Random numbers that do not match each other

Question

I want to produce different numbers with C. We can generate a random number using the stdlib library and the srand function.

For example; I want to produce a random number between 0 and 5.

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

int main(void)
{
int i;
int n = 4;
int array[3];

srand(time(NULL));

for(i = 0; i < n; i++)
{
   array[i] = rand() % 5; 
   printf("%d\n", array[i]);
}
return 0;

But the same numbers may coincide here.Like this:

How can I prevent this?

If you want them to be unique fill an array with the values 0..5 then shuffle it, e.g. fisher-yates http://stackoverflow.com/questions/6127503/shuffle-array-in-c — Alex K., Mar 14 '17 at 15:22
Just because the same number appears twice in a row doesn't mean it's not random. — dbush, Mar 14 '17 at 15:23
Why do you think a number should not be repeated. If the number is a (pseudo) random number then you should expect *any* result, including the same as the last. The probability will be increasing when you reduce the number space. — harper, Mar 14 '17 at 15:23

score 0 · Answer 1 · answered Mar 14 '17 at 15:32

Maybe you can use something like this:

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

int main(void)
{
    int i;
    int n = 4;
    int array[4];

    // Fill an array with possible values
    int values[5] = {0, 1, 2, 3, 4};

    srand(time(NULL));

    for(i = 0; i < n; i++)
    {
       int t1 = rand() % (5-i);   // Generate next index while making the
                                  // possible value one lesser for each
                                  // loop

       array[i] = values[t1];     // Assign value
       printf("%d\n", array[i]);

       values[t1] = values[4-i];  // Get rid of the used value by
                                  // replacing it with an unused value
    }
    return 0;
}

score -1 · Answer 2 · answered Mar 14 '17 at 15:36

-1

Instead of random number you can generate random non-zero shift from the previous number:

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

int myrand() {
        static int prev = -1;
        if (prev < 0)
               prev = rand() % 5;
        prev = (prev + 1 + rand() % 4) % 5;
        return prev;
}

int main(void) {
        int i;
        for (i = 0; i < 20; i++)
                printf("%d\n", myrand());
}

answered Mar 14 '17 at 15:36

avysk

1,973
12
18

Interesting. BTW to help prevent overflow, could use `else prev = (prev + 1u + rand() % 4) % 5;` – chux - Reinstate Monica Mar 14 '17 at 16:12
@chux where is overflow? Prev is always between 0 and 4. – avysk Mar 14 '17 at 16:17
True, OF prevention idea is for the general case of rather than 5, some positive `int`. `else prev = (prev + 1u + rand() % (N-1)) % N;` – chux - Reinstate Monica Mar 14 '17 at 16:18
And how is that supposed to prevent the same value showing up more than once?? It does not change anything. – too honest for this site Mar 14 '17 at 16:26
@Olaf oh, you are right, I maybe misunderstood the question. This code prevents getting the same number twice in a row, that's all. – avysk Mar 14 '17 at 16:28
Not even that . – too honest for this site Mar 14 '17 at 16:30
@Olaf oh really? Please explain. – avysk Mar 14 '17 at 16:30
This is exactly the solution I'm looking for.It will be random numbers, but the same numbers will not repeat each other. @avysk – bburak Mar 14 '17 at 16:33
Sorry, it will, but it will also generate a bad distribution. There is only one correct way, but apparently OP is not really interested about the quality. – too honest for this site Mar 14 '17 at 16:34
@Olaf oh really? Please explain about bad distribution. – avysk Mar 14 '17 at 16:35
The modulo is the crux. And the fact that `rand` is a PRNG. – too honest for this site Mar 14 '17 at 16:36
@Olaf oh, so you are complaining here that we are not using the real randomness, from nuclear decay or something? Well. – avysk Mar 14 '17 at 16:38

Random numbers that do not match each other

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