<span style="font-size:18px;">/*	Name:		find_nbr.c
 *	Purpose:	This program picks a random number and then
 *				lets the user try to guess it
 *	Returns:	Nothing
 */


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

#define NO 0
#define YES 1

int main(void)
{
	int guess_value = -1;
	int number;
	int nbr_of_guesses;
	int done = NO;

	printf("\n\nGetting	a Random number\n");

	/* use the time to seed the random number generator*/

	srand(	(unsigned) time(	NULL ) );
	number = rand();

	nbr_of_guesses = 0;
	while ( done == NO)
	{
		printf("\nPick a number between 0 and %d> ",RAND_MAX);
		scanf(	"%d",	&guess_value); /*Get a number*/

		nbr_of_guesses++;

		if ( number == guess_value)
		{
			done = YES;
		}
		else
		if ( number < guess_value)
		{
			printf("\nYou guessed high !");
		}
		else
		{
			printf("\nYou guessed low!");
		}
	}

	printf("\n\nCongratulations ! You guessed right in %d Guesses! ",
			nbr_of_guesses);
	printf("\n\nThe number was %d\n\n",number);

	return 0;
}</span>
 

srand()就是給rand()提供種子seed

如果srand每次輸入的數值是一樣的，那麼每次執行產生的隨機數也是一樣的，

也就是說，以一個固定的數值作為種子是一個缺點。 通常的做法是  以這樣一句程式碼srand((unsigned) time(NULL));來取代，這樣將使得種子為一個不固定的數， 這樣產生的隨機數就不會每次執行都一樣了。

/* RAND.C: This program seeds the random-number generator
 * with the time, then displays 10 random integers.
 */

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

void main( void )
{
   int i;

   /* Seed the random-number generator with current time so that
    * the numbers will be different every time we run.
    */
   srand( (unsigned)time( NULL ) );

   /* Display 10 numbers. */
   for( i = 0;   i < 10;i++ )
      printf( "  %6d\n", rand() );
}