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iterating bit-by-bit across int?

Paul Watson pwatson at redlinec.com
Thu Oct 23 23:22:54 EDT 2003 
"Diez B. Roggisch" <deets_noospaam at web.de> wrote in message
news:bn9acd$4s6$06$1 at news.t-online.com...
> > I'm thinking about writing a function that eats integers and poops out
> > lists of bools; and then I can iterate through that, and change values
> > in there.  But before I do that, does anyone have a better idea?
>
> For speed, you should use shift and boolean ops - like this:
>
> def mutate(seq, n=32, prob=0.05):
>   for bit in xrange(n):
>     if random.random() <= prob:
>       seq ^= 1 << bit
>   return seq
>
> Regards,
>
> Diez

And you might want to make a list of precalculated masks using the shift
operator for even more speed.

#! /usr/bin/env python

import sys

print sys.maxint

i = sys.maxint
bitcount = 0

while (i != 0):
    i >>= 1
    bitcount += 1

print "i = %d" % (i)
print "bitcount = %d" % (bitcount)

masklist = []
for i in range(bitcount):
    masklist.append(1 << i)
masklist.append(sys.maxint + 1)

for i in masklist:
    print hex(i)

thelist = [0x7F, 0x5A5A5A5A]

for i in thelist:
    for mask in masklist:
        if ((i & mask) <> 0):
            # do True thing
            print "True"
        else:
            # do False thing
            print "False"
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