in reply to Bitwise & with large numbers

meditate on the output of this: :) 
use strict;

for (0..20,) {
   $a = dec2bin($_+1);
   $b = dec2bin($_);
   print "$a = ", $_ + 1, "\n",
         "$b = ", $_, "\n",
         "--------\n", 
         dec2bin(($_+1) & $_), " = ", 
         (($_+1) & $_), "\n\n";

}

# Perl Cookbook recipe 2.4.
sub dec2bin {
   my $s = unpack("B32", pack("N",shift));
   #$s =~ s/^0+(?=\d)//;
   $s = substr($s,-8,8);
   return $s;
}

[download]
UPDATE: thanks tachyon!

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)
-->

Replies are listed 'Best First'.
Re: (jeffa) Re: Bitwise & with large numbers
by tachyon (Chancellor) on Apr 03, 2002 at 04:29 UTC

    Actually Anonymonk has a valid point as this is a buffer overflow type bug. 4294967296 is a 33 bit integer, which is of course one bit more than 32. Perhaps you should meditate on this: 

    print "2**32 is: ", 2**32, "\n";
print <<TEXT;
This is what we should have:
100000000000000000000000000000000 = 4294967296
011111111111111111111111111111111 = 4294967295
----------------------------------------------
000000000000000000000000000000000 = 0

But we get a buffer overflow like so, resulting in erroneous output:
TEXT

for (4294967295) {
    $a = dec2bin($_+1);
    $b = dec2bin($_);
    print "$a = ", $_ + 1, "\n",
    "$b = ", $_, "\n",
    "--------\n", 
    dec2bin(($_+1) & $_), " = ", 
    (($_+1) & $_), "\n\n";
    
}

sub dec2bin {
   return unpack("B*", pack("N",shift));
}

    [download]

    I have no call for it personally but Math::BigInt might be useful.

    cheers

    tachyon

    s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

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[d/l]

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