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         


        

Answer 1

Rand does not get you a random number. It gives you the next number in a sequence generated by a pseudorandom number generator. To get a different sequence every time you start your program, you have to seed the algorithm by calling srand.

A (very bad) way to do it is by passing it the current time:

#include 
#include 
#include 

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

Why this is a bad way? Because a pseudorandom number generator is as good as its seed, and the seed must be unpredictable. That is why you may need a better source of entropy, like reading from /dev/urandom.