY2 ...
Creates an array iterator to return the elements of the list of arrays
ARRAY1, ARRAY2 throughout ARRAYn in turn. That is, the first time it
is called, it returns the first element of each array. The next time,
it returns the second elements. And so on, until all elements are
exhausted.
This is useful for looping over more than one array at once:
my $ea = each_array(@a, @b, @c);
while ( my ($a, $b, $c) = $ea->() ) { .... }
The iterator returns the empty list when it reached the end of all
arrays.
If the iterator is passed an argument of '"index"', then it returns the
index of the last fetched set of values, as a scalar.
each_arrayref LIST
Like each_array, but the arguments are references to arrays, not the
plain arrays.
natatime EXPR, LIST
Creates an array iterator, for looping over an array in chunks of $n
items at a time. (n at a time, get it?). An example is probably a
better explanation than I could give in words.
Example:
my @x = ('a' .. 'g');
my $it = natatime 3, @x;
while (my @vals = $it->())
{
print "@vals\n";
}
This prints
a b c
d e f
g
Searching
bsearch BLOCK LIST
Performs a binary search on LIST which must be a sorted list of values.
BLOCK must return a negative value if the current element (stored in
$_) is smaller, a positive value if it is bigger and zero if it
matches.
Returns a boolean value in scalar context. In list context, it returns
the element if it was found, otherwise the empty list.
bsearchidx BLOCK LIST
bsearch_index BLOCK LIST
Performs a binary search on LIST which must be a sorted list of values.
BLOCK must return a negative value if the current element (stored in
$_) is smaller, a positive value if it is bigger and zero if it
matches.
Returns the index of found element, otherwise "-1".
"bsearch_index" is an alias for "bsearchidx".
firstval BLOCK LIST
first_value BLOCK LIST
Returns the first element in LIST for which BLOCK evaluates to true.
Each element of LIST is set to $_ in turn. Returns "undef" if no such
element has been found.
"first_value" is an alias for "firstval".
onlyval BLOCK LIST
only_value BLOCK LIST
Returns the only element in LIST for which BLOCK evaluates to true.
Sets $_ for each item in LIST in turn. Returns "undef" if no such
element has been found.
"only_value" is an alias for "onlyval".
lastval BLOCK LIST
last_value BLOCK LIST
Returns the last value in LIST for which BLOCK evaluates to true. Each
element of LIST is set to $_ in turn. Returns "undef" if no such
element has been found.
"last_value" is an alias for "lastval".
firstres BLOCK LIST
first_result BLOCK LIST
Returns the result of BLOCK for the first element in LIST for which
BLOCK evaluates to true. Each element of LIST is set to $_ in turn.
Returns "undef" if no such element has been found.
"first_result" is an alias for "firstres".
onlyres BLOCK LIST
only_result BLOCK LIST
Returns the result of BLOCK for the first element in LIST for which
BLOCK evaluates to true. Sets $_ for each item in LIST in turn. Returns
"undef" if no such element has been found.
"only_result" is an alias for "onlyres".
lastres BLOCK LIST
last_result BLOCK LIST
Returns the result of BLOCK for the last element in LIST for which
BLOCK evaluates to true. Each element of LIST is set to $_ in turn.
Returns "undef" if no such element has been found.
"last_result" is an alias for "lastres".
indexes BLOCK LIST
Evaluates BLOCK for each element in LIST (assigned to $_) and returns a
list of the indices of those elements for which BLOCK returned a true
value. This is just like "grep" only that it returns indices instead of
values:
@x = indexes { $_ % 2 == 0 } (1..10); # returns 1, 3, 5, 7, 9
firstidx BLOCK LIST
first_index BLOCK LIST
Returns the index of the first element in LIST for which the criterion
in BLOCK is true. Sets $_ for each item in LIST in turn:
my @list = (1, 4, 3, 2, 4, 6);
printf "item with index %i in list is 4", firstidx { $_ == 4 } @list;
__END__
item with index 1 in list is 4
Returns "-1" if no such item could be found.
"first_index" is an alias for "firstidx".
onlyidx BLOCK LIST
only_index BLOCK LIST
Returns the index of the only element in LIST for which the criterion
in BLOCK is true. Sets $_ for each item in LIST in turn:
my @list = (1, 3, 4, 3, 2, 4);
printf "uniqe index of item 2 in list is %i", onlyidx { $_ == 2 } @list;
__END__
unique index of item 2 in list is 4
Returns "-1" if either no such item or more than one of these has been
found.
