Beefy Boxes and Bandwidth Generously Provided by pair Networks
Welcome to the Monastery
 
PerlMonks  

Re^3: Efficient bit counting with a twist.

by Anonymous Monk
on Jun 01, 2013 at 00:41 UTC ( #1036355=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Efficient bit counting with a twist.
in thread Efficient bit counting with a twist.

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?


Comment on Re^3: Efficient bit counting with a twist.
Download Code
Replies are listed 'Best First'.
Re^4: Efficient bit counting with a twist.
by BrowserUk (Pope) on Jun 01, 2013 at 09:49 UTC
    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.

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://1036355]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others rifling through the Monastery: (12)
As of 2015-07-28 09:10 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









    Results (254 votes), past polls