在 C 的 rand() 中，我每次都得到相同的值

Question

In this program, I need to take 2 random numbers and one of them must be equal to 3. To make them, the randomSet function works with a while loop, but the problem is in the main function: the loop gives me same 2 numbers every time. How do I get different numbers