struct and typedef initialization

Question

I am not sure why I am not getting a display on the console for the value of maxProd. I think I am declaring the arr_integer array variable wrong in main?

I ran a couple tests and the program looks like it does not even get to call solution().

Any help would be greatly appreciated. I am trying to get this to work and get rid of the following warnings;

Adjacent_Element_Product.c:31:40: note: (near initialization for 'array')
Adjacent_Element_Product.c:31:43: warning: excess elements in struct initializer

 #include <stdio.h>
    
    
    typedef struct arr_integer {
      int size;
      int *arr;
    } arr_integer;            
    
    arr_integer alloc_arr_integer(int len) {
      arr_integer a = {len, len > 0 ? malloc(sizeof(int) * len) : NULL};
      return a;
    }                                
    
    
    int solution(arr_integer inputArray) {       
    
    int maxProd = inputArray.arr[0] * inputArray.arr[1];
      for (int i = 1; i < inputArray.size - 1; i++)
      {
        int product = inputArray.arr[i] * inputArray.arr[i + 1];        //multiple next door neighbours.
        if (product > maxProd)
          maxProd = product;
      }
      
      return printf("maxProd: %d\n", maxProd);
    }
    
    
    int main()
    {
        arr_integer array = {3, 6, -2, -5, 7, 3};            
        solution(array);            
        return 0;
    }

Answer 1

The problem is that you don't allocate memory for the array and that it then tries to use the list of integers as an int*.

A possible remedy:

#define Size(x) (sizeof (x) / sizeof *(x))

int main() {
    int ints[] = {3, 6, -2, -5, 7, 3};
    arr_integer array = {.size = Size(ints), .arr = ints};
    solution(array);
}

Answer 2

Here are the relevant changes:

1. added <stdlib.h> for malloc
2. eliminated alloc_arr_integer() which was not used
3. maxProd is initialized to INT_MIN and tweaked initial loop condition to reduce duplication
4. to make the solution a little smaller passing the value inline to solution (@TedLyngmo probably gave you a better solution)

Not fixed:

5. arr_integer doesn't seem to add any value so maybe just pass size and array to solution(). You could create a macro to initialize it using the approach @TedLyngmo shared with you. It has to be a macro as int [] degrades to int * in a function call, and the latter (silently?) gives you the wrong result which makes it error prune.
6. Does negative size make sense? If not then use an unsigned type.
7. What is the solution if size < 2?
8. solution() returns the value from printf() which is strange. You might want to return maxProd and have main() print out the value. This makes your function testable

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>

typedef struct arr_integer {
    int size;
    int *arr;
} arr_integer;

int solution(arr_integer inputArray) {
    int maxProd = INT_MIN; // what is answer if size < 2?  error?
    for (int i = 0; i < inputArray.size - 1; i++) {
        int product = inputArray.arr[i] * inputArray.arr[i + 1];        //multiple next door neighbours.
        if (product > maxProd)
            maxProd = product;
    }
    return printf("maxProd: %d\n", maxProd);
}

int main() {
    solution((arr_integer) {
        6,
        (int []) { 3, 6, -2, -5, 7, 3 }
    });
    return 0;
}

and the output is:

maxProd: 21