Adding one to number represented as array of digits in C Program?

Initially the default value of index is 0
begin
   if index < n, then
      if arr[index] < 9, then
         arr[index] := arr[index] + 1
      else
         arr[index] := 0
         increment(arr, n, index + 1)
   end if
   if index = n, then
      arr[n] := 1
      n := n + 1
   end if
end

#include <iostream>
#include <cmath>
#define MAX 20
using namespace std;
void increment(int num_arr[], int &n, int index = 0){
   if(index < n){
      if(num_arr[index] < 9){ //if digit is less than 9, add 1
         num_arr[index]++;
      }else{ //otherwise increase number recursively
         num_arr[index] = 0;
         increment(num_arr, n, index+1);
      }
   }
   if(index == n){
      num_arr[n] = 1; //add extra carry
      n++; //increase n
   }
}
void dispNumber(int num_arr[], int n){
   for(int i = n-1; i>= 0; i--){
      cout << num_arr[i];
   }  
   cout << endl;
}
int numToArr(int num_arr[], int number){
   int i = 0;
   int n = log10(number) + 1;
   for(int i = i; i< n; i++){
      num_arr[i] = number % 10;
      number /= 10;
   }
   return n;
}
main() {
   int number = 1782698599;
   int num_arr[MAX];
   int n = numToArr(num_arr, number);
   cout << "Initial Number: "; dispNumber(num_arr, n);
   increment(num_arr, n);
   cout << "Final Number: "; dispNumber(num_arr, n);
}

Initial Number: 1782698599
Final Number: 1782698600