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Re^2: Efficient bit counting with a twist.

by BrowserUk (Pope)
on Jan 28, 2013 at 14:00 UTC ( #1015707=note: print w/ replies, xml ) Need Help??


in reply to Re: Efficient bit counting with a twist.
in thread Efficient bit counting with a twist.

Note that that sequence of unpack/pack/unpack achieves exactly the same thing as AnomalousMonk's unpack "%32b$p".

But the latter avoids turning a 500MB string into a 4 billion element list of 1s and 0s before converting it back to the 500MB string; which is what that sequence of unpack/pack/unpack does.


With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.


Comment on Re^2: Efficient bit counting with a twist.
Re^3: Efficient bit counting with a twist.
by Anonymous Monk on Jun 01, 2013 at 00:41 UTC
    Maybe I'm crazy, but this doesn't seem to work:
    #!/usr/bin/perl use strict; use warnings; my $mask = hex($ARGV[0]); my $setbits = unpack("%32b*", $mask); printf("0x%x has %d set\n", $mask, $setbits);

    Then I run it:
    ./bittest 0xff
    0xff has 11 set

    Or worse:
    ./bittest 0x1001
    0x1001 has 14 set

    Even the simple case:
    ./bittest 0
    0x0 has 2 set

    What is going on here?

      What is going on here?

      I tweaked your code by adding a couple of extra print statements:

      my $mask = hex($ARGV[0]); print $mask; print unpack 'H*', $mask; my $setbits = unpack("%32b*", $mask); printf("0x%x has %d set", $mask, $setbits);

      And when I run it I get this output:

      C:\test>junk 0xff 255 ## 3 bytes 323535 ## 6 nybbles with bit patterns 0011 0010 0011 0101 0011 0101 0xff has 11 set

      hex converts the string '0xff' to a number which you store in $mask;

      unpack expects a string, so perl helpfully converts the number stored in $mask to a string in the default decimal representation '255'.

      You are counting the bits in that 3 byte string.

      To count the bits in the number, you need to tell perl that you want the binary representation of that number:

      #! perl -slw use strict; my $mask = hex($ARGV[0]); print $mask; print unpack 'H*', pack 'C', $mask; my $setbits = unpack("%32b*", pack 'C', $mask ); printf("0x%x has %d set", $mask, $setbits); __END__ C:\test>junk 0xff 255 ff 0xff has 8 set

      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.

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