Why do I always get the same sequence of random numbers with rand()?

Question

This is the first time I'm trying random numbers with C (I miss C#). Here is my code:

int i, j = 0; for(i = 0; i <= 10; i++) {     j = rand();     printf("j = %d\n", j); } 

with this code, I get the same sequence every time I run the code. But it generates different random sequences if I add srand(/*somevalue/*) before the for loop. Can anyone explain why?

Answer 1

You have to seed it. Seeding it with the time is a good idea:

srand()

#include <stdio.h> #include <stdlib.h> #include <time.h>  int main () {   srand ( time(NULL) );   printf ("Random Number: %d\n", rand() %100);   return 0; } 

You get the same sequence because rand() is automatically seeded with the a value of 1 if you do not call srand().

Edit

Due to comments

rand() will return a number between 0 and RAND_MAX (defined in the standard library). Using the modulo operator (%) gives the remainder of the division rand() / 100. This will force the random number to be within the range 0-99. For example, to get a random number in the range of 0-999 we would apply rand() % 1000.