Thread: Program about pointers

Hi, everyone. I have here a code of a program that interchanges the values of two int variables, x and y, by using pointers:

Code:

#include <stdio.h>
void interchange(int * u, int * v);

int main(void)
{
    int x = 5, y = 10;
    printf("Originally, x = %d and y = %d.\n", x, y);
    interchange(&x, &y); //send addresses to function
    printf("Now, x = %d and y = %d.\n", x, y);

    return 0;
}

void interchange(int * u , int * v)
{
    int temp;

    temp = *u; //temp = x    //temp gets value that u points to
    *u = *v;   //x = y
    *v = temp; //y = temp
}

The thing is: how are the values of x and y changed in the main function, if the funtion interchange() doesn't return anything? Is it because this one works with pointers that point to x and y, in such a way that these pointers are the "bridge" between the main() and the interchange() functions?

Thanks in advance!

The thing is you are not swapping the pointers... you are swapping the values at the addresses in the pointers... The pointers in the function are copies of the ones in the main program... so the change is relected back to the calling function.

Originally Posted by CommonTater

The thing is you are not swapping the pointers... you are swapping the values at the addresses in the pointers... The pointers in the function are copies of the ones in the main program... so the change is relected back to the calling function.

Thanks!

Originally Posted by quzah

Yes. Without a pointer, you pass the value stored in a variable. Through a pointer, you pass the value that represents where that varaible is stored in memory. Since you know where it is in memory, you can change what value it holds (since it's not declared as constant).

It sounds like you need to reread whatever it is you read about pointers again.


Quzah.

So I am interchanging the values in the addresses of x and y?

Originally Posted by quzah

It sounds like you need to reread whatever it is you read about pointers again.
Quzah.

Thanks; actually, I am still starting on this subject. I guess I'll have to read more about it.

Originally Posted by AndrewHunter

For a quick review on pointers, read through Lesson 6- Pointers For a more thorough treatment of the subject I would suggest Prelude's Pointer Tutorial.

Thanks!

Originally Posted by quzah

We had a good analogy going once before ... and here it is: Questions to understand malloc better


Quzah.

Thanks