List::Pairwise - map/grep arrays and hashes pairwise
use List::Pairwise qw(mapp grepp); my %hash = ( foo => 4, bar => 2, baz => 6, ); my @list = %hash; # increment values in-place mapp {++$b} %hash; # copy with modifications on keys %hash = mapp {lc($a) => $b} %hash # iterate pairwise mapp { print "$a: $b\n" } %hash; # reverse array pairs in-place mapp { ($a, $b) = ($b, $a) } @list; # list "keys" and "values" my @keys = mapp {$a} @list; my @values = mapp {$b} @list; # grep hash subset my %subset1 = grepp {$a =~ /^ba/} %hash; my %subset2 = grepp {$b < 5} %hash;
List::Pairwise
provides functions to map and grep lists two elements at a time, setting $a and $b to each pair instead of setting $_ to each element.
As of version 1.01, List::Pairwise now tries to use the newly implemented XS functions pairmap, pairgrep, pairfirst and pairs from List::Util 1.31 and up, resulting in a major speedup.
New code should now preferably use List::Util functions directly, with the added benefit of relying on a Perl core module.
/!\ as of version 1.03 List::Pairwise does not use List::Util, because version up to the current one (1.39) presents a strange bug where a key can get undefined after an assignement (see t/listutil.t with paimap instead of mapp) /!\
Evaluates the BLOCK for each pair of LIST (locally setting $a and $b to each pair) and returns the list value composed of the results of each such evaluation. In scalar context, returns the total number of elements so generated (not pairs). Evaluates BLOCK in list context, so each element of LIST may produce zero, one, or more elements in the returned value.
Note that $a and $b are aliases to the list elements, so they can be used to modify the elements of the LIST, exept for hash keys ($a when LIST is a hash).
mapp
is optimized in void context, and can thus be used to iterate lists pairwise.
map_pairwise
is an alias for mapp
.
keys/values emulation (only slower):
my @keys = mapp {$a} %hash; my @keys = mapp {$a} @list; # same my @values = mapp {$b} %hash; my @values = mapp {$b} @list; # same
copy (only slower):
my %b = mapp {$a, $b} %hash;
modify values in-place:
mapp {$b = lc($b)} %hash; mapp {$b = lc($b)} @list; # same
modifying hash keys in-place does not work with a hash:
mapp {$a = lc($a)} %hash; # wrong my %b = mapp {lc($a) => $b} %hash; # ok %hash = mapp {lc($a) => $b} %hash; # also ok (copy)
modify array "keys" in-place does work:
mapp {$a = lc($a)} @list;
modify keys and copy:
%hash = mapp {lc($a) => $b} %hash; @hash = mapp {lc($a) => $b} @list; # same
reverse hash (does not work in-place):
my %reverse_a = mapp {$b, $a} %hash;
reverse array pairs in-place:
mapp { ($a, $b) = ($b, $a) } @list;
each emulation, iterating a list pairwise:
mapp { print "$a: $b\n"; } %hash; mapp { print "$a: $b\n"; } @list;
Evaluates the BLOCK in scalar context for each pair of LIST (locally setting $a and $b to each pair) and returns the list value consisting of those pairs for which the expression evaluated to true. In scalar context, returns the number of valid pairs, ie the number of times the expression was true.
So this equality stands:
(grepp BLOCK LIST) == 1/2 * scalar(my @list = (grepp BLOCK LIST))
Note that $a and $b are aliases to the list elements, so they can be used to modify the elements of the LIST, exept for hash keys ($a when LIST is a hash).
grep_pairwise
is an alias for grepp
.
grep hash subset:
my %subset1 = grepp {$a =~ /^ba/} %hash; my %subset2 = grepp {$b < 5} %hash;
grep specific values:
my @values = mapp {$b} grepp {$a =~ /^ba/} %hash;
This does not work:
values grepp {$a =~ /^ba/} %hash;
values() and keys() expect a hash, whereas grepp returns a list
Evaluates the BLOCK in scalar context for each pair of LIST (locally setting $a and $b to each pair) and returns the first pair for which the expression evaluated to true. In scalar context, returns 1 if a valid pair was found.
firstp
can be used to iterate lists pairwise as does mapp
, but with the additional option of using the value returned by the BLOCK as a last
statement
my $i; firstp { print "$a: $b\n"; ++$i==5 # last after 5 iterations } %hash;
Evaluates the BLOCK in scalar context for each pair of LIST (locally setting $a and $b to each pair) and returns the last pair for which the expression evaluated to true. In scalar context, returns 1 if a valid pair was found.
Returns a list of pairs as array references.
my @pairs = pair @list; my @pairs = mapp {[$a, $b]} @list; # same, but slower
pair
can be used in combination with sort, map and grep to do ordered hash-like manipulations in long chains/streams:
my @ranges = sort { $a->[0] <=> $b->[0] or $a->[1] <=> $b->[1] } grep { $_->[0] < $_->[1] } pair /\b(\d+)-(\d+)\b/g ;
Nothing by default. Functions can be imported explicitely
use List::Pairwise qw(mapp grepp first_pairwise);
You can use the :all tag to import all functions, including *_pairwise aliases
use List::Pairwise qw(:all);
In prior versions, List::Pairwise function did croak when given a list with an odd number of elements. This is not the case anymore: a warning will now be emitted if warnings of the 'misc' category are enabled, and the last pair will be completed with an undefined value. The old behavior can be restored by making these misc warnings FATAL:
use warnings FATAL => 'misc';
As of List::Pairwise version 0.28:
---------------------------- ------ ------ ------ ------ ------ ------ ------ File stmt bran cond sub pod time total ---------------------------- ------ ------ ------ ------ ------ ------ ------ lib/List/Pairwise.pm 100.0 100.0 100.0 100.0 100.0 88.0 100.0 t/01load.t 100.0 n/a n/a 100.0 n/a 0.6 100.0 t/context.t 100.0 n/a n/a 100.0 n/a 0.6 100.0 t/coverage.pl 100.0 100.0 n/a 100.0 n/a 4.2 100.0 t/firstp.t 100.0 n/a n/a 100.0 n/a 1.2 100.0 t/grepp.t 100.0 n/a n/a 100.0 n/a 1.2 100.0 t/lastp.t 100.0 n/a n/a 100.0 n/a 1.2 100.0 t/mapp.t 100.0 n/a n/a 100.0 n/a 1.4 100.0 t/pair.t 100.0 n/a n/a 100.0 n/a 1.6 100.0 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 ---------------------------- ------ ------ ------ ------ ------ ------ ------
List::Util, List::MoreUtils, grep
, map
The author wishes to thank:
pair
idea and implementation, as well as numerous other contributions (see changelog)Thomas Drugeon, <tdrugeon@cpan.org>
Copyright (C) 2006 by Thomas Drugeon
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.8 or, at your option, any later version of Perl 5 you may have available.