Thanks for your suggestions,I'll try working on them tomorrow morning and I'll tell you how they go.

If we wanted to do the exact same thing as the above but this time sort a list of a KNOWN size(say 1000 numbers in each file),could we use a bubble sort function to do so?

We have this code...

Code:
#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
#include <cstdlib>
#include <vector>
#include <list>

int main( )
{

  using namespace std; 
  
  ifstream in_stream1;
  ofstream out_stream;
  ifstream in_stream2;

  in_stream1.open("prob2list1.txt");
  in_stream2.open("prob2list2.txt");
  out_stream.open("prob2merged.txt");
  
  int numbers[2000];
  int array_size;
  int num;
  int i,j;

    while(i<2000)
    {
      in_stream1>>num;
      numbers[i]=num;
      i++;
    }
  

  void bubbleSort(int numbers[], int array_size);
  {
  int i, j, temp;

  for (i = (array_size - 1); i >= 0; i--)
  {
    for (j = 1; j <= i; j++)
    {
      if (numbers[j-1] > numbers[j])
      {
        temp = numbers[j-1];
        numbers[j-1] = numbers[j];
        numbers[j] = temp;
      }
    }
  }
  }

   bubbleSort(numbers, 2000);

    while(j<2000)
      {
	numbers[j]=num;
	out_stream<<num<<"\n";
	  }
  
  return 0;
 }
We only have one error and its this...

"undefined reference to 'bubbleSort<int*, int>'
collect2: ld returned 1 exit status"


I have never seen this error before,whats up?