Question

I have a function that decrements a whole number parameter represented by a string. For instance, if I pass in the string “100”, it should return “99.”

def dec(s):
    i = len(s) - 1
    myString = ""
    while (i >= 0):
        if s[i] == '0':
            s[i] = '9'
            i -= 1
        else:
            s[i] = chr(int(s[i]) - 1)
            break
    return s

However, Python issues this error.

s[i] = '9'
TypeError: 'str' object does not support item assignment 

I am assuming that s[i] cannot be treated as an lvalue. What is a solution around this?

Answer 1

You can’t. In Python, strings are immutable — once created, they can’t be changed.

You have two options without changing your function entirely.

Convert the string to a list and back:

def dec(s):
    s = list(s)
    i = len(s) - 1
    myString = ""
    while (i >= 0):
        if s[i] == '0':
            s[i] = '9'
            i -= 1
        else:
            s[i] = chr(int(s[i]) - 1)
            break
    return ''.join(s)

Create a new string each time you want to make a change:

def dec(s):
    i = len(s) - 1
    myString = ""
    while (i >= 0):
        if s[i] == '0':
            s = s[:i] + "9" + s[i+1:]
            i -= 1
        else:
            s = s[:i] + chr(int(s[i]) - 1) + s[i+1:]
            break
    return s