Can you define the size of an array at runtime in C

Question

New to C, thanks a lot for help.

Is it possible to define an array in C without either specifying its size or initializing it.

For example, can I prompt a u

Answer 1

For something like this, you might want to look into data structures such as: Linked Lists (Ideal for this situation) Various Trees (Binary Trees, Heaps, etc) Stacks & Queues

But as for instantiating a variable sized array, this isn't really possible.

The closest to a dynamic array is by using malloc and it's associated commands (delete, realloc, etc).

But in this situation, using commands like malloc may result in the need to expand the array, an expensive operation where you initialize another array and then copy the old array into that. Lists, and other datatypes, are generally much better at resizing.

Answer 2

You can use malloc to allocate memory dynamically (i.e. the size is not known until runtime).

C is a low level language: you have to manually free up the memory after it's used; if you don't, your program will suffer from memory leaks.

UPDATE

Just read your comment on another answer.

You're asking for an array with a dynamically-changing-size.

Well, C has no language/syntactic facilities to do that; you either have to implement this yourself or use a library that has already implemented it.

See this question: Is there an auto-resizing array/dynamic array implementation for C that comes with glibc?

Answer 3

If you're a beginner, maybe you don't want to deal with malloc and free yet. So if you're using GCC, you can allocate variable size arrays on the stack, just specifying the size as an expression.

For example:

#include <stdio.h>
void dyn_array(const unsigned int n) {
        int array[n];
        int i;

        for(i=0; i<n;i++) {
                array[i]=i*i;
        }
        for(i=0; i<n;i++) {
                printf("%d\n",array[i]);
        }
}
int main(int argc, char **argv) {
        dyn_array(argc);
        return 0;
}

But keep in mind that this is a non standard extension, so you shouldn't count on it if portability matters.

Answer 4

If you're looking for array facilities and don't want to roll your own, try the following:

1. Glib
2. Apache APR
3. NSPR