Math with strings

[TedBronson]

Hello,

I am trying to make a function which takes two very large integers inside of strings, subtracts them, and returns the result as a string.

One goal is to do this without using third-party libraries, so please don't tell me to use Boost, MathTT etc.

I was able to make a function which adds 1 to a string:

void IncrementString(string& s)
{
    string::reverse_iterator i = s.rbegin(), end = s.rend();
    int carryOver = 1;
    while (carryOver && i != end)
    {
        int val = (*i - '0') + carryOver;
        carryOver = (val / 10);
        *i = '0' + (val % 10);
        ++i;
    }
    if (carryOver)
        s.insert(0, "1");
}

But my code to subtract strings doesn't work:

string StringSubtract(string& y, string& z) {
    string::reverse_iterator i = z.rbegin(), iend = z.rend();
    string::reverse_iterator j = y.rbegin(), jend = y.rend();
    string answer;
    bool borrow = false;
    while (j != jend) {
        int p, q;
        p = *i - '0';
        q = *j - '0';
        if (borrow) {
            p--;
            borrow = false;
        }
        if (q > p) {
            borrow = true;
            p += 10;
        }
        int r = p - q;
        answer = answer.insert(0,to_string(r)); //PROBLEM!
        i++;
        j++;
    }
    while (i != iend) {
        if (borrow) {
            *i--;
            borrow = false;
        }
        const char* s = *i;
        answer = answer.insert(0,&s);
    }
    return answer;
}
 

The error I get is with the string.insert() function - invalid conversion from char to const char*.

I've tried sprintf(), string_to() and using a char, nothing seems to work! Is there any other way to put an integer in the front of a string that is not a constant?

[weaknessforcats]

What do you mean by "very large integers"?

Using strings and iterators will be very very slow.

Instead consider an array of unsigned int that you create on the heap. Now you can work at the binary bit level so your addition and subtraction is very straightforward. An array is guaranteed contiguous allowing you to scan bit-by-bit the entire array.

You can aleays write a function to convert your array to a basic_string<char> (aka a string object).