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Destructive Unmerge List

by CharlesClarkson (Curate)
on Feb 06, 2002 at 10:07 UTC ( #143591=snippet: print w/ replies, xml ) Need Help??

Description:

This routine unmerges a list into more than one list, but destroys the original list. A non-destructive version is available.

my @arr = qw ( apples oranges bananas pears berries apricots peaches ) +; my @arr_of_columns = unmerge_list_d( \@arr, 2 );
giving us:
['apples', 'bananas','berries', 'prunes'] ['oranges','pears', 'apricots']

If you want to pad the end of uneven arrays place the padding as the third argument

my $arr_of_arr = unmerge_list_d( [ 1 .. 7 ], 3, 'goofy' );
which yields:
[ [1, 4, 7] [2, 5, 'goofy'] [3, 6, 'goofy'] ]

I don't know if unmerge_list_d will allow very large arrays to be unmerged, but it should help reduce memory usage.

sub unmerge_list_d {
    my ( $list_ref, $qty, $pad ) = @_;
    return unless ref $list_ref eq 'ARRAY';
    my $arr;
    push @$arr, [] for 1 .. $qty;

    if ( @$list_ref % $qty &&  @_ > 2) {
        my $padded = $qty - @$list_ref % $qty ;
        push @{ $list_ref }, ($pad) x $padded;
    }

    LOOP:
    while (1) {
        foreach ( @$arr ) {
            push @{ $_ }, shift @$list_ref;
            last LOOP unless @$list_ref;
        }
    }

    return @$arr if wantarray;
    return $arr;
}
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