Question

So I’m trying to learn C right now, and I have two functions: one that shifts elements in an array:

void shift(int a[], int n) {
   int i;
   for(i = 0; i != n-1; i++){
      a[i] = a[i+1];
   }
}

and a version of the same function, except pointer-based:

void pointer_shift(int *a[], int n) {
   int i;
   for (i = 0; i != n - 1; i++) {
      *a[i] = *a[i + 1];
   }
}

I don’t know whether the pointer-based version is correct or not, but I guess my most important question is how I’m supposed to actually test that both/either work. Besides the definition of these two functions, I have:

#include <stdio.h>

void shift(int a[], int n);
void pointer_shift(int *a[], int n);

int main(void) {
    printf("Begin execution of testing Problem 1\n");
    int a1[] = {100, 101, 102};
    int i;
    for(i = 0; i<3;i++)
        printf("Before Shift: " "%d\n", a1[i]);
    //shift(a1, 3);
    pointer_shift(&a1, 3);
    for(i = 0; i<3;i++)
        printf("After Shift In Main: " "%d\n", a1[i]);
    return 0;
}

shift(a1, 3)

works fine, but I, for the life of me, can’t figure out how to correctly test pointer_shift.

I get two errors; one is that in the line

pointer_shift(&a1, 3)

I am passing argument 1 from an incompatible pointer type. The other error is indecipherable, but I was hoping the problem would be obvious enough that someone would be able to help me. So… how to test my two functions in my main?

Thanks!

Answer 1

Change your pointer based function like this:

void pointer_shift(int *a, int n) {
   int i;
   for (i = 0; i != n - 1; i++) {
      *(a+i) = *(a+i+1);
   }
}

What you are receiving is an array of pointers whereas you pass an array from main. Since an array decays into pointer, calling shift(a1, 3) is sufficient here.