How can I use the rand() function to generate a different number that hasn't been generated before?

Question

// What I mean by this is shown by my example:

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

int i;
int a;


for (a = 0;a <10;a ++) {
    i = (rand()%10)+1; // generates a number from 1-10
    printf("%d\n", i);
}

// I would like for the loop to generate a number that gives a number that was not generated before. For example, an output such as:

1,3,6,2,8,9,4,10,5,7

instead of:

3,9,10,3,7,9,2,7,10,1

In other words, I would like no copies.

It seems that you are trying to *shuffle* the array (like shuffling a deck of cards). Google "shuffling algorithm". — Robert Lozyniak, Jul 21 '18 at 15:20
in general, just remember previous numbers in an array and compare, rejecting duplicates. But your special case seems to be you want every number in a specific range exactly once, and that's what @RobertLozyniak already addressed. — , Jul 21 '18 at 15:25
See (amongst other SO questions), [Shuffle array in C](https://stackoverflow.com/questions/6127503) and [Is this C implementation of the Fisher-Yates shuffle correct](https://stackoverflow.com/questions/3343797), and links from those pages, including [The Danger of Naivete](http://www.codinghorror.com/blog/2007/12/the-danger-of-naivete.html) — Jonathan Leffler, Jul 22 '18 at 00:12

score 2 · Answer 1 · answered Jul 21 '18 at 15:48

You obviously don't just want no copies, but you want every number in a given set exactly once. This is, as commented by Robert, similar to shuffling a deck of cards. You don't have "decks" in C, but you can model one as an array:

int deck[] = {1,1,1,1,1,1,1,1,1,1};

This should represent 10 different "cards" (identified by their index in the array), each available one time. Now, just write code that "draws" cards:

int i = 0;  // starting point for searching for the next card to draw
for (int n = 10; n > 0; --n)  // how many cards are left
{
    int skip = rand() % n;  // randomly skip 0 .. n cards
    while (1)
    {
        if (deck[i])             // card still available?
        {
            if (!skip) break;    // none more to skip -> done
            --skip;              // else one less to skip
        }

        if (++i > 9) i = 0;      // advance index, wrapping around to 0
    }
    deck[i] = 0;              // draw the card
    printf("%d\n", i+1);      // and print it out
}

of course, seed the PRNG (e.g. srand(time(0))) first, so you don't get the same sequence every time.

Weather Vane · Answer 2 · 2018-07-21T17:54:42.210

The idea shown in the question is to print numbers within a range, without repetition. Here is one way to do that, by putting each value into an array and swapping its elements around.

A variation could be that you don't want to use all the possible numbers, in that case just change PICKED.

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

#define ARRLEN 10
#define PICKED 10

int main(void) { 
    int array[ARRLEN];

    srand((unsigned)time(NULL));        // seed the PRNG

    for(int i = 0; i < ARRLEN; i++) {       // generate the numbers
        array[i] = i + 1;
    }

    for(int i = 0; i < ARRLEN; i++) {       // shuffle the array
        int index = rand() % ARRLEN;
        int temp = array[i];
        array[i] = array[index];        // by randomly swapping
        array[index] = temp;
    }

    for(int i = 0; i < PICKED; i++) {       // output the numbers
        printf("%d ",  array[i]);
    }
    printf("\n");
}

Program output:

9 8 4 5 1 10 7 3 6 2

The library's PRNG is not very random, but for many cases that is not important. If it is, better algorithms are available.

This code can be adapted to the scenario shown in [The Danger of Naiveté](http://www.codinghorror.com/blog/2007/12/the-danger-of-naivete.html) with ARRLEN and PICKED both set to 3. It can then be either run repeatedly or modified further to extract most of what's in `main()` into a function `shuffle_once()` that can be called many times from `main()` — leaving `srand()` in the revised `main()`, of course. When run with 60000 iterations, then run through `sort | uniq -c | sort -n`, you get skewed output such as: `8754 321` — `8869 312` — `8966 123` — `11005 231` — `11126 132` — `11280 213`. — Jonathan Leffler, Jul 23 '18 at 12:29

How can I use the rand() function to generate a different number that hasn't been generated before?

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