Question

I want to make an array whose size is to be determined during run time i.e. user input.

I tried to do it like this:

printf("enter the size of array \n");

scanf("%d",&n);

int a[n];

But this resulted in an error.

How do I set the size of an array like this?

Answer 1

Unless you are using C99 (or newer) you need to allocate memory manually, e.g. using calloc().

int *a = calloc(n, sizeof(int)); // allocate memory for n ints
// here you can use a[i] for any 0 <= i < n
free(a); // release the memory

If you do have a C99-compliant compiler, e.g. GCC with --std=c99, your code works fine:

> cat dynarray.c
#include <stdio.h>
int main() {
        printf("enter the size of array \n");
        int n, i;
        scanf("%d",&n);
        int a[n];
        for(i = 0; i < n; i++) a[i] = 1337;
        for(i = 0; i < n; i++) printf("%d ", a[i]);
}
> gcc --std=c99 -o dynarray dynarray.c
> ./dynarray
enter the size of array
2
1337 1337