Using pointers in C programming?

Question

Right now I'm working on a program in C that takes 3 parameters; the address of a "first" integer, address of the "second" integer and the address in which to store the maximum of the two integers. So far I have the following basic code:

void max(int* x, int* y, int* z)
{

    if (x > y) {
        z = x;
    }
    else if (y > x){
        z = y;
    }
}

int main()
{
    int x = 6;
    int y = 4;
    int z;
    max(&x, &y, &z);
    printf("max of %d and %d = %d\n", x, y, z);

    x = 12;
    y = 17;
    max(&x, &y, &x);
    printf("x = %d, y = %d\n", x, y);
}

When executed it outputs the following:

max of 6 and 4 = 32767
x = 12, y = 17

HOWEVER! I want it to output this instead:

max of 6 and 4 = 6
x = 17, y = 17

I'm not sure where I'm going wrong in my max function. Z should not be so huge and in the second print statement x should equal y. Any help is greatly appreciated, thanks!

Why are you passing the addresses of x and y? You only need to pass an address if you need to modify the parameter, so only z needs to be an address. — Barmar
– Barmar, Commented Nov 18, 2015 at 22:08

Michi · Accepted Answer · 2015-11-18 22:25:00Z

As you probably already know, a pointer is a variable which contains the address in memory of another variable.

To access that memory we use The Unary operator & which gives us the address of a variable.

But accessing that memory is not all what a Pointer can do, we can with that pointer modify the value of that variable where the pointer points to. For that we need to supply * to that pointer like this *ptr.

Let's take a look at the following program:

#include<string.h>
#include<stdlib.h>
#include<stdio.h>

int main(void){

    int a = 5;
    int b = 10;

    printf("A = %d\nB = %d\n\n\n",a,b);

    int *pa = &a;
    int *pb = &b;

    *pa = 50;
    *pb = 100;

    printf("A = %d\nB = %d\n*PA = %d\n*PB = %d\n",a,b,*pa,*pb);

    return 0;
}

The output will be:

A = 5
B = 10

A = 50
B = 100
*PA = 50
*PB = 100

As you can see A and B got new values. I hope you understand that.

Keith Nicholas · Accepted Answer · 2015-11-18 22:05:48Z

2

when you pass things by pointer, if you want to get to the values, you need to do

(*x > *y)

Which gets the value pointed at by the pointer. (x and y are pointers, so they are going to contain memory addresses of the where the values are stored)

answered Nov 18, 2015 at 22:05

Keith Nicholas

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1 Comment

Johnathan Scott Over a year ago

WOW well that was way easier than I expected! Thank you!

kcraigie · Accepted Answer · 2015-11-18 22:06:50Z

2

Needs to be:

if (*x > *y) {
    *z = *x;
}
else if (*y > *x){
    *z = *y;
}

answered Nov 18, 2015 at 22:06

kcraigie

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Comments

mohkhan · Accepted Answer · 2015-11-18 22:07:22Z

2

You are comparing pointer addresses. You should de-reference the pointers for comparisons and assignments.

void max(int* x, int* y, int* z)
{

    if (*x > *y) {
        *z = *x;
    }
    else if (*y > *x){
        *z = *y;
    }
}

answered Nov 18, 2015 at 22:07

mohkhan

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