Binary Subtraction - Python

Question

I want to make a binary calculator and I have a problem with the subtraction part. Here is my code (I have tried to adapt one for sum that I\'ve found on this website).

Answer 1

I hope the answer below it helps.

def binarySubstration(str1,str2):
if len(str1) == 0:
    return
if len(str2) == 0:
    return 

str1,str2 = normaliseString(str1,str2)
startIdx = 0
endIdx = len(str1) - 1
carry = [0] * len(str1)
result = ''


while endIdx >= startIdx:
    x = int(str1[endIdx])
    y = int(str2[endIdx])
    sub = (carry[endIdx] + x) - y

    if sub == -1:
        result += '1'
        carry[endIdx-1] = -1

    elif sub == 1:
        result += '1'
    elif sub == 0:
        result += '0'
    else:
        raise Exception('Error')

    endIdx -= 1
   
return result[::-1]

normalising the strings

def normaliseString(str1,str2):
    diff = abs((len(str1) - len(str2)))
    if diff != 0:
        if len(str1) < len(str2):
            str1 = ('0' * diff) + str1
            
        else:
            str2 = ('0' * diff) + str2
           
    return [str1,str2]

Answer 2

Short answer: Your code is wrong for the case when s1[i] == s2[i] and carry == 1.

Longer answer: You should restructure your code to have three separate cases for s==-1, s==0, and s==1, and then branch on the value of carry within each case:

if s == -1:  # 0-1
    if carry == 0:
        ...
    else:
        ...
elif s == 0:  # 1-1 or 0-0
    if carry == 0:
        ...
    else:
        ...
else:  # 1-0
    if carry == 0:
         ...
    else:
        ...

This way you have a separate block for each possibility, so there is no chance of overlooking a case like you did on your first attempt.