Number of elements in an array in C, without sizeof

Question

I've looked through the site to find a way to get the number of elements of an array with data in it. All I can find is people using sizeof, but that isn't an option for me due to the teachers limitations on what we can access. I need a way of doing it in a loop, preferably the same loop where I'm reading data from.

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

#define SIZE 10

int readData();

int
main(void)
{
double data[SIZE];
int size;

readData(double data[], &size);

return 0;

}

int readData(double data[], int* size)
{
FILE* fp;
fp = fopen("data.txt", "r");
int i;

for (i = 0; i < SIZE; i++)
{
    if( fscanf(fp, "%lf", &data[i]) < 1)
    break;
    if(data[i] < 0)
        printf("Negative data at %d\n", i);
}
size = i;

printf("The array is: ");
for (i = 0; i < size; i++)
{
    printf("%f ", data[i]);
}
fclose(fp);

printf("\n\n");

return;
}

The array is reading from a file that can contain anywhere from 1-SIZE, so I need to be able to make the program adapt to anything in that range. — user3857017, Jul 20 '14 at 03:43
You need to check whether `fscanf` succeeded or failed. Then you need to `return` what `i` was up to at that point when it failed. (So your function returns `int` instead of `void`) — M.M, Jul 20 '14 at 03:51
You need to rephrase the question. The size of the array is not the same as the number of elements that you have stored in the array. — William Pursell, Jul 20 '14 at 04:37
@WilliamPursell good point, I always forget the two aren't interchangeable in this case. — user3857017, Jul 20 '14 at 04:42

Mohit Jain · Answer 1 · 2014-12-12T07:20:46.597

2

int arr[] = {1, 2, 3, 4, 5};
size_t size = ( (char *)(&arr + 1) - (char *) (&arr) ) / sizeof arr[0];
printf("%zu\n", size);

live code here

For reuse you can write a macro as below

#define ARR_SZ(ARR) ( ( (char *)(&ARR + 1) - (char *) (&ARR) ) / sizeof ARR[0] )

EDIT
If you are reading from a file at run time, you can keep a track of succesful read which will be the size of your array.

size_t size = 0;
while( fread(arr + size, sizeof arr[0], 1, fp) ) ++size;

size will contain the count of succesfully read items.

EDIT2
Here is a live code example of returning the size and below is the modified code.

int readData(double data[], int size)   /* Change return type */
{
    FILE* fp;
    fp = fopen("data.txt", "r");
    int i;

    for (i = 0; i < size; i++)
    {
        if( fscanf(fp, "%lf", &data[i]) < 1) break; /* Added */
        if(data[i] < 0)
            printf("Negative data at %d\n", i);
    }
    size = i; /* Added */

    printf("The array is: ");
    for (i = 0; i < size; i++)
    {
        printf("%f ", data[i]);
    }
    fclose(fp);
    printf("\n"); /* Added */

    return size; /* Added */
}

In above example you can increase or decrease the number of inputs.

edited Dec 12 '14 at 07:20

answered Jul 20 '14 at 03:45

Mohit Jain

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Edited code above for completeness as well, maybe it will make more sense in full – user3857017 Jul 20 '14 at 04:00
I adapted this code to mine as best I could, and for some reason when I try to test the value of size in main (via printf("The size of size is %d", size);) it says 1989992264. – user3857017 Jul 20 '14 at 04:13
I've managed to get it to something more logical, it's 49 now but I feel like that's just the ASCII code for the character s in int form. – user3857017 Jul 20 '14 at 04:30
thanks for all of your help, you were very helpful and very thorough! – user3857017 Jul 20 '14 at 05:04

score 2 · Answer 2 · edited May 23 '17 at 10:26

Using info from this question:

int size = (&data)[1] - data;

The [1] treats &data as an array. Arrays are indexed by starting with the location of index 0, adding the index number multiplied by the size of an element, and dereferencing the result.
Because the element in this case is the original array, (&data)[1] returns the address of the index after the end of the array. Subtracting the address of the array returns the number of elements.
(see link for slightly less awkwardly worded explanation).

Number of elements in an array in C, without sizeof

2 Answers2