Simple example illustrating how to dynamically allocate array in C. Number of elements in array provided via command-line argument.

arraydyn.c

/*
Gets a command-line argument that provides the number of
elements in an array. Dynamically allocates the array.
Generates random numbers and print them.

Usage:
    
    args 10

will print out the 10 random numbers.
*/

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(int argc, char *argv[]){
    int n, i;
    int *ran;

	// reads command-line argument
    sscanf(argv[1], "%i", &n); 

    // Allocates array with n elements
    ran = (int *)malloc(sizeof(int)*n); 

    // initialize pseudo-random number generator
    srand(time(NULL)); 
    // fills array with random integers
    for (i = 0; i < n; i++) {
        ran[i] = rand() % 1000;
   	}

   	// prints array values
   	for (i = 0; i < n; i++) {
    	printf("array[%i] = %i \n", i, ran[i]);
   	}

    return(0);
}