Generating unequal random numbers in C

Question

I have written a program that generates 4 random digits, that should appear on the screen like this: from 0000 to 9999 (not in ascending order necessarily, of course!).

The problem is that I have encountered numbers that are equal with each other. How can I fix it? I just want to produce 10.000 numbers, in the range 0000 to 9999, but not in any order: just "random".

Here is what I have written so far:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <assert.h>
#define SIZE 10000

int main(){

    srand(time(NULL));

    int a[SIZE], b[SIZE], c[SIZE], d[SIZE], i;
    FILE *fd = fopen("combinations_rand.txt", "w");
    assert(fd);

    for(i=0; i<SIZE; i++){      
        a[i] = rand()%10; //array of first digits
        b[i] = rand()%10; //array of second digits, etc...
        c[i] = rand()%10;
        d[i] = rand()%10;
        fprintf(fd, "%d%d%d%d\n", a[i], b[i], c[i], d[i]);
     }
     fclose(fd);
}

You have the answer, you say not in any order ... it's just about reordering. — Iharob Al Asimi, Nov 18 '15 at 23:30
If you don't want duplicates then you don't want a *random number*, you want a well defined *set* of numbers in *random order*. — J..., Nov 18 '15 at 23:32
When I ran the program, I had the number 0844 two times. And I don't think it's only this number that appears twice. Let's say that I have produced numbers in this way: 0000, 0001, ..., 9999, not randomly. How can I reorder them in order to seem random? (question to your comment.) — George, Nov 18 '15 at 23:32

score 6 · Answer 1 · edited May 23 '17 at 11:59

6

Take an array of 10000 length and store the elements from 0 to 9999 in those. Then shuffle the array Shuffle array in C.

Then print the answer with leading zeros when needed Printing leading 0's in C?

edited May 23 '17 at 11:59

Community

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answered Nov 18 '15 at 23:33

Bug Hunter 219

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This is an excelente answer. I had mine ready so I posted it anyway. – Iharob Al Asimi Nov 18 '15 at 23:39

Iharob Al Asimi · Answer 2 · 2015-11-18T23:40:22.587

What you should do is generate an array with the numbers from 0 to 9999 and then mix it up, like this

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

#define SIZE 10000

int main()
{
    int numbers[SIZE];

    srand(time(NULL));
    /* Fill the array */
    for (int i = 0 ; i < SIZE ; ++i)
        numbers[i] = i;
    /* Mix it */
    for (int i = 0, j = rand() % SIZE ; i < SIZE ; ++i, j = rand() % SIZE)
    {
        int swap;
        swap = numbers[i];
        numbers[i] = numbers[j];
        numbers[j] = swap;
    }
    /* Display it */
    for (int i = 0 ; i < SIZE ; ++i)
        fprintf(stdout, "%04d\n", numbers[i]);
    return 0;
}

except I believe this introduces bias: see Fisher-Yates shuffle. — Mitch Wheat, Nov 19 '15 at 00:42

Generating unequal random numbers in C

2 Answers2