Unknown size of int array

Question

I have a struct which contains an array of ints. I do not want to fix the size of the array as I don't know how many elements will go into it and do not want to waste memory.

An array of int pointers would be the same essentially.

I tried a pointer to an array of integers and a double pointer to no avail.

How could I accomplish this?

NOTE:

What I mean by array of int pointers is that if I set a fixed size array of int pointers then I am still wasting space, so in that sense it makes no difference with a fixed size array of ints. The only difference is that an int and int pointer may be different in size.

It's pointless to have an array of int pointers instead of an array of ints. — Jabberwocky, Oct 15 '14 at 07:51

David Ranieri · Accepted Answer · 2014-10-15T05:52:47.353

4

An array of int pointers would be the same essentially.

No, you mean a pointer (int *):

int *arr = malloc(sizeof(*arr) * n);

If you don't know n beforehand:

int n = 0, *arr = NULL;

while (condition) {
    arr = realloc(arr, sizeof(*arr) * (n + 1));
    n++;
}

edited Oct 15 '14 at 05:52

answered Oct 15 '14 at 05:38

David Ranieri

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I don't know `n` beforehand though. – Mars Oct 15 '14 at 05:40
I want to grow the array like a `vector`. – Mars Oct 15 '14 at 05:40
@Zaphod: That's what dynamic memory is all about. If you want that sort of data structure than you can A) implement it, or B) find an existing implementation. Nothing in standard C will do this for you. – Ed S. Oct 15 '14 at 05:41
But you do will know 'n' (number of array elements) runtime right ?? – vvvv Oct 15 '14 at 05:41
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@Zaphod Are you asking for a linked list? – Peter R Oct 15 '14 at 05:44
@Zaphod: It is not possible to have an array and at the same time not to know the number of elements. It is self-contradictory. The number of elements can change at run-time, but an any given moment it must be a specific number. – AnT stands with Russia Oct 15 '14 at 05:45
@AndreyT. Sure, but that's not the point. The point is I do not want to fix the number initially. – Mars Oct 15 '14 at 05:49
2

@Zaphod: You don't have to *fix* it neither initially nor at any other moment. This means that at any run-time moment you can increase it or decrease it using `realloc`. That's the only way "not to fix" the array size in C. Yet at any moment (including "initially") the array will have to have a specific size. Which means that you will have to *choose* some initial size in any case. Later you will be able to change it if it proves insufficient (or excessive). – AnT stands with Russia Oct 15 '14 at 06:02
@AndreyT. I ended up doing slightly different but yes, `realloc` did the trick. – Mars Oct 15 '14 at 06:07

score 1 · Answer 2 · edited May 23 '17 at 11:43

1

An array of int pointers would be the same essentially

Not really. An array of pointers has absolutely nothing to do with it.

What you need is an int *a pointer, which you will use as a pointer to the first element of an array of integers. That array of integers you will allocate yourself at run-time thorough malloc with appropriate size. You will be able to grow your array, if the need arises, using realloc.

If your struct type has only one run-time-sized array in it, you have another option known as a "struct hack". There is plenty of information on "struct hack" that you can search for. Here's one example: C struct hack at work

edited May 23 '17 at 11:43

Community

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answered Oct 15 '14 at 05:38

AnT stands with Russia

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You're saying something like `int (*a)[100]`? – Mars Oct 15 '14 at 05:39
@Zaphod: No, he's saying `int *a` – Ed S. Oct 15 '14 at 05:40
No. This is a pointer to an array of *fixed* compile-time size `100`. You said that your array size is unknown at compile time. For that you need `int *a` specifically and a `malloc` of appropriate size. Later you will access it as `a[i]`. – AnT stands with Russia Oct 15 '14 at 05:40
@Zaphod: Exactly. That pointer will point to the element number `0` of your run-time allocated array. – AnT stands with Russia Oct 15 '14 at 05:42

score 0 · Answer 3 · answered Oct 15 '14 at 05:44

You can use this trick:

struct stuff
{
  int arr[1];
};

Then when you create one of these, you use malloc(sizeof(struct stuff) + n*sizeof(int)) where n is how many extra integers you want to store. Then you can access them as usual, e.g. mystuff.arr[100] so long as n was at least 100.

In C99 you can instead say int arr[]; and prior to C99 if you have GCC you can do int arr[0];. This of course makes it easier to create empty arrays if you want that.

Unknown size of int array

3 Answers3

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