CODEWITHSUNDEEP
c
Filip Biljic
Filip Biljic

Reputation: 11

Changing the value of a variable with pointers not working

Basically I have a function called MinSubTab that is supposed to calculate the sum of the array passed and also to change the value passed in the first argument from inside the function without using return. This is done with pointers. Anyway, I think it'd be easier if I just showed you the code so here it is:

maintab.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include "tab.h"

int main(){
    int *reftab;
    int min;
    reftab = (int *)malloc(sizeof(int) * NMAX);
    InitTab(reftab,NMAX);
    printf("\n Total: %d et min: %d", MinSumTab(&min, reftab, NMAX), min);
    free(reftab);
    return 0;
}

tab.c

void InitTab(int *tab, int size){
    srand(time(NULL));
    for (int i=0; i<size; i++){
        *(tab+i) = rand() % 10;
    }
}
int MinSumTab(int *min, int *tab, int size){
    int total=0;
    int minimum = NMAX;
    int temp = *min;
    for (int i=0; i<size; i++){
        total += *(tab+i);
    }
    for (int i=0; i<size; i++){
        if(*(tab+i)<minimum){
            minimum = *(tab+i);
        }
    }
    *min = minimum;
    return total;
}

So the expected result here is that the sum is printed (which it is) and the minimum value of the array is printed (which it is not). Every single time the min variable equals 8 and I've no idea how to actually change the value of min from within that function.

Please help as my brain has no more capacity for rational thought, it's been 1.5 hrs and no solution in sight. Thanks

Upvotes: 0

Views: 51

Answers (2)

0___________
0___________

Reputation: 67476

In C order of evaluation and argument passing is undefined.

You can of course the order yourself but it only to feed your curiosity.

#include <stdio.h>

volatile char *message[] = {
    "fisrt", "second", "third", "fourth"
};

int print(size_t x)
{
    printf("%s\n", message[x]);
    return x;
}

int main()
{
    printf("%d %d %d %d\n", print(0), print(1), print(2), print(3));

    return 0;
}

Note. There is one exception from this rule.

Logical operators are evaluated form the left to the right.

if( x != NULL && *x == 5)is safe because x will not be dereferenced if it is NULL

Upvotes: 0

Codo
Codo

Reputation: 78825

Looks like a small mistake:

You initialize minimum with NMAX, which I assume is 8 (the size of the array). 99.9% of the random numbers will be bigger. So 8 is chosen as the minimum.

What you really want is to initialize it with RAND_MAX – the maximum value rand() can return.

Upvotes: 1

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