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=head1 TITLE
Builtin: partition
=head1 VERSION
Maintainer: Jeremy Howard <[EMAIL PROTECTED]>
Date: 11 August 2000
Version: 1
Mailing List: [EMAIL PROTECTED]
Number: 91
=head1 ABSTRACT
It is proposed that a new function, C<partition>, be added to Perl.
C<partition($partition_size, \@list)> would return @list broken into
references to sub-lists, each one $list_size in size.
=head1 DESCRIPTION
In order to work with lists of arbitary size, it is often necessary to
split a list into equal sized sub-lists. A C<partition> function is
proposed that achieves this:
@list = (1,2,3,4,5,6);
@partitioned_list = partition(2, \@list); # ([1,2],[3,4],[5,6])
This is useful to provide tuples to functions that can operate on lists of
lists, for instance:
@sum_pairs = map {$_->[0] + $_->[1]} @partitioned_list; # (3,7,11)
If the list to be unzipped is not an exact multiple of the partition size,
the final list reference is not padded. For example:
@list2 = (1..7);
@partitioned_list2 = partition(3, @list2); # ([1,2,3], [4,5,6], [7])
C<zip> (see RFC 90) and C<partition> can work together to allow manipulation
of arbitary sized lists. For instance, we can extend the $sum_xy function
used as an example in the C<zip> RFC, which takes two lists and returns
the sum of them multiplied together component-wise:
$sum_xy = sub {reduce ^last+^x*^y, zip($_[0], $_[1])};
to a function $sum_mult that does the same with an arbitary number of
lists:
# Multiply all the elements of a list together, returning the result
$apply_times = reduce (^total * ^element, @^multiplicands);
# Swap the rows and columns of a list of lists
$transpose = partition(
# Find the size of each column
scalar @^list_of_lists,
# Interleave the rows
zip(@^lists_of_lists);
)
# Take a list of references to lists, multiply them component-wise,
# and return their sum
$sum_mult = reduce (
^total + $apply_times->( @^next_list ),
$transpose->(^list_of_lists),
);
# Example usage of $sum_mult
@a = (1,3,5);
@b = (2,4,6);
@c = (-1,1,-1);
$answer = $sum_mult->(\@a, \@b, \@c); # 1*2*-1+3*4*1+5*6*-1 = -20
=head1 IMPLEMENTATION
The C<partition> functions should be evaluated lazily. Because it is used
in common operations such as the transposition of a matrix, its efficiency
is particularly important.
=head1 REFERENCES
RFC 23: Higher order functions
RFC 76: Builtin: reduce
RFC 90: Builtins: zip() and unzip()
=head1 ACKNOWLEDGEMENTS
Damian Conway: Numerous comments on first draft
