List::MoreUtils(3) User Contributed Perl Documentation List::MoreUtils(3)
NAME
List::MoreUtils - Provide the stuff missing in List::Util
SYNOPSIS
# import specific functions
use List::MoreUtils qw(any uniq);
if ( any { /foo/ } uniq @has_duplicates ) {
# do stuff
}
# import everything
use List::MoreUtils ':all';
# import by API
# has "original" any/all/none/notall behavior
use List::MoreUtils ':like_0.22';
# 0.22 + bsearch
use List::MoreUtils ':like_0.24';
# has "simplified" any/all/none/notall behavior + (n)sort_by
use List::MoreUtils ':like_0.33';
DESCRIPTION
List::MoreUtils 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.
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::MoreUtils ':all';
use List::MoreUtils qw{ any firstidx };
Because historical changes to the API might make upgrading
List::MoreUtils difficult for some projects, the legacy API is
available via special import tags.
Like version 0.22 (last release with original API)
This API was available from 2006 to 2009, returning undef for empty
lists on "all"/"any"/"none"/"notall":
use List::MoreUtils ':like_0.22';
This import tag will import all functions available as of version 0.22.
However, it will import "any_u" as "any", "all_u" as "all", "none_u" as
"none", and "notall_u" as "notall".
Like version 0.24 (first incompatible change)
This API was available from 2010 to 2011. It changed the return value
of "none" and added the "bsearch" function.
use List::MoreUtils ':like_0.24';
This import tag will import all functions available as of version 0.24.
However it will import "any_u" as "any", "all_u" as "all", and
"notall_u" as "notall". It will import "none" as described in the
documentation below (true for empty list).
Like version 0.33 (second incompatible change)
This API was available from 2011 to 2014. It is widely used in several
CPAN modules and thus it's closest to the current API. It changed the
return values of "any", "all", and "notall". It added the "sort_by"
and "nsort_by" functions and the "distinct" alias for "uniq". It
omitted "bsearch".
use List::MoreUtils ':like_0.33';
This import tag will import all functions available as of version 0.33.
Note: it will not import "bsearch" for consistency with the 0.33 API.
FUNCTIONS
Junctions
Treatment of an empty list
There are two schools of thought for how to evaluate a junction on an
empty list:
o Reduction to an identity (boolean)
o 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
Applies BLOCK to each item in LIST and returns a list of the values
after BLOCK has been applied. In scalar context, the last element is
returned. 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 }
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 '', 'S1', A5' and complains about "Use of uninitialized value"
my @s = distinct '', undef, 'S1', 'A5'
# returns undef, 'S1', A5' and complains about "Use of uninitialized value"
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 { $idx[$++ % 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.
ENVIRONMENT
When "LIST_MOREUTILS_PP" is set, the module will always use the pure-
Perl implementation and not the XS one. This environment variable is
really just there for the test-suite to force testing the Perl
implementation, and possibly for reporting of bugs. I don't see any
reason to use it in a production environment.
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.
BUGS
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).
SUPPORT
Bugs should always be submitted via the CPAN bug tracker.
You can find documentation for this module with the perldoc command.
perldoc List::MoreUtils
You can also look for information at:
o RT: CPAN's request tracker
<http://rt.cpan.org/NoAuth/Bugs.html?Dist=List-MoreUtils>
o AnnoCPAN: Annotated CPAN documentation
<http://annocpan.org/dist/List-MoreUtils>
o CPAN Ratings
<http://cpanratings.perl.org/l/List-MoreUtils>
o CPAN Search
<http://search.cpan.org/dist/List-MoreUtils/>
o Git Repository
<https://github.com/perl5-utils/List-MoreUtils>
Where can I go for help?
If you have a bug report, a patch or a suggestion, please open a new
report ticket at CPAN (but please check previous reports first in case
your issue has already been addressed) or open an issue on GitHub.
Report tickets should contain a detailed description of the bug or
enhancement request and at least an easily verifiable way of
reproducing the issue or fix. Patches are always welcome, too - and
it's cheap to send pull-requests on GitHub. Please keep in mind that
code changes are more likely accepted when they're bundled with an
approving test.
If you think you've found a bug then please read "How to Report Bugs
Effectively" by Simon Tatham:
<http://www.chiark.greenend.org.uk/~sgtatham/bugs.html>.
Where can I go for help with a concrete version?
Bugs and feature requests are accepted against the latest version only.
To get patches for earlier versions, you need to get an agreement with
a developer of your choice - who may or not report the issue and a
suggested fix upstream (depends on the license you have chosen).
Business support and maintenance
Generally, in volunteered projects, there is no right for support.
While every maintainer is happy to improve the provided software, spare
time is limited.
For those who have a use case which requires guaranteed support, one of
the maintainers should be hired or contracted. For business support
you can contact Jens via his CPAN email address rehsackATcpan.org.
Please keep in mind that business support is neither available for free
nor are you eligible to receive any support based on the license
distributed with this package.
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::Utils 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::MoreUtil" 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::MoreUtils - 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:
o List::Util export pass-through
Allow List::MoreUtils to pass-through the regular List::Util
functions to end users only need to "use" the one module.
o uniq_by(&@)
Use code-reference to extract a key based on which the uniqueness
is determined. Suggested by Aaron Crane.
o delete_index
o random_item
o random_item_delete_index
o list_diff_hash
o list_diff_inboth
o list_diff_infirst
o list_diff_insecond
These were all suggested by Dan Muey.
o 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(3), List::AllUtils(3), List::UtilsBy(3)
AUTHOR
Jens Rehsack <rehsack AT cpan.org>
Adam Kennedy <adamk@cpan.org>
Tassilo von Parseval <tassilo.von.parseval@rwth-aachen.de>
COPYRIGHT AND LICENSE
Some parts copyright 2011 Aaron Crane.
Copyright 2004 - 2010 by Tassilo von Parseval
Copyright 2013 - 2015 by Jens Rehsack
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself, either Perl version 5.8.4 or, at
your option, any later version of Perl 5 you may have available.
perl v5.16.3 2015-03-30 List::MoreUtils(3)
list-moreutils 0.410.0 - Generated Wed Apr 1 05:54:00 CDT 2015