How to count the number of elements of an array

Question

I encountered a problem while testing a code.I define a macro for obtaining the number of elements of an array as follows:

#define ARRAY_SIZE(arr) sizeof(arr) / sizeof(arr[0])

This macro works fine for counting the number of elements of an array whose initializers match the storage capacity(e.g. int buf[] = {1,2,3};),but not very effective with arrays declared as : int buf[20] = {1,2,3};

Now i know counting array elements like these is pretty easy,but what about large number of elements? How do you count them? counting could be a killer,you know!

Consider the following code:

#include <stdio.h>
#include <string.h>

#define ARRAY_SIZE(arr) sizeof(arr) / sizeof(arr[0])

void g_strcat(void *_Dst, size_t dstSize, size_t bytes, const void *_Src, size_t srcSize);

int main(void)
{
    int dst[20] = { 1,2,3 };
    int src[] = { 4,5,6 };

    size_t dstSize = 3; // dstSize = ARRAY_SIZE(dst) doesn't work
    size_t srcSize = ARRAY_SIZE(src);

    g_strcat(dst, dstSize, sizeof(int), src, srcSize);

    size_t n, newSize = dstSize + srcSize;
    for (n = 0; n < newSize; n++) {
        printf("%d ", dst[n]);
    }
    putchar('\n');
    return 0;
}

void g_strcat(void *_Dst, size_t dstSize, size_t bytes, const void *_Src, size_t srcSize)
{
    memcpy((char *)_Dst + (dstSize * bytes), _Src, srcSize * bytes);
}

Answer 1

If you only partially initialize a list of primitive data types (ie: array of ints), the remaining elements get initialized to 0.

C99 Standard 6.7.8.21

If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

In your case, you're trying to determine the size of the initializer list. Can't really think of a valid reason to need to do this, but you could simply check when elements start being equal to zero consistently. Of course, this fails if you've deliberately set an element to zero.

The macro you wrote will work correctly (ie: return the number of elements in the array), but it will fail if you use it in a function that accepts a pointer-to-array as an argument, as pointer decay causes sizeof to act differently than one might expect.

All that being said, you can't determine the size of the initializer list in any meaningful sense, unless you do something like so, where you define the initializer list as a macro:

---

Code Listing

---

#include <stdio.h>
#define LIST    {1,2,3}

int main(void)
{
    int i[20] = LIST;
    int t[] = LIST;

    printf("elements in i: %d\n", sizeof(i)/sizeof(int));
    printf("elements in t: %d\n", sizeof(t)/sizeof(int));

    return 0;
}

---

Sample output

---

elements in i: 20
elements in t: 3

---

You can minimize wasted memory by putting the throw-away array into a new block scope, ie:

---

#include <stdio.h>
#define LIST    {1,2,3}

int main(void)
{
    int i[20] = LIST;
    int initListSize = 0;

    {
        int t[] = LIST;
        initListSize = sizeof(t) / sizeof(int);
    }

    printf("elements in t: %d\n", initListSize);

    return 0;
}

---

This will limit the storage lifetime of the temporary array to the narrow scope between the braces. Again, I can see this being useful maybe as an experiment, but can't see this ever finding its way into production code.

Answer 2

Your macro is working fine. The statement:

int dst[20] = { 1,2,3 };

Creates the following in stack memory:

|1|2|3|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|

The array size is still 20 even if it has only been initialized with the first three values

Regarding your question in the comments: How will i concatenate to the array? The array must be big enough to hold the other string:

If you are working with strings (and not number arrays) the string function int len = strlen(string); can be used to test the existing usage of a string variable before concatenating.

Unlike the sizeof macro, strlen is a function that looks for the first NULL character in an array of characters:

char string[20] = {"yes"};

Creates the following in memory:

|y|e|s|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|

strlen starts at the address of string and counts characters until it encounters the first NULL. (or 0):

int len strlen(string); //will yield a len of 3

you can then use strncat(string, newStr, count); to concatenate a specified numbers of characters, count, to a string of a known capacity, thus avoiding a buffer overflow.

Keep in mind that a string variable of size == 20 for example is limited to containing a string of length == 19. The 20th position must be reserved for the NULL character.

Answer 3

This will do it if we can assume that the last initialized element is not zero (because that's indistinguishable from being implicitly initialized to zero):

size_t trailing_zero_bytes(const void* data, size_t size) {
    for (; size > 0; size--) {
        if (((const char*)data)[size - 1] != 0) {
            break;
        }
    }
    return size;
}

#define ARRAY_SIZE(arr) \
    ((sizeof(arr) - trailing_zero_bytes(arr, sizeof(arr)) + sizeof(arr[0]) + 1) / sizeof(arr[0]))

If you want to count these two cases differently, you're completely out of luck (unless you parse the code using Clang or GCC-XML or whatever):

int s1[5] = { 4,5,6 }; // 2 zeros implied
int s2[5] = { 4,5,6,0 }; // 1 zero implied

Both of the above will give 3 with my approach, and there is nothing that can be done about it.

Answer 4

If you want to determine the number of initializers of an array,declare an array to hold the initializers elements and apply the macro ARRAY_SIZE() on it.Then use memcpy() to copy the initializers to the dst[20] array.

Now you have the number of elements without hassle.

#include <stdio.h>
#include <string.h>

#define ARRAY_SIZE(arr) sizeof(arr) / sizeof(arr[0])

void g_strcat(void *_Dst, size_t dstSize, size_t bytes, const void *_Src, size_t srcSize);

int main(void)
{
    int dst[20], src[] = { 4,5,6 };
    int initializer_list[] = { 1,2,3 };

    size_t init_size = ARRAY_SIZE(initializer_list);
    memcpy(dst, initializer_list, init_size * sizeof(int));

    size_t dstSize = init_size;
    size_t srcSize = ARRAY_SIZE(src);

    g_strcat(dst, dstSize, sizeof(int), src, srcSize);
    dstSize += srcSize;

    size_t n;
    for (n = 0; n < dstSize; n++) {
        printf("%d ", dst[n]);
    }
    putchar('\n');
    return 0;
}

void g_strcat(void *_Dst, size_t dstSize, size_t bytes, const void *_Src, size_t srcSize)
{
    memcpy((char *)_Dst + (dstSize * bytes), _Src, srcSize * bytes);
}