Thanks a lot.

Davidos solution is faster than mine, and LanX' and CountZeros is the fastest.

For some reason Devel::NYTProf gives better results for C-style loops over 'for my $i (...)'.

The hashref is a relict from having this part in an sub, then refactored down, then inlined, now 1-lined. See here the same logic in Algorithm::Diff:

sub _withPositionsOfInInterval
{
    my $aCollection = shift;    # array ref
    my $start       = shift;
    my $end         = shift;
    my $keyGen      = shift;
    my %d;
    my $index;
    for ( $index = $start ; $index <= $end ; $index++ )
    {
        my $element = $aCollection->[$index];
        my $key = &$keyGen( $element, @_ );
        if ( exists( $d{$key} ) )
        {
            unshift ( @{ $d{$key} }, $index );
        }
        else
        {
            $d{$key} = [$index];
        }
    }
    return wantarray ? %d : \%d;
}

[download]

Having an arrayref comes from the two input-parameters, sequences X, Y or sometimes also called A, B in the descriptions of diff/LCS/align-algorithms.

Helmut Wollmersdorfer

In reply to Re^2: Faster indexing an array by wollmers
in thread Faster indexing an array by wollmers

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