Getting number of elements in array not working as expected

Question

Here I am implementing a simple sort (insertion), but I'm not getting the correct size of the array. Namely, my length variable is getting set equal to 1. Also, what ways could I optimize my code for this insertion sort algorithm? Thanks

void insertionSort(int a[])
{
    int sorted = 1;
    int length = sizeof(a) / sizeof(int);
    int i = 0;
    int tmp;

    printf("%d   ", length);

    if (length < 2)
        return;

    for (i = 1; i < length - 1; i++)
    {
        sorted = i;
        while (a[sorted] < a[sorted - 1] && sorted > 0)
        {
            tmp = a[sorted];
            a[sorted] = a[sorted - 1];
            a[sorted - 1] = tmp;
            sorted--;
        }
    }
}

Do you have an aversion of passing the array length as an argument? — Fiddling Bits, Dec 15 '13 at 04:00

score 0 · Answer 1 · answered Dec 15 '13 at 03:56

0

int length = sizeof(a) / sizeof(int);

This returns the size of a pointer a, divided by the size of an int. Always equal to one.

You will have to pass length to the function, void insertionSort(int a[], size_t length){

The function that creates the array has to keep track of the number of elements that are active in the array in order to do this.

answered Dec 15 '13 at 03:56

jim mcnamara

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Not necessarily 1: if this is a 64-bit compilation on Unix, the size of the pointer will usually be 8 and the size of the integer will be 4 so the answer will be 2. It won't often be the size of the array, though. – Jonathan Leffler Dec 15 '13 at 06:56

score 0 · Answer 2 · answered Dec 15 '13 at 09:14

C arrays doesn't really exists. This essentially means that you don't have good abstract support for arrays in C. As an example of this, if I give you (you are the function insertionSort) an array (say a) you won't be able to determine it size! Arrays have no size in C.

So what is sizeof(a)? First a is not an array but a constant pointer to the memory address of the first element; that's all. Then sizeof(a) just gives you the size of the pointer a.

If you want to manage arrays in C, you must use a variable to store its length. This is why most of functions that deals with arrays have additional parameters length.

Getting number of elements in array not working as expected

2 Answers2