variable modification inside functions

This is a discussion on variable modification inside functions within the C++ Programming forums, part of the General Programming Boards category; Hello, I am trying to find a way to let functions edit variables outside of main(). Is there anyway to ...

  1. #1
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    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?

  2. #2
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    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;
    }

  3. #3
    Hi ay_okay's Avatar
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    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.

  4. #4
    Hi ay_okay's Avatar
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    you got there before me

  5. #5
    Hardware Engineer
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    Exclamation There are two issues...

    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.

  6. #6
    Registered User hk_mp5kpdw's Avatar
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    Some other comments:

    1. Your initialization of y is unnecessary. You don't do anything with it before the user stuffs a value into it so it's not critical that it have a value prior to that point (although it doesn't hurt either).

    2. The a variable is unnecessary. It seems you really only have it as a method of pausing the program before exiting by getting user input. A simple cin.get(); call should work just as well.

    3. As far as z goes, you can declare and initialize the variable all in a single step by just saying int z = 0; instead of having two statements to declare and then initialize.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
    -Christopher Hitchens

  7. #7
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    Quote 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.

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