firstArray[1] = firstArray[1] + firstArray[0];

firstArray[1] = FirstArray[0];

#include <iostream>
#include<string>
#include<vector>
#include<fstream>
using namespace std;

string firstArray = {0,0, '/', 0, 0, '/', 0, 0};
int main(){

  cout << firstArray <<endl;
  firstArray[1] = firstArray[0]; //this works
  cout << firstArray << endl;
  cout << firstArray <<endl;
  firstArray[1] = firstArray[0] + firstArray[1]; //this is the bit that doesn't work

  cout << "thanks guys :) <<endl;

  return 0;
}