add subtract large numbers C++

[rangerSkip]

I'm looking to find the sums and differences of large numbers inputed by the user. all numbers must by positive and whole and the first number must be larger than the second. I'm having a couple problems:

1) I'm currently using arrays and saving the input into a string. I think that, when copying the data from the string to the array, I need to have the numbers at the very end of the array. I think I found a good way but its not working correctly.

2) This might be linked to #1 but I think my programming is wrong when I'm calculating the sum and differences for the two numbers. I get really long outputs that don't make sense.

Here is all the code:

#include<string>
#include<iostream>
 
using namespace std;
 
int main() {
    //Declare Variables
    int fnum[512] = {0};
    int snum[512] = {0};
    int sum[512] = {0};
    int diff[512] = {0};
    string first;
    string second;
 
 
    //Prompt user for first integer
    cout << "Please enter a positive whole number no larger than 512 digits long." << endl;
    cin >> first;
 
    if (first[0] == '-') {
        cout << "Your number must be positive.";
        return -1;
    }
 
    for (int i = 0; i < sizeof(first); i++) {
        fnum[sizeof(first) - i] = first[i];
    }
 
 
    //Prompt user for second integer
    cout << endl << "Please enter a positive whole number no larger than 512 digits long" << endl;
    cout << "AND is smaller than the first number." << endl;
    cin >> second;
    if (second[0] == '-') {
        cout << "Your number must be positive.";
        return -1;
    }
 
    for (int j = 0; j < sizeof(second); j++) {
        snum[sizeof(second) - j] = second[j];
    }
 
 
    //If second integer is larger than first return error
    if (first < second) {
        cout << "The second integer must be smaller than the first integer.";
        return -1;
    }
 
    //TEST: make sure input copied into string correctly
    cout << endl;
    cout << "Your first number is " << first << "." << endl;
    cout << "Your second number is " << second << "." << endl;
    cout << endl;
 
    //TEST: make sure string copied into array correctly
    cout << "first number is: ";
    for(int n = 0; n < sizeof(first); n++) {
        cout << fnum[n];
    }
 
    cout << endl << "second number is: ";
    for(int m = 0; m < sizeof(second); m++) {
        cout << snum[m];
    }
 
 
    //Display sum of two integers
    cout << endl;
    /*for(int s = 0; s < 513; s++) {
       if ((512 - s) == 0) {
          sum[512 - s] = fnum[512 - s] + snum[512 - s];
       }       
       sum[512 - s] = fnum[512 - s] + snum[512 - s];
       if (sum[512 - s] >= 10) {
          sum[512 - s] = sum[512 - s] - 10;
          sum[512 - s - 1] = sum[512 - s - 1] + 1;
       }
    }
    cout << first << " + " << second << " = ";
    for (int a = 0; a < 512; a++) {
        cout << sum[a];
    }*/
 
 
    //Display difference of two integers
    cout << endl;
 
    /*for(int d = 0; d < 513; d++) {
       diff[512 - d] = fnum[512 - d] - snum[512 - d];
       if (fnum[512 - d] > snum[512 - d]) {
          fnum[512 - d - 1] = fnum[512 - d - 1] - 1;
          fnum[512 - d] = fnum[512 - d] + 10;
       }
    }
    cout << first << " - " << second << " = ";
    for (int b = 0; b < 512; b++) {
        cout << diff[b];
    }
    cout << endl;*/
 
 
    //Pause and exit
    system("PAUSE");
    return 0;
}
 

[weaknessforcats]

Is this a homework problem?

If not, then you should use one of the big number libraries already written for C++. Large number operaions have already been coded up

[rangerSkip]

alas, it is a homework assignment. However, it is extra credit.

I'm only allowed to use iostream, strings and cmath libraries.

[weaknessforcats]

In that case I would use std::strings for your numbers instead of int arrays.

Then to add two numbers I would write a function that takes two strings by reference. Create two iterators and point them to string::rbegin so you can iterte from the right to he left.

You should be able to add chars until you get to string::rend.

The sum is returned as a string.