Prime numbers

[Beginner1551]

Why does my code print so many of the same prime number?

 #include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

int main()
{		
	int num;	
										
	cout << endl << "Please enter an integer larger than 50: ";
	cin >> num;
	cout << endl; 
	
	while (num <= 50)		
	{						
		cout << "You have entered a value less than 50. ";
		cout << "Please enter a valid value: ";
		cin >> num;
	}
	
	int a[20];
	ofstream outfile;					
	outfile.open("Random.txt");			
	
	for(int i = 0; i < 20; i++)			
	{											
		outfile << rand()%(num) + 1 << endl;	
	}
	
	outfile.close();					
	
	ifstream infile;					
	infile.open("Random.txt");			
	
	while(infile >> num)				
	{
		//if(num == 1)							
		
		for(int i = 2; i < num - 1; ++i)		
		{
			if(num % i == 0)					
			{
				//nothing
			}
			
			else if (num % i != 0)
			{
				cout << num << endl;
			}
		}
				
	}

	infile.close();		
	return 0;
}

[SamuelAdams]

Dupe post
http://www.cplusplus.com/forum/beginner/177314/