NAME

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

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) /!\

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

    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

    use warnings FATAL => 'misc';

TEST COVERAGE

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

SEE ALSO

ACKNOWLEDGMENT

• 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
• Paul "LeoNerd" Evans for having implemented XS versions of these functions in the core module List::Util

AUTHOR

COPYRIGHT AND LICENSE

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.