This snippet parses an int32 from a string.

string2int.cpp

#include <cstdio>
#include <cstring>
using namespace std;

int multiplier(int n)
{
    if (n == 0)
        return 1;

    int q = 10;
    for (int i = 0; i < n - 1; i++)
    {
        q *= 10;
    }

    return q;
}

int string2int(char s[])
{
    if (strlen(s) == 0)
        return 0;

    bool r = s[0] == '-'; int n = 0;
    for (int i = r; i < strlen(s); i++)
    {
        int c = (s[i] - 48) * multiplier(strlen(s) - i - 1);
        if (!r)
            n += c;
        else
            n -= c;
    }

    return n;
}

int main()
{
    char s[64];
    scanf("%s", s);
    printf("%d", string2int(s));
    return 0;
}