Convert String to Int without int()

python, python-3.x

Solution

Refer to a basic `atoi` in C:

int myAtoi(char *str)
{
    int res = 0; // Initialize result

    // Iterate through all characters of input string and update result
    for (int i = 0; str[i] != '\0'; ++i)
        res = res*10 + str[i] - '0';

    // return result.
    return res;
}

Which translates into the Python:

def atoi(s):
    rtr=0
    for c in s:
        rtr=rtr*10 + ord(c) - ord('0')

    return rtr

Test it:

>>> atoi('123456789')
123456789

If you want to accommodate an optional sign and whitespace the way that `int` does:

def atoi(s):
    rtr, sign=0, 1
    s=s.strip()
    if s[0] in '+-':
        sc, s=s[0], s[1:]
        if sc=='-':
            sign=-1

    for c in s:
        rtr=rtr*10 + ord(c) - ord('0')

    return sign*rtr

Now add exceptions and you are there!

Problem

I'm trying to implement `add2strings`, `sub2strings`, `mult2strings` functions in Python. They're all very easy if you just do `int(string)`, but I want to do them without that and without importing another cheating thing like `Decimal`. My current idea is to use `bytes`. Is there another way to do this?

Original source