Size of Operator and trying it to print array

August 3, 2024

Consider this C program

#include <stdio.h>
#define TOTAL_ELEMENTS (sizeof(arr))/sizeof(arr[0])
int arr[] = {23, 34, 12, 17, 204, 99, 16};
int main() {
 int d;
 printf("%zu\n", TOTAL_ELEMENTS);
 for (d = -1; d <= (TOTAL_ELEMENTS - 2); d++) {
  printf("%d ", arr[d + 1]);
 }
 return 0;
}

compilers are giving output as 7
according to my dry run and reasoning output should be
7
23 34 12 17 204 99 16

Now in order to see what is going wrong I applied some Variation
consider this code

#include <stdio.h>
#define TOTAL_ELEMENTS (sizeof(arr))/sizeof(arr[0])
int arr[] = {23, 34, 12, 17, 204, 99, 16};
int main() {
 int d;
 printf("%zu\n", TOTAL_ELEMENTS);
 for (d = -1; d <=(int) ((TOTAL_ELEMENTS) - 2); d++) {
  printf("%d ", arr[d + 1]);
 }
 return 0;
}

Notice that the only change I done is introducing type casting with int, and now it’s giving expected results.

Now as much as I know when we compare in C program it can do conversion to bigger data-type right, so suppose TOTAL_ELEMENT even has bigger type like long then d should be converted to long and program should run fine but that is not happening. What points I am missing here?

>Solution :

In the comparison d <= (TOTAL_ELEMENTS - 2) the rank of d is lower than the rank of (TOTAL_ELEMENTS - 2) as int has lower precision than size_t on your platform per 6.3.1.1:

The rank of a signed integer type shall be greater than the rank of any signed integer type with less precision.

int d = -1 is cast to size_t which yield a very large number (SIZE_MAX from stdint.h) so the loop condition is false and the body of the loop is never executed.

Prefer the standard for-loop idiom. Then you can use size_t d which type matches the type of your upper bound. Avoid global variables. Remove unused variables. Minimize scope of variables; in this case by declaring d in the for-loop. Use the correct format string %zu for size_t. I also left out the optional {} and return 0.

#include <stdio.h>

#define TOTAL_ELEMENTS (sizeof (arr) / sizeof *(arr))

int main() {
    int arr[] = {23, 34, 12, 17, 204, 99, 16};
    printf("%zu\n", TOTAL_ELEMENTS);
    for (size_t d = 0; d < TOTAL_ELEMENTS; d++)
        printf("%d ", arr[d]);
}

and you get the expected output: