Having a problem with gcd

I'm having a problem with gcd. I don't know why it gives me a red squiggly under the gcd(lower case) at this line
GCD = gcd(y, x % y);

Any help?

Code:

---

// Greatest Common Divisor.cpp : Defines the entry point for the console application.
//

//        =========
//        Libraries
//        ==================
  #include "stdafx.h"
  #include <iostream>
  #include <math.h>
  #include <stdio.h>
//        ==================

//  ==========
//  Namespaces
//        ====================
        using namespace std;
//  ====================

//  ===================
//        Function Prototypes
//        =====================
        void WelcomeBanner();
        void GetIntegers(int&, int&);
//        =====================



// ==============

        int main( ) { // main()

                int x;
                int y;
                int GCD;

                WelcomeBanner();
                GetIntegers(x, y);

                if (y == 0 ){
                       
                        cout << "GCD = " << x << endl;
                }
                else
                        GCD = gcd(y, x % y);
                        cout << "GCD = " << GCD << endl;
               
       
               


        return 0;
        }// main()
//        ========


//  =========================
//        Function WelcomeBanner()
//        =========================
        void WelcomeBanner(){

                cout << "================================================" << endl;
                cout << "Welcome to my program that will give you the  |" << endl << "greatest common divisor of your two numbers =) |" << endl;
                cout << "================================================"<< endl << endl;

        }// Welcome banner
//        ==================

//        ====================
//        Function GetIntegers
//        ====================
        void GetIntegers(int& num1, int& num2){

                cout << "Please enter an integer that you want me to find the GCD of." << endl;
                cin >> num1;

                cout << "Please enter another integer that you want me to find the GCD of." << endl;
                cin >> num2;

        }//GetIntegers
//        ==============

---

Bump!

Have you written the gcd function somewhere becuase there is no gcd function in cmath.

Well in my book it says

gcd(x, y) = x if y = 0
gcd(y,x%y) if y =/= 0

So I'm a little confused..

Your book gave you the mathematics that can be used to implement the function. It's your turn now :)

That's the concept of GCD. You have to write that gcd function yourself.

Code:

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int gcd(int x, int y)
{
  translate that part into code here
}

---

Awwww. Okokok. I can do that ;)