What are pointers in C

Question

I have this task:

Make corrections in the maxmin.c program so it compiles itself and works correctly (finds max and min out of 3 integer numbers and displays them on the screen). Do not change the structure of the program. In case of incorrect input, you must output n/a.

I've been experimenting with this code below for a couple of hours, but I still can't figure out what exactly is wrong here

#include <stdio.h>

void maxmin(int prob1, int prob2, int prob3, int *max, int min);

/* Find a max & min probabilities */
int main()
{
    int x, y, z;
    scanf("%d %d %d", x, y, z);

    int max, min;

    maxmin(x, y, z, max, min);

    printf("%d %d", max, min);

    return 0;
}


/* This function should be kept !!! (Your AI) */
/* But errors & bugs should be fixed         */
void maxmin(int prob1, int prob2, int prob3, int *max, int min)
{
    *max = min = prob1;
    
    if(prob2 > max)
        max = prob2;
    if(prob2 < min)
        min = prob2;
    
    if(prob3 > max)
        max = prob3;
    if(prob3 < min)
        min = prob3;    
}

A "pointer" is very much what it sounds like: It's something that *points* to something else. To visualize how a pointer works, take a pencil and a paper, and draw two boxes on the paper. Now label one box "ptr" and the other "var" (representing a pointer and another variable). Then draw an arrow from the "ptr" box to the "var" box. That's very much how a pointer works. — Some programmer dude, Aug 08 '22 at 09:53
Here are the compiler warnings: https://godbolt.org/z/83M63YYsP For learning purpose I suggest you try to fix them on your own. — mch, Aug 08 '22 at 09:54
Did you get that task before taking the corresponding lessons about pointers? Did you try to compile that code? The compiler should already tell you about some problems — Gerhardh, Aug 08 '22 at 09:54
If you don't know about dynamic memory allocation, don't worry about that constraint. You won't use it by accident. — Gerhardh, Aug 08 '22 at 09:55
bowtie, Save time and enable all warnings, like from `int x, y, z; scanf("%d %d", x, y, z);` — chux - Reinstate Monica, Aug 08 '22 at 09:55
Is that some exam question that you have to solve without acutally using a compiler? — Gerhardh, Aug 08 '22 at 09:56
You need to read a C tutorial that covers arrays and pointers. It makes no sense to try to debug pointer code without knowing what a pointer is. So this question should really be "Can someone recommend a good C tutorial that explains pointers?" And the answer is "No, Stack Overflow does not accept questions asking for recommendations for books". But, that having been said, you should be able to find one yourself pretty easily. — Tom Karzes, Aug 08 '22 at 10:04
I can't help but vote keep closed. There's nothing to be gleaned from the code. Bowtie, simply asking to the tune of “I cannot understand how pointers work” would be better. As is, the code is not helpful, it is, unfortunately, _not even wrong._ There's absolutely nothing bad about not knowing anything, but some basics are better learned not by asking but thru more systematic learning. The adage “to ask a question, one must know half an answer“ is true, IMO. Also, there is nearly a duplicate, with an excellent answer, in line with your stab at the question: https://stackoverflow.com/q/4955198/ — kkm inactive - support strike, Sep 15 '22 at 00:40
A piece of advice to you as a very welcome newcomer. One, don't use emoticons, as there is nothing emotional about any question topic. Two, do not post text output as graphics; modify code to output text (unless the Q is about graphics, naturally), because graphics snips of text are not searchable. And thank you for joining! Don't take the closing of a question as something negative. The _core tenet_ of SO is to ask and answer in a form _useful to others in future,_ thus not simply getting one's questions answered, but also building a large base of knowledge in the process. — kkm inactive - support strike, Sep 15 '22 at 00:52

Vlad from Moscow · Accepted Answer · 2022-08-08T10:34:40.160

By default arguments are passed to functions by values. It means that functions dial with copies of values of argument expressions.

Changing a copy of the value of an object within a function does not change the value of the original object.

To change the original object in a function you need to pass it to the function by reference.

In C passing by reference means passing an object indirectly through a pointer to it. So dereferencing the pointer within the function you get a direct access to the original object and can change its value.

In the function maxmin you want to change the objects min and max declared in main

int max, min;

So the function needs to accept these objects by reference that is through pointers to them.

It means that the function must be declared like

void maxmin(int prob1, int prob2, int prob3, int *max, int *min);

and called in main like

maxmin(x, y, z, &max, &min);

As it was already mentioned to access the original objects min and max you need to dereference the pointers.

Thus the function definition will look like

/* This function should be kept !!! (Your AI) */
/* But errors & bugs should be fixed         */
void maxmin( int prob1, int prob2, int prob3, int *max, int *min )
{
    *max = *min = prob1;
    
    if ( prob2 > *max )
        *max = prob2;
    else if ( prob2 < *min )
        *min = prob2;
    
    if ( prob3 > *max )
        *max = prob3;
    else if ( prob3 < *min )
        *min = prob3;    
}

The function scanf also needs to accept its arguments by reference. So in main you have to write

scanf("%d %d %d", &x, &y, &z);

and in the format string to specify three conversion specifiers.

Also bear in mind that according to your assignment you need

In case of incorrect input, you must output n/a.

It means that you need to check whether the call of scanf was successful that is whether all three values were inputted correctly.

It can be done the following way

int x, y, z;

if ( scanf( "%d %d %d", &x, &y, &z ) == 3 )
{
    int max, min;

    maxmin( x, y, z, &max, &min );
    //...
}
else
{
    puts( "n/a" );
}

What are pointers in C

1 Answers1