I have pairs of same-lengthed strings which I am counting the orientation of the paired characters via bitwise operators. (Based on a great suggestion way back in time from BrowserUk: Re: Speeding permutation counting).

The previous solution works great in my application, but I'd now like to make the solution more flexible. I'm tring to do this with eval for a tr expression, and setting counted terms from variable. However, I'm doing something wrong, as I'm not able to return the value from the eval. Here is a sample of my code. 

my @num = ( "0000", "0101", "1010", "1111" );

my ( $n1, $n2 ) = ( "0", "1" );

my $ndiff = abs( $n2 - $n1 );

for ( my $i = 0 ; $i < @num ; $i++ ) {
    for ( my $j = $i + 1 ; $j < @num ; $j++ ) {

       # Count oriented matches and mismatches along string
        print join(
            "\t",
            $num[$i],
            $num[$j],
            ( ( $num[$i] | $num[$j] )  =~ tr[0][0] ),      #count 00
            ( ( ~$num[$i] & $num[$j] ) =~ tr[\1][\1] ),    #count 01
            ( ( $num[$i] & ~$num[$j] ) =~ tr[\1][\1] ),    #count 10
            ( ( $num[$i] & $num[$j] )  =~ tr[1][1] )       #count 11
          ),
          "\n";

        # Attempt at flexible version via eval
        my ( $string1, $string2 ) = ( $num[$i], $num[$j] );
        print join( "\t",
            $string1,
            $string2,
            ( eval "( $string1 | $string2 )  =~  tr[$n1][$n1]" ),
            ( eval "( ~$string1 & $string2 ) =~ tr[\$ndiff][\$ndiff]" 
+),
            ( eval "( $string1 & ~$string2 ) =~ tr[\$ndiff][\$ndiff]" 
+),
            ( eval "( $string1 & $string2 )  =~ tr[$n2][$n2]" ) ),
          "\n\n";
    }
}

__END__
# Output from example
# In each pair of lines, the first is the desired.

0000    0101    2    2    0    0
0000    0101    0    0    0    0

0000    1010    2    2    0    0
0000    1010    2    0    0    0

0000    1111    0    4    0    0
0000    1111    0    0    0    0

0101    1010    0    2    2    0
0101    1010    1    0    0    0

0101    1111    0    2    0    2
0101    1111    0    0    0    0

1010    1111    0    2    0    2
1010    1111    1    0    0    0

[download]
I'm fairly sure that there may be a trivial solution to this, but I'm not versed enough with eval to figure out how to get the flexible version working. Any help in improving this is greatly appreciated.

-albert

In reply to Returning transliteration from eval by albert

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