How can I solve this problem with pointers in C?

Question

In the code below, I'm trying to implement a function that finds the maximun and minimun of a given array. But I was trying to use a different approach. I was trying to change the memory adress of the pointers for the maximum and minimum.

Why this is not working? Should I use pointers to pointers for solving this problem?

#include <stdio.h>
#define M 5

void maxMin(int *v, int N, int *max, int *min){
    int i;
    printf("%d\n",*v);
    printf("%d\n",*max);
    for(i = 0; i < M; i++){
        if(*max < *(v+i)){
            max = (v+i);
        }
        if(*min > *(v+i)){
            min = (v+i);
        }
    }
}

int main(){
    int v[M] = {1, 3, 5, 7, 8}, *max=v, *min=v;
    maxMin(v, M, max, min);

    printf("MAX %d\n", *max);
    printf("MIN %d\n", *min);
    return 0;
}

`max` points at the array. When you pass `max` you pass a pointer. The function receives a _copy_ of that pointer. When you modify it inside the function, the caller won't see it. To modify the pointer and let the caller see it, pass a "double pointer", i.e., the _address_ of the pointer. — Paul Ogilvie, Nov 01 '18 at 13:22
This is unrelated to your problem, but there is some confusion between the argument N and the global macro M in your code. It causes no problems in your example, because they are identical, but maxMin should refer to "N" in its body rather than "M". — jwimberley, Nov 01 '18 at 13:22
Remember that C passes arguments *by value*, which means the argument variables inside a function are ***copies***. And modifying a copy will not modify the original. Please do some research about *emulating pass by reference in C*. — Some programmer dude, Nov 01 '18 at 13:22
Maybe a duplicate of https://stackoverflow.com/questions/766893/how-do-i-modify-a-pointer-that-has-been-passed-into-a-function-in-c — Jabberwocky, Nov 01 '18 at 13:37

score 3 · Answer 1 · answered Nov 01 '18 at 13:33

3

You should use double pointers because passing it like this will copy the value of the pointers.

void maxMin(int *v, int N, int **max, int **min){
    int i;
    int minVal = *v;
    int maxVal = *v;
    for(i = 0; i < M; i++){
        if(maxVal < *(v+i)){
            maxVal = *(v+i);
            *max = (v+i);
        }
        if(minVal > *(v+i)){
            minVal = *(v+i);
            *min = (v+i);
        }
    }
}

Then you call the function like this.

maxMin(v, M, &max, &min);

The rest stays the same.

answered Nov 01 '18 at 13:33

Petar Velev

2,305
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1

I think there's a typo. The parameter is `int N` but the for-loop refers to `M`. – Austin Mullins Nov 01 '18 at 13:48
1

@AustinMullins Its like this in the question. It is probably a typo but it is not mine :). Look at the macro M in the question. – Petar Velev Nov 01 '18 at 14:04

score 1 · Answer 2 · answered Nov 01 '18 at 14:15

The only reason why you need to use pointers here, is because you wish to return both min and max, but a function only has one return value. Therefore, you have to return them through the parameters.

#include <stdio.h>
#define M 5

void maxMin(size_t n, const int v[n], int *max, int *min)
{
  *max = v[0];
  *min = v[0];

  for(size_t i = 1; i < n; i++)
  {
    if(*max < v[i])
    {
      *max = v[i];
    }
    if(*min > v[i]){
      *min = v[i];
    }
  }
}

int main (void)
{
  int v[M] = {1, 3, 5, 7, 8};
  int max;
  int min;

  maxMin(M, v, &max, &min);

  printf("MAX %d\n", max);
  printf("MIN %d\n", min);
  return 0;
}

(Returning two pointers that point at the max and min elements would be a weird interface - in that case you would rather return the indices where max and min could be found.)

score 0 · Answer 3 · answered Nov 01 '18 at 17:25

You are passing the value(that is the starting address of array) stored in pointers max and min to the function maxMin() with maxMin(v, M, max, min) . What this means is that the pointer variables min and max in main() are related to min and max pointer variables in maxMin() argument list. So any change done to these variables in maxMin() will not be reflected in main().

What you need to do is pass the address of min and max variables and update the values of the *min and *max in the function maxMin(See Petar Velev answer).

How can I solve this problem with pointers in C?

3 Answers3