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[python] Bitwise Operations

  1. Jan 14, 2013 #1
    I'm playing around in Python at the moment, and I came across an exercise to perform bitwise operations manually (without the built in & | ^ ~ operators).

    I understand the operations on paper, and I have a function here that performs them (& in this case, the others are similar):

    Code (Text):
    def bitwise_and(num1, num2):

        result = "" # the result of the bitwise and operation

        while num1 > 0 or num2 > 0:
            # num % 2  != 0 implies a 1-bit in the lowest position
            if num1 % 2 != 0 and num2 % 2 != 0:
                result = "1" + result # both bits are 1, so 1 & 1 == 1, append 1 to result
            else:
                result = "0" + result# at least one 0 bit, 1 & 0 == 0, 0 & 0 == 0, append 0

            num1 = num1 >> 1 # drop the last bit
            num2 = num2 >> 1 # drop the last bit

        return int(result,2) # return an int representation of the result binary string
     
    I'd like some help modifying this implementation to use an integer type for the result instead of a string type which I then have to convert. I was trying to set the current result bit and then left shift (<<) but it didn't work when I only had 0 bits to work with because 0b0 == 0b00 == 0b0000000 so there's no significant 1 bit to shift. Is there a neater way do this?
     
    Last edited: Jan 14, 2013
  2. jcsd
  3. Jan 15, 2013 #2
    Re: Bitwise Operations

    I've not programmed in Python, however, ... Have you considered using the divmod operator, with the 'div' bit providing the shifted number and then multiplying the 'mod' bits together (then iterating until the 'div' bit of either number is 0). It seems to work in Mathcad ...

    attachment.php?attachmentid=54739&stc=1&d=1358249555.jpg
     

    Attached Files:

  4. Jan 18, 2013 #3
    Re: Bitwise Operations

    Thanks, that's nice and simple :) I can stare at problems forever and just not grok things properly sometimes! Here's some Python code:

    Code (Text):
    #divmod version

    def divmod_and(x, y):
        z = 0
        p = 1
        while x or y:
            x, xd = divmod(x, 2)
            y, yd = divmod(y, 2)
            z = p * xd * yd + z
            p <<= 1

        return z

    # manual mod and divide for clarity

    def bitwise_and(x, y):
        result = 0
        power = 1
        while x or y:
            bit = x % 2 * y % 2 # both bits on == 1
            result = bit * power + result
            x /= 2
            y /= 2
            power <<= 1

        return result
     
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