in reply to An array of boolean values in a bitfield

did you consider using a lookup-table to calculate the number of bits, changing time with memory-complexity?

for example 

 # for each character, count the on bits in that character
my %bits_in;
foreach my $char ( map { chr } 0 .. 255 ) {
    foreach my $bit ( 0 .. 7 ) {
        $bits_in{ $char } += vec( $char, $bit, 1 );
    }
}

[download]
could be written 
# for each character, count the on bits in that character
my %bits_in;
foreach my $char ( map { chr } 0 .. 255 ) {
        $bits_in{ $char } = $sum[ $char ];
}

[download]

Cheers Rolf

Replies are listed 'Best First'.
Re^2: An array of boolean values in a bitfield
by kyle (Abbot) on Dec 03, 2008 at 18:08 UTC

    Where do you get the array @sum? The point of the loop that you're commenting on is specifically to build a lookup table that maps a chr to the number of '1' bits that are in it. That table gets used in some of the bit counting methods that follow.

[reply]
[d/l]
      I get it, so @sum and %bits_in have the same purpose!

      I'm just confused, why do you care about the speed to calculate a constant table with only 256 entries? And why do you use a hash if speed counts? ... arrays are much faster!

      Cheers Rolf

[reply]

        I'm not trying to optimize the creation of the lookup table. The lookup table is used to (more quickly) count bits in the bit field. I don't use an array for that because then the lookups would have to call ord to get the index into the array.

[reply]

In Section Meditations