/*
 * demonstrate the use of qsort by sorting an array
 * of random numbers
 *
 * Matt Bishop, ECS 36A, Fall 2019
 */
#include <stdio.h>
#include <stdlib.h>

/*
 * supposedly declared in stdlib.h, but not on this system
 * generate a random number between 0 and RAND_MAX inclusive
 * (RAND_MAX is defined in stdlib.h!)
 */
long int random(void);

/*
 * the list of random numbers
 */
int list[100];

/*
 * initialize the array with random numbers between 0 and 999
 * inclusive
 */
void fillup(void)
{
	int i;			/* counter in a for loop */

	/* put random numbers into the array */
	for(i = 0; i < 100; i++)
		list[i] = (int) random() % 1000;
}

/*
 * print the array of random numbers
 */
void prlist(void)
{
	int i;			/* counter in a for loop */

	/* print the array 10 numbers per line */
	for(i = 0; i < 100; i++){
		printf("%6d ", list[i]);
		if (i % 10 == 9) putchar('\n');
	}
}

/*
 * qsort comparison function: return <0, 0, >0
 * depending on whether the first integer pointed to
 * is less than, equal to, or greater than the second
 * integer pointed to
 * 
 * note this takes pointers to the things to be sorted
 * the pointers here are done correctly, so the
 * compiler does not complain
 */
int intcmp(const void *x, const void *y)
{
	return(*(const int *)x - *(const int *)y);
}

/*
 * the main routine to pull this together
 */
int main(void)
{
	/* initialize the array */
	fillup();

	/* print it so user can see contents are out of order */
	prlist();

	/* separator to make it easier to see the unsorted and sorted */
	printf("------------------\n");
	
	/* now sort the array */
	qsort(list, 100, sizeof(int), intcmp);

	/* print it out */
	prlist();

	/* bye! */
	return(0);
}