Using while loop and if statement to tell if a binary has an odd or even number of 1's.

Mark Wooding mdw at distorted.org.uk
Sat Feb 7 03:42:44 CET 2009


Duncan Booth <duncan.booth at invalid.invalid> writes:

> Mark Dickinson <dickinsm at gmail.com> wrote:
[snip]
>>     while n:
>>         count += 1
>>         n &= n-1
>>     return count
>> 
>> is_even = count_set_bits(the_int) % 2 == 0
>> 
>> ...but anyone submitting this as a homework
>> solution had better be prepared to explain why
>> it works.
>> 
>
> I remember a programming exercise when I was an undergraduate and anyone 
> who *didn't* use that trick got marked down for writing inefficient
> code.

Curious.  I don't see why

        def parity(x):
          b = 2
          l = 1
          while True:
            b <<= l
            if x < b: break
            l <<= 1
          while l:
            b >>= l
            x ^= x >> l
            x &= b - 1
            l >>= 1
          return x & 1

is any less efficient.  Indeed, it seems more so to me.  The number of
top-level loop iterations is bounded by the logarithm of the total
number of bits in the input rather than the Hamming weight.  In terms of
single-precision operations (if we're dealing with bignums) the analysis
is more complicated; but the number of single-precision operations in
one of my loops is a linear function of l (assuming that the comparison
is done sensibly), and l increases and decreases geometrically, so I
have performance which is O(log x).  Assuming no special Hamming-weight
distribution on the input, the `efficient' version takes O(log^2 x)
single-precision operations.

Given an /a-priori/ upper bound on the length of the input, e.g., it
fits in a machine word, the above technique is still probably faster.
That is, assuming arithmetic and bitwise operations on integers take
constant time, my version runs in O(log log x) time whereas the
`efficient' version takes O(log x) time.

(My function returns the complement of the value requested.  Fixing it
is obviously trivial.)

-- [mdw]



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