NAME

List::Pairwise - map/grep arrays and hashes pairwise

SYNOPSIS

   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;

DESCRIPTION

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.

mapp BLOCK LIST

map_pairwise BLOCK LIST

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;

grepp BLOCK LIST

grep_pairwise BLOCK 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

firstp BLOCK LIST

first_pairwise BLOCK 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;

lastp BLOCK LIST

last_pairwise BLOCK LIST

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.

pair LIST

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
;

EXPORTS

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);

CAVEATS

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';

TEST COVERAGE

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
---------------------------- ------ ------ ------ ------ ------ ------ ------

TODO

• XS implementation

SEE ALSO

List::MoreUtils, List::Util, grep, map

ACKNOWLEDGMENT

The author wishes to thank:

• Johan Lodin for the pair idea and implementation, as well as numerous other contributions (see changelog)

• Andreas J. Koenig for his advices on documentation and his insight on how to keep perl 5.10 compatibility

• Slaven Rezic for discovering the issues that module has with pre-5.6 versions of perl

AUTHOR

Thomas Drugeon, <tdrugeon@cpan.org>

COPYRIGHT AND LICENSE

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.

cpanm List::Pairwise

perl -MCPAN -e shell
install List::Pairwise