Greatest Common Divisor problem

I'm having trouble with creating a program that calculates the greatest common divisor between two numbers. I would greatly appreciate some help.

Code:

`#include <iostream>`

using std::cout;

using std::cin;

using std::endl;

double gcd ( int x, int y );

int main()

{

int a;

int b;

// allow the five sets of numbers to be input

for ( int j = 1; j <= 5; ++j ) {

cout << "Enter two integers: ";

cin >> a >> b;

cout << "The greatest common divisor of " << a

<< " and " << b << " is "

<< gcd ( a, b ) << "\n\n";

} // end for

return 0;

} // end main

// function gcd definition

double gcd( int x, int y )

{

int greatest = 1;

for ( int i = 2; i <= ( ( x < y ) ? x : y ); ++i );

if 0 = ( x % i ) + ( y % 1 );

greatest = i;

return greatest;

} // end function gcd