I am having a hard time understanding how to establish arrays for this problem. We're supposed to write a program that reads in numbers from 2 different files, and it averages one of the files while outputting both files for each month of the year. I have been reading the tutorial but the array doesn't make sense to me.

Code:
#include <iostream>
#include <fstream>

using namespace std;

const int Months = 12;

void get_stats ( double Previous_data, double current_data );

void output_previous ( double Previous_data );

int main()
{
int months [12];
double Previous_data, current_data, average;
cout << "We are going to get the data required for this lab. One moment please..." << endl;
    get_stats( Previous_data, current_data );
    output_previous( Previous_data );

return 0;
}

void get_stats( double Previous_data, double current_data)
{
ifstream current;

current.open ( "rainfall_current.dat" );

    // If the file fails to open it will close the program and output the statement.
    if ( current.fail( ) )
    {
        cout << "Please check if the file is saved properly. It could not open." << endl;
    }

    double temp, sum = 0;
    int i = 0;
    double average;

    // Loops the file until no more numbers can be read
    while (current >> temp)
    {
    sum += temp;
    ++i;
    }

    // Final average
    average = sum / i;

    // Outputs the average to the screen
    cout << "The average of the numbers is ";
    cout << average;
    cout << endl;
}

void output_previous( double Previous_data )
{
ifstream previous;

previous.open ( "rainfall_previous.dat" );

    // If the file fails to open it will close the program and output the statement.
    if ( previous.fail( ) )
    {
        cout << "Please check if the file is saved properly. It could not open." << endl;
    }

}