Thread: variable modification inside functions

Hello,

I am trying to find a way to let functions edit variables outside of main(). Is there anyway to do this?

My code currently look like this

Code:

#include <iostream>

using namespace std;

int Runme(int y, int z); 

int main()
{
int y; y = 0; //define variables
int z; z = 0;
int a; a = 0;


cin>> y;
cout<<"\n"<<y<<"\n";

Runme(y, z);

cout<<"\n"<< y <<" "<< z;
cin >> a;

}

int Runme(int y, int z)
{
   cin>>z;
   cout <<"\n"<<y<<" "<<z;
   y = y + z;
   cout <<"\n"<< y <<" "<< z;
return y, z;
}

If you complie this, you'll notice the last cout statement shows int y and z as they are in main, not with the last input recived from Runme()...

Is there anyway for function Runme() to pass it's variables back to main()? What would someone recommend I do when trying to work with variables inside main() like this?

The problem is the variables y and z in your function "Runme" are just copys of y and z from main, not the originals. To modify the variables like you are asking one way would be to pass them by reference.

Code:

#include <iostream>

using namespace std;

void Runme(int &y, int &z); 

int main()
{
	int y = 0; //define variables
	int z = 0;
	int a = 0;

	cout << "Enter a value for \"y\" in main : " << endl;
	cin >> y;
	cout << "\n\"y\" in main " << y << endl;

	Runme(y, z);

	cout << "\n\"y\" and \"z\" in main after Runme " << y << " " << z << endl;
	cin >> a;
}

void Runme(int &y, int &z)
{
	cout << "Enter a value for \"z\" in Runme : " << endl;
	cin >> z;
	cout << "\n\"y\" in Runme " << y << " z in Runme "<< z << endl;
	y = y + z;
	cout << "\nAfter y = y + z " << y << " " << z << endl;
	//return y, z;
}

if you want to edit the variables in main directly from a function, then include them in the functions argument and add a & infront of them

Code:

int Runme(&y, &z);

I'm not sure if you need to include those & when you declare the function. Try it out and write back.

you got there before me

1- When you pass a variable into a function, you are not passing the variable... you are only passing it's value. This is called passing by value. That value is assigned to a new variable inside the function. Sometimes the variable inside the function has the same variable-name, but it's a different X and a different Y!

2- A function can only return one value. Again, it's returning a value of a particular type.

There are two ways around this - Use pointers or better yet, references which allow you to "get-to" all of the variables by passing-in their addresses. Hmmm... no reference tutorial???

Most beginning C++ books will introduce pointers and references with a function that swaps the values of X & Y. You can't do that with regular variables.

Pointers are used with C-style strings, because a function cannot return an entire array of characters.

Originally Posted by hk_mp5kpdw

Some other comments:

1. Your initialization of y is unnecessary.

In my opinion, telling beginners not to initialize their variables is bad advice.

Always initialize your variables, period.