Iterate through a C array

Question

I have an array of structs that I created somewhere in my program.

Later, I want to iterate through that, but I don\'t have the size of the array.

How can I

Answer 1

I think you should store the size somewhere.

The null-terminated-string kind of model for determining array length is a bad idea. For instance, getting the size of the array will be O(N) when it could very easily have been O(1) otherwise.

Having that said, a good solution might be glib's Arrays, they have the added advantage of expanding automatically if you need to add more items.

P.S. to be completely honest, I haven't used much of glib, but I think it's a (very) reputable library.

Answer 2

You can store the size somewhere, or you can have a struct with a special value set that you use as a sentinel, the same way that '\0' indicates the end of a string.

Answer 3

If the size of the array is known at compile time, you can use the structure size to determine the number of elements.

struct foo fooarr[10];

for(i = 0; i < sizeof(fooarr) / sizeof(struct foo); i++)
{
  do_something(fooarr[i].data);
}

If it is not known at compile time, you will need to store a size somewhere or create a special terminator value at the end of the array.

Answer 4

It depends. If it's a dynamically allocated array, that is, you created it calling malloc, then as others suggest you must either save the size of the array/number of elements somewhere or have a sentinel (a struct with a special value, that will be the last one).

If it's a static array, you can sizeof it's size/the size of one element. For example:

int array[10], array_size;
...
array_size = sizeof(array)/sizeof(int);

Note that, unless it's global, this only works in the scope where you initialized the array, because if you past it to another function it gets decayed to a pointer.

Hope it helps.