Writing a .txt file

I generate in a for loop:
1- An array made of random numbers
2- Add the elements of the array and store the
result in a number S
3- finally write in a .txt file the
number j(number of time that the for-loop has been executed)
and the number S for each j like this:
j=0 S=x
j=1 S=x
j=2 S=x
Steps 1 and 2 work fine.
Does someone has an idea how I can write
the js and the number S in a .txt file?
here is how my code till now looks like:
#include <stdlib.h>
#include <time.h>
#include <iostream>

using namespace std;

// The function RandomNum creates random numbers
// in the range between small and big

void RandomNum(double small,double big ,int p, double *array)
{
     double range=(big-small)+1;
    for (int i=0; i<p; i++){
   array[i]=small+int(range*rand()/(RAND_MAX + 1.0));
    }
}

//the function AdditionArr Add the elements of an
//array and store the result in a number S

double AdditionArr (double *Arr,int k)
{
      int i;
      double Sum=0.0;
      for (i=0; i<k; i++){
       Sum+=Arr[i];
        }
  return Sum;
}

int main()
    {
srand((unsigned)time(0));
int p=3;
int j;
double S,array[3];
        for(j=0;j<3;j++){
        RandomNum(-1.5,1.5 ,p,&array[0]);
       S=AdditionArr (&array[0],p);
    for (int k=0; k<p; k++){
    cout << "array"<<"["<< k << "]" <<" = " << array[k] << endl;
                }
    cout << "S=" << S << endl;
    cout << endl;
        }
        return 0;
    }