1

I am trying to add the Two's Complement to a Binary number represented with a string. Assuming the string has already been flipped, how would I go about "adding" 1 to the last character, and replacing the other characters in the string as needed?

Example: 100010 is flipped to 011101, and is represented as a string. How would you apply the Two's Complement to the 011101 string?

One part of this that really has me puzzled is if the user enters a binary number that, when the two's complement is applied, involves a lot of carrying.

Improve this question
3
  • 2
    A very impractical thing to do -- is this homework? Commented Oct 13, 2010 at 5:13
  • A follow-on from your previous question, presumably. Commented Oct 13, 2010 at 10:30
  • It is. I've been working on it for several days with no luck. Commented Oct 13, 2010 at 14:58

3 Answers 3

Reset to default
2

I'd just do it as a number, then convert it back.

def tobin(x, count=8):
    # robbed from http://code.activestate.com/recipes/219300/
    return "".join(map(lambda y:str((x>>y)&1), range(count-1, -1, -1)))

def twoscomp(num_str):
    return tobin(-int(num_str,2),len(num_str))

print twoscomp('01001001') # prints 10110111
print twoscomp('1000')     # prints 1000 (because two's comp is cool like that)
print twoscomp('001')      # prints 111
Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

2

Just for variety, here's yet another way based on the fact the Two's Complement is defined as the One's Complement plus one. This cheats a little and converts the intermediate one's complement string value into an integer to add one to it, and then converts it back to a binary string using the new built-in bin() function added in Python 2.6.

def onescomp(binstr):
    return ''.join('1' if b=='0' else '0' for b in binstr)

def twoscomp(binstr):
    return bin(int(onescomp(binstr),2)+1)[2:]

print twoscomp('01001001') # prints 10110111
print twoscomp('011101')   # prints 100011
print twoscomp('001')      # prints 111
Improve this answer

3 Comments

When trying to use "print twoscomp(<my flipped string>), I get an error saying that strings cannot be called.
@Anonymous: Sorry I cannot reproduce the problem you describe. What version of Python are you using and what exactly are you passing to twoscomp() as the argument (i.e <my flipped string>)?
@Anonymous: Here's a shot in the dark, but my wild guess is that you might have created a string variable named bin, which hides the built-in bin function. That, or possibly int. Change your variable name (to bin_ perhaps), and give it another shot.
1

if you want to do it without converting back to a number, start from the right of the string until you find the first 1, then flip all chars to its left.

Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.