"only_index" is an alias for "onlyidx".
lastidx BLOCK LIST
last_index BLOCK LIST
Returns the index of the last element in LIST for which the criterion
in BLOCK is true. Sets $_ for each item in LIST in turn:
my @list = (1, 4, 3, 2, 4, 6);
printf "item with index %i in list is 4", lastidx { $_ == 4 } @list;
__END__
item with index 4 in list is 4
Returns "-1" if no such item could be found.
"last_index" is an alias for "lastidx".
Sorting
sort_by BLOCK LIST
Returns the list of values sorted according to the string values
returned by the KEYFUNC block or function. A typical use of this may be
to sort objects according to the string value of some accessor, such as
sort_by { $_->name } @people
The key function is called in scalar context, being passed each value
in turn as both $_ and the only argument in the parameters, @_. The
values are then sorted according to string comparisons on the values
returned. This is equivalent to
sort { $a->name cmp $b->name } @people
except that it guarantees the name accessor will be executed only once
per value. One interesting use-case is to sort strings which may have
numbers embedded in them "naturally", rather than lexically.
sort_by { s/(\d+)/sprintf "%09d", $1/eg; $_ } @strings
This sorts strings by generating sort keys which zero-pad the embedded
numbers to some level (9 digits in this case), helping to ensure the
lexical sort puts them in the correct order.
nsort_by BLOCK LIST
Similar to sort_by but compares its key values numerically.
Counting and calculation
true BLOCK LIST
Counts the number of elements in LIST for which the criterion in BLOCK
is true. Sets $_ for each item in LIST in turn:
printf "%i item(s) are defined", true { defined($_) } @list;
false BLOCK LIST
Counts the number of elements in LIST for which the criterion in BLOCK
is false. Sets $_ for each item in LIST in turn:
printf "%i item(s) are not defined", false { defined($_) } @list;
minmax LIST
Calculates the minimum and maximum of LIST and returns a two element
list with the first element being the minimum and the second the
maximum. Returns the empty list if LIST was empty.
The "minmax" algorithm differs from a naive iteration over the list
where each element is compared to two values being the so far
calculated min and max value in that it only requires 3n/2 - 2
comparisons. Thus it is the most efficient possible algorithm.
However, the Perl implementation of it has some overhead simply due to
the fact that there are more lines of Perl code involved. Therefore,
LIST needs to be fairly big in order for "minmax" to win over a naive
implementation. This limitation does not apply to the XS version.
mode LIST
Calculates the most common items in the list and returns them as a
list. This is effectively done by string comparisons, so references
will be stringified. If they implement string overloading, this will be
used.
If more than one item appears the same number of times in the list, all
such items will be returned. For example, the mode of a unique list is
the list itself.
This function returns a list in list context. In scalar context it
returns a count indicating the number of modes in the list.
MAINTENANCE
The maintenance goal is to preserve the documented semantics of the
API; bug fixes that bring actual behavior in line with semantics are
allowed. New API functions may be added over time. If a backwards
incompatible change is unavoidable, we will attempt to provide support
for the legacy API using the same export tag mechanism currently in
place.
This module attempts to use few non-core dependencies. Non-core
configuration and testing modules will be bundled when reasonable; run-
time dependencies will be added only if they deliver substantial
benefit.
KNOWN ISSUES
There is a problem with a bug in 5.6.x perls. It is a syntax error to
write things like:
my @x = apply { s/foo/bar/ } qw{ foo bar baz };
It has to be written as either
my @x = apply { s/foo/bar/ } 'foo', 'bar', 'baz';
or
my @x = apply { s/foo/bar/ } my @dummy = qw/foo bar baz/;
Perl 5.5.x and Perl 5.8.x don't suffer from this limitation.
If you have a functionality that you could imagine being in this
module, please drop me a line. This module's policy will be less strict
than List::Util's when it comes to additions as it isn't a core module.
When you report bugs, it would be nice if you could additionally give
me the output of your program with the environment variable
"LIST_MOREUTILS_PP" set to a true value. That way I know where to look
for the problem (in XS, pure-Perl or possibly both).
THANKS
Tassilo von Parseval
Credits go to a number of people: Steve Purkis for giving me namespace
advice and James Keenan and Terrence Branno for their effort of keeping
the CPAN tidier by making List::Util obsolete.
Brian McCauley suggested the inclusion of apply() and provided the
pure-Perl implementation for it.
Eric J. Roode asked me to add all functions from his module
"List::SomeUtil" into this one. With minor modifications, the pure-Perl
implementations of those are by him.
The bunch of people who almost immediately pointed out the many
problems with the glitchy 0.07 release (Slaven Rezic, Ron Savage, CPAN
testers).
A particularly nasty memory leak was spotted by Thomas A. Lowery.
Lars Thegler made me aware of problems with older Perl versions.
Anno Siegel de-orphaned each_arrayref().
David Filmer made me aware of a problem in each_arrayref that could
ultimately lead to a segfault.
Ricardo Signes suggested the inclusion of part() and provided the Perl-
implementation.
Robin Huston kindly fixed a bug in perl's MULTICALL API to make the XS-
implementation of part() work.
Jens Rehsack
Credits goes to all people contributing feedback during the v0.400
development releases.
Special thanks goes to David Golden who spent a lot of effort to
develop a design to support current state of CPAN as well as ancient
software somewhere in the dark. He also contributed a lot of patches to
refactor the API frontend to welcome any user of List::SomeUtils - from
ancient past to recently last used.
Toby Inkster provided a lot of useful feedback for sane importer code
and was a nice sounding board for API discussions.
Peter Rabbitson provided a sane git repository setup containing entire
package history.
TODO
A pile of requests from other people is still pending further
processing in my mailbox. This includes:
• List::Util export pass-through
Allow List::SomeUtils to pass-through the regular List::Util
functions to end users only need to "use" the one module.
• uniq_by(&@)
Use code-reference to extract a key based on which the uniqueness
is determined. Suggested by Aaron Crane.
• delete_index
• random_item
• random_item_delete_index
• list_diff_hash
• list_diff_inboth
• list_diff_infirst
• list_diff_insecond
These were all suggested by Dan Muey.
• listify
Always return a flat list when either a simple scalar value was
passed or an array-reference. Suggested by Mark Summersault.
SEE ALSO
List::Util, List::AllUtils, List::UtilsBy
HISTORICAL COPYRIGHT
Some parts copyright 2011 Aaron Crane.
Copyright 2004 - 2010 by Tassilo von Parseval
Copyright 2013 - 2015 by Jens Rehsack
SUPPORT
Bugs may be submitted at
<https://github.com/houseabsolute/List-SomeUtils/issues>.
I am also usually active on IRC as 'autarch' on "irc://irc.perl.org".
SOURCE
The source code repository for List-SomeUtils can be found at
<https://github.com/houseabsolute/List-SomeUtils>.
DONATIONS
If you'd like to thank me for the work I've done on this module, please
consider making a "donation" to me via PayPal. I spend a lot of free
time creating free software, and would appreciate any support you'd
care to offer.
Please note that I am not suggesting that you must do this in order for
me to continue working on this particular software. I will continue to
do so, inasmuch as I have in the past, for as long as it interests me.
Similarly, a donation made in this way will probably not make me work
on this software much more, unless I get so many donations that I can
consider working on free software full time (let's all have a chuckle
at that together).
To donate, log into PayPal and send money to autarch@urth.org, or use
the button at <http://www.urth.org/~autarch/fs-donation.html>.
AUTHORS
• Tassilo von Parseval <tassilo.von.parseval@rwth-aachen.de>
• Adam Kennedy <adamk@cpan.org>
• Jens Rehsack <rehsack@cpan.org>
• Dave Rolsky <autarch@urth.org>
CONTRIBUTORS
• Aaron Crane <arc@cpan.org>
• BackPan <BackPan>
• bay-max1 <34803732+bay-max1@users.noreply.github.com>
• Brad Forschinger <bnjf@bnjf.id.au>
• David Golden <dagolden@cpan.org>
• jddurand <jeandamiendurand@free.fr>
• Jens Rehsack <sno@netbsd.org>
• J.R. Mash <jrmash@cpan.org>
• Karen Etheridge <ether@cpan.org>
• Ricardo Signes <rjbs@cpan.org>
• Toby Inkster <mail@tobyinkster.co.uk>
• Tokuhiro Matsuno <tokuhirom@cpan.org>
• Tom Wyant <wyant@cpan.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2019 by Dave Rolsky <autarch@urth.org>.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.
The full text of the license can be found in the LICENSE file included
with this distribution.
perl v5.30.0 2019-12-11 List::SomeUtils(3pm)
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