How to use pointers with arrays?

Question

I have tried to solve this problem with pointers, but I couldn’t. The requirement was writing a function in which you see if the array is

sorted increase (return 1)
sorted decrease (return -1)
not sorted at all (return 0)

Here is what I wrote:

int *test(int l,int *T)
{
   int i,A[l],sign;
   int *t=&A[0];
   for (i=0; i<l; i++)
   {
       if(A[i]>A[i+1]){
           t++;
       sign =-1;
       }
       if (A[i]<A[i+1]){
           t++;
       sign =1;
       }
       if (A[i]!=A[i+1]){
            t++;
          sign =0;
       }
   }
   return sign;
}

The compiler is giving

returning ‘int’ from a function with return type ‘int *’ makes pointer from integer without a cast [-Wint-conversion]
   61 |     return sign;


error: ld returned 1 exit status

int test(int l, int *T) ... <= remove the star from the return type of the function — pmg
– pmg, Commented Dec 19, 2021 at 11:33
sign is int you want to return pointer to the int (see the star) — 0___________
– 0___________, Commented Dec 19, 2021 at 11:34
could you please tell me what i can do to let the micro pro compare all the elements of the array then return the value — lunix girl
– lunix girl, Commented Dec 19, 2021 at 20:00

user7372188 · Accepted Answer · 2021-12-24 16:00:43Z

A few things to notice,

The 'T' parameter wasn't used, so removed.
The 't' variable wasn't used, so removed.
The function return type shouldn't be a pointer to integer, it should be just an integer, from your requirements.
Your code is testing against an array declared in the function scope, and since it is an automatic variable, it is not initialized and may contain garbage values.
Your code is testing against an out of bounds index when using 'i < len' as the loop condition (ex.: considering that the array length is 3, when i == 2, comparing a[i] with a[i + 1] would access a[3], which is not within array boundaries that goes from index 0 to index 2.

With that in mind, a possible implementation with some tests is provided below, from what I can see from the requirements list, but bear in mind that I made some assumptions, since there was no restriction about them.

#include <assert.h>

#define SORTED_ASC 1
#define SORTED_DES -1
#define UNSORTED 0

int is_sorted(int *arr, int len)
{
    int sorted = 0;

    // I am assuming that this approach is reasonable, check your requirements.
    if (len <= 1)
        return UNSORTED;

    for (int i = 0; i < len - 1; i++)
    {       
        // Previous iteration detected order as 'descending', but current 
        // is 'ascending'.
        if (sorted == SORTED_DES && arr[i] < arr[i + 1])
            return UNSORTED;

        // Previous iteration detected order as 'ascending', but current 
        // is 'descending'.
        if (sorted == SORTED_ASC && arr[i] > arr[i + 1])
            return UNSORTED;

        // I am assuming that arrays with repeated values should remain classified 
        // as 'unsorted' until a different value appears, check your requirements.
        if (arr[i] > arr[i + 1])
            sorted = SORTED_DES;
        else if (arr[i] < arr[i + 1])
            sorted = SORTED_ASC;
   }

   return sorted;
} 

void test_unsorted()
{
    int arr[4][3] = {
        { 1, 3, 2 },
        { 2, 1, 3 },
        { 2, 3, 1 },
        { 3, 1, 2 }
    };

    for (int row = 0 ; row < 4 ; row++)
    {
        int res = is_sorted(arr[row], 3);
        assert(res == UNSORTED);
    }
}

void test_sorted_ascending()
{
    int arr[] = { 1, 2, 3 };
    int res = is_sorted(arr, 3);
    assert(res == SORTED_ASC);
}

void test_sorted_descending()
{
    int arr[] = { 3, 2, 1 };
    int res = is_sorted(arr, 3);
    assert(res == SORTED_DES);
}

void test_with_repeated_values()
{
    int sorted_asc[] = { 1, 1, 2 };
    int sorted_asc_res = is_sorted(sorted_asc, 3);
    assert(sorted_asc_res == SORTED_ASC);

    int sorted_des[] = { 3, 3, 2 };
    int sorted_des_res = is_sorted(sorted_des, 3);
    assert(sorted_des_res == SORTED_DES);

    int unsorted[] = { 1, 1, 1 };
    int unsorted_res = is_sorted(unsorted, 3);
    assert(unsorted_res == UNSORTED);
}

int main(void)
{   
    test_unsorted();
    test_sorted_ascending();
    test_sorted_descending();
    test_with_repeated_values();
}

And last, but not least, the error your compiler is giving is because you are trying to return an integer (int sign) when your return type is an integer pointer ;]

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