Function to add up an array

I am just about done but I am having a hard time thinking of how to add up numbers in an array from a file. I want it as a user-defined function. I have done it through predefined methods but I was wondering how to make a function that does the job.

#include <iostream>
#include <fstream>
#include <iomanip>
#include <string>
#include <stdlib.h>
using namespace std;

const int OCCUPATION_MAX = 10;
void sort(string occupation[], double salary[]);
void sum(double salary[], double sum, int index);
double calc(double avgSalary, double medSalary);

int main()
{   
  string occupation[OCCUPATION_MAX];    //array of type string for the occupation names
  double salary[OCCUPATION_MAX];
  ifstream input;
  ofstream output;
  int index = 0;
  double sum = 0;
  string fname;
  double average;
  double median;

  //Intro
  cout << "Welcome to this program.\n";
  cout << "Please enter text file you want to use.\n";
  cin >> fname;
  //opening file of correct
  input.open(fname.c_str());

  if(input.fail())
  {
    cout << "Not a valid filename\n";
    exit(0);
  }

  //If input file exists, read all records from file
  while (input >> occupation[index])
  {
    input >> salary[index];
    sum = sum + salary[index];
    index++;
  }
  //Close input file
  input.close();

  //Call function to sort arrays in descending order by salary
  sort(occupation, salary);

  //Call function to calculate average and median of salaries
  double calc(double sum, double salary[], double& average, double& median);

  //Open output file, remember to look at output.txt for the result)
  output.open("output.txt");

  //Write file to output
  output << setw(15) << "OCCUPATION" << setw(15)<< "SALARY" << endl;
  output << "------------------------------------" << endl;
  for (index = 0; index<OCCUPATION_MAX; index++)
  {
    output << setw(15) << occupation[index] << setw(10) <<setprecision(2) << fixed << "$ " << salary[index] << endl;
  }
  output << endl << setw(15) << "Average Salary" << setw(10) <<setprecision(2) << fixed << "$ " << average << endl;
  output << setw(15) << "Median Salary" << setw(10) <<setprecision(2) << fixed << "$ " << median << endl;

  //Close output file
  output.close();

  return 0;
}

//Function to sort both arrays in descending order by salary
void sort(string occupation[], double salary[])
{
  for (int i=0; i<OCCUPATION_MAX; i++)
  {
    for (int j=i+1; j<OCCUPATION_MAX; j++)
    {
      if (salary[i] < salary[j])
      {
        //Swap salary
        double temp = salary[i];
        salary[i] = salary[j];
        salary[j] = temp;

        //Swap occupations
        string t = occupation[i];
        occupation[i] = occupation[j];
        occupation[j] = t;
      }
    }
  }
}
void sum(double salary[], double sum, index=0)
double salary;
double sum=0;
int index=0;
{
 while(!eof)
 {
    output >> salary[index];
    sum = sum + salary[index];
    index++;
  }
}
//Function to calculate average and median
double calc(double sum, double salary[], double& average, double& median)
{
  //Calculate average of occupations
  average = sum / salary[OCCUPATION_MAX];

  //Calculate median of occupations
  median = (salary[OCCUPATION_MAX/2] + salary[(OCCUPATION_MAX-1)/2])/2;
}

So far I have been told to dereference my pointers.

void sum(double salary[], double sum, index=0)
    double salary;
    double sum=0;
    int index=0;
    {
     while(!eof)
     {
        output >> salary[index];
        sum = sum + salary[index];
        index++;
      }
    }