CODEWITHSUNDEEP
crandomnumberssrand
litegait
litegait

Reputation: 11

Even with srand (time(NULL)) out of the loop, numbers are the same for a period of time

I have a simple code in C and there is a random number generation code in the middle.. problem is that for a long period of time that number stays the same and then goes one up! rand_pick might be 5 for over 30 times you run the C program and then will change finally to 6,... I wanted to know why and if there is another way to produce a really random 1 to 9 digit.

int rand_pick;
srand (time(NULL));
rand_pick = 1 + (int) (9.0*rand()/(RAND_MAX+1.0));

Thanks a lot ;)

Upvotes: 0

Views: 1393

Answers (2)

izidor
izidor

Reputation: 4144

No, numbers should be quasi random. You can run this snippet of code:

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

#define RANDOM_COUNT 10000

int main(void) {
    int rand_count[10] = {};
    int rand_pick;
    int i;

    srand (time(NULL));

    // run number of experiments
    for(i = 0; i < RANDOM_COUNT; i++) {
        rand_pick = 1 + (int) (9.0*rand()/(RAND_MAX+1.0));
        rand_count[rand_pick]++;
    }

    for(i = 1; i <= 9; i++) {
        printf("%d => %.3f %%\n", i, rand_count[rand_pick] /((float) RANDOM_COUNT));
    }   
}

It will randomly pick 10,000 numbers and count their distribution. I always get uniform distribution:

1 => 0.111 %
2 => 0.111 %
3 => 0.111 %
4 => 0.111 %
5 => 0.111 %
6 => 0.111 %
7 => 0.111 %
8 => 0.111 %
9 => 0.111 %

Upvotes: 2

CrazyCasta
CrazyCasta

Reputation: 28362

You only want to initialize srand once in your program. If you are running the program several times a second for some reason then you need to get a random number from somewhere else (like /dev/random on linux) to seed your random number generator.

Upvotes: 1

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