Sum of two arrays, carry over operation C++ - c++

Question

Beginner here, and I'm stuck. The main program is provided to us, and we're supposed to write 3 functions. readBig(), addBig(), and printBig(). I'm stuck on the addBig() function. It's supposed to sum two arrays, and perform the carry operation. I cannot figure out where I'm going wrong. The carry operation is working out for me.
Any help/direction is appreciated.
#include <iostream>
using namespace std;
//This program will test three functions capable of reading, adding,
//and printing 100-digit numbers.
// Do not change these function prototypes:
void readBig(int[]);
void printBig(int[]);
void addBig(int[], int[], int[]);
// This constant should be 100 when the program is finished.
const int MAX_DIGITS = 100;
//There should be no changes made to the main program when you turn it
in.
int main(){
// Declare the three numbers, the first, second and the sum:
int num1[MAX_DIGITS], num2[MAX_DIGITS], sum[MAX_DIGITS];
bool done = false;
char response;
while (not done)
{
cout << "Please enter a number up to "<<MAX_DIGITS<< " digits: ";
readBig(num1);
cout << "Please enter a number up to "<<MAX_DIGITS<< " digits: ";
readBig(num2);
addBig(num1, num2, sum);
printBig(num1);
cout << "\n+\n";
printBig(num2);
cout << "\n=\n";
printBig(sum);
cout << "\n";
cout <<"test again?";
cin>>response;
cin.ignore(900,'\n');
done = toupper(response)!='Y';
}
return 0;
}
//ReadBig will read a number as a string,
//It then converts each element of the string to an integer and stores
it in an integer array.
//Finally, it reverses the elements of the array so that the ones digit
is in element zero,
//the tens digit is in element 1, the hundreds digit is in element 2,
etc.
void readBig(int num[])
{
for(int i = 0; i < MAX_DIGITS; i++){
num[i] = 0;
}
string numStr;
getline(cin,numStr);
string temp;
//store into array
for (int i = 0; i < numStr.length(); i++){
temp = numStr.at(i);
num[i] = stoi(temp);
}
int arrayLength = MAX_DIGITS;
int temp2;
for (int i = 0; i < (arrayLength/2); i++){
temp2 = num[i];
num[i] = num[(arrayLength - 1) - i];
num[(arrayLength - 1) - i] = temp2;
}
}
//AddBig adds the corresponding digits of the first two arrays and
stores the answer in the third.
//In a second loop, it performs the carry operation.
void addBig(int num1[], int num2[], int sum[])
{
for (int i = 0; i < MAX_DIGITS; i++){
sum[i] = num1[i] + num2[i];
if (sum[i] > 9){
sum[i] = sum[i] - 10;
sum[i+1] = sum[i+1] + 10;
}
}
}
//PrintBig uses a while loop to skip leading zeros and then uses a for
loop to print the number.
void printBig(int array[])
{
int i = 0;
while (array[i] == 0){
i++;
}
for (int j = i; j < MAX_DIGITS;j++){
cout << array[j] << endl;
}
}

Answer 1

Looks like readBig function isn't correct, it stores least significiant digit into num[numStr.length()-1], after reversing it became num[MAX_DIGITS -1 - ( numStr.length()-1], but addNum assumes last digit is num[0].
Correct variant:
void readBig(int num[])
{
//clear num, read numStr...
//store into array
int count = 0;
for (int i = numStr.length()-1; i >= 0; --i){
temp = numStr.at(i);
num[count++] = stoi(temp);
}
|

Answer 2

So this
sum[i] = sum[i] - 10;
sum[i+1] = sum[i+1] + 10;
should most likely be this
sum[i] = sum[i] - 10;
sum[i+1] = sum[i+1] + 1;
Since its the next decimal place it shouldnt be incremented by 10
Also when you get to the last cell in your array
sum[i+1] = sum[i+1] + 1;
this will be out of bounds, so depending on the requirments you will want to change this