srand in C - with just one repetition

Question

I'm trying to build a memory game and I want to ask how I can generate a randomic number with just one repetition. Like 1-1, 2-2, 3-3. I will paste here my function that I created and tell me if I have to create another function just to create a condition to create just a pair from numbers.

// function to fulfill the table
void preencher_mesa(int matriz[4][4], int dificuldade)
{
    int i, j;
    int lim_col, lim_linha; // limits of the matriz

    for(i=0; i<4; i++)
        for(j=0; j<4; j++)
            matriz[i][j] = 0;

    if(dificuldade == 1)
    {
        lim_col = 3;
        lim_linha = 2;
    }
    else if(dificuldade == 2)
    {
        lim_col = 4;
        lim_linha = 2;
    }
    else if(dificuldade == 3)
    {
        lim_col = 4;
        lim_linha = 4;
    }

    srand(time(NULL));
    for(i=0;i<lim_linha;i++)
    {
        for(j=0; j<lim_col;j++)
        {
            if(dificuldade == 1) // difficulty == 1
            {
                matriz[i][j] = (rand()%3)+1;
            }
            else if(dificuldade == 2) // difficulty == 2
            {
                matriz[i][j] = (rand()%6)+1;
            }
            else if (dificuldade == 3) // difficulty == 3
            {
                matriz[i][j] = (rand()%8)+1;
            }
        }
    }

    mostrar_mesa(matriz); //showtable
}

Answer 1

If you have a 3x2 matrix that should be filled with the digits/numbers 1, 1, 2, 2, 3, 3 in some random permutation, then you could do something like:

1. Allocate an array (vector) of the right size — 6 for the current example.
2. Populate the array with the correct values — 1, 1, 2, 2, 3, 3 for the current example.
3. Use an appropriate technique to shuffle the array, and then copy the shuffled data into the target 2D array.
4. Or select a digit at random from the initial 6 options, then (if necessary) move the last digit into the hole and select the next digit from the remaining 5 options, etc.

You can use the Fisher-Yates shuffling algorithm. You might check in your copy of Knuth The Art of Computer Programming, Volume 2: Seminumerical Algorithms. Or you can look for expositions on Stack Overflow (such as Algorithm to select a single random combination of values, chosen because one of my Google searches also came across it).

---

Judging from the comments, you want duplicates from your rand() surrogate, so this should work:

int duprand(void)
{
    static int mode = 0;
    static int value = 0;
    if (mode == 0)
    {
        mode = 1;
        value = rand();
    }
    else
    {
        mode = 0;
    }
    return value;
 }

Or, more succinctly:

int duprand(void)
{
    static int mode = 0;
    static int value = 0;
    if (mode == 0)
        value = rand();
    mode = !mode;
    return value;
 }

Simply call duprand() each time you want a random number. You will get the same value twice in a row. This code doesn't provide a resynchronization method; if you want one, you can write one easily enough:

void sync_duprand(void)
{
    int i = duprand();
    int j = duprand();
    if (i != j)
       i = duprand();
}

---

What I really wanted was ...

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

extern void shuffle(int *array, int n);
/*
** rand_int() and shuffle() copied verbatim (but reformatted) from
** https://stackoverflow.com/a/3348142 - an answer by Roland Illig
** (https://stackoverflow.com/users/225757/roland-illig).
*/

static int rand_int(int n)
{
    int limit = RAND_MAX - RAND_MAX % n;
    int rnd;

    do
    {
        rnd = rand();
    } while (rnd >= limit);
    return rnd % n;
}

void shuffle(int *array, int n)
{
    int i, j, tmp;

    for (i = n - 1; i > 0; i--)
    {
        j = rand_int(i + 1);
        tmp = array[j];
        array[j] = array[i];
        array[i] = tmp;
    }
}

/* New code - but hardly novel code */
static void dump_matriz(int matriz[4][4])
{
    for (int i = 0; i < 4; i++)
    {
        for (int j = 0; j < 4; j++)
            printf("  %d", matriz[i][j]);
        putchar('\n');
    }
}

int main(void)
{
    int matriz[4][4];

    int *base = &matriz[0][0];
    for (int i = 0; i < 8; i++)
    {
        *base++ = i + 1;
        *base++ = i + 1;
    }

    printf("Before:\n");
    dump_matriz(matriz);

    shuffle(&matriz[0][0], 16);

    printf("After:\n");
    dump_matriz(matriz);

    return 0;
}

Sample output:

Before:
  1  1  2  2
  3  3  4  4
  5  5  6  6
  7  7  8  8
After:
  1  7  8  6
  6  2  5  8
  2  4  7  3
  3  5  1  4

Note that because there's no call to srand(), the permutation is fixed. (You might get a different result from what I show, but running this test multiple times will produce the same result each time on your machine.) Add a call to srand() with an appropriate initialization, and you get different sequences. Hack and chop to suit your requirements for smaller matrices.