Getting IndexError: string index out of range for a python problem

Question

the problem I am solving is to find the sum of two large numbers, where the numbers are given as strings. the algorithm is :

Reverse both strings.
Keep adding digits one by one from 0’th index (in reversed strings) to end of smaller string, append the sum % 10 to end of result and keep track of carry as sum / 10.
Finally reverse the result.

This was the code I tried:

def stringadd(s1, s2):
    s1 = s1[::-1]
    s2 = s2[::-1]
    
    m = len(s1)
    n = len(s2)
    carry = 0
    s = ""
    if len(s1) > len(s2):
        temp = s2
        s2 = s1
        s1 = temp
    for i in range(m):
        su, = (ord(s1[i]) - 48) + (ord(s2[i]) - 48) + carry
        s += chr(sum % 10 + 48)
        carry = int(sum / 10)
        
    for i in range(m, n):
        sum = (ord(s2[i]) - 48) + carry
        s += chr(sum % 10 + 48)
        carry = int(sum / 10)
        i += 1
    if carry:
        s += chr(carry + 48)
    s = s[::-1]
    return s


print(stringadd("32134", "23456782"))

I am getting a string index out of range error in the line 17 which is

sum = (ord(s1[i]) - 48) + (ord(s2[i]) - 48) + carry

You can try with int("32134") + int("23456782"). if the given string is not convertible to int then it raises error. — Mahesh Anakali
– Mahesh Anakali, Commented Jan 16, 2021 at 13:23

mensi · Accepted Answer · 2021-01-16 14:03:15Z

A few things:

You are taking the lengths for m and n before swapping s1 and s2 around, so they do not necessarily match anymore.
You have a i+=1 in the for loop which doesn't do anything
You are using sum as a variable name, shadowing the built-in sum function.

I would propose to rewrite it like this:

def stringadd(s1, s2):
    shorter, longer = (x[::-1] for x in sorted((s1, s2), key=len))
    carry = 0
    result = ""

    for i in range(len(shorter)):
        s = (ord(shorter[i]) - 48) + (ord(longer[i]) - 48) + carry
        result += chr((s % 10) + 48)
        carry = int(s / 10)

    for i in range(len(shorter), len(longer)):
        s = (ord(longer[i]) - 48) + carry
        result += chr(s % 10 + 48)
        carry = int(s / 10)

    if (carry):
        result += chr(carry + 48)

    return result[::-1]


if __name__ == '__main__':
    print(stringadd("32134", "23456782"))
    assert(32134 + 23456782 == int(stringadd("32134", "23456782")))
    print(stringadd("999", "9"))
    assert(999 + 9 == int(stringadd("999", "9")))

Some of the advantages:

More descriptive variable names
Use len(shorter) or len(longer) instead of n and m to make it clear what is meant

You could even replace the magic constant 48 with ord("0") to make it clearer what that number does. And for fun, we can also use a more functional style:

from functools import reduce
def stringadd2(s1, s2):
    # Make s1 and s2 the same length by appending 0s to the reversed string,
    # then pair them up.
    digits = zip(s1[::-1] + "0" * (max(len(s1), len(s2)) - len(s1)),
                 s2[::-1] + "0" * (max(len(s1), len(s2)) - len(s2)))

    # The function for our reducer which has state in part and gets each pair
    # of digits in x iteratively.
    def add(part, x):
        s = (ord(x[0]) - ord("0")) + (ord(x[1]) - ord("0")) + part[1]
        return part[0] + chr((s % 10) + ord("0")), int(s / 10)

    result, carry = reduce(add, digits, ("", 0))
    if carry:
        result += chr(ord("0") + carry)

    return ''.join(result[::-1])

@prison-mike in case you're interested, I also added an implementation that is a bit more functional-programming style.

Prakhar Rathi · Accepted Answer · 2021-01-16 13:38:40Z

I tried running the code and I did get the correct answer when I send the inputs as print(stringadd("32134","23456782")) but when I change it to print(stringadd("23456782", "32134")), I face the error. This is because you're using m and n for looping but you are interchanging s1 and s2 after that in some cases (when len(s1)>len(s2)).

You should take the value of s1 and s2 after the interchange. This will fix the code. Please find the correct code below.

def stringadd(s1,s2):
    
    s1=s1[::-1]
    s2=s2[::-1]
    
    
    carry=0
    s=""
    
    if len(s1)>len(s2):
        
        temp=s2
        s2=s1
        s1=temp
    
    # Value of m and n taken after interchaning
    m=len(s1)
    n=len(s2)
    
    for i in range(m):
        sum=(ord(s1[i])-48)+(ord(s2[i])-48) +carry
        s+=chr(sum%10 +48)
        carry=int(sum/10)
        
    for i in range(m,n):
        sum=(ord(s2[i])-48)+carry
        s+=chr(sum%10 +48)
        carry=int(sum/10)

    if(carry):
        s+=chr(carry+48)
    s=s[::-1]
    return s


print(stringadd("23456782", "32134"))

A couple of other things that you could change as are:-

Don't use sumas a variable because it is a reserved keyword.
Remove i += 1 from the code since it serves no purpose in the forloop.

Hope this answers your question.

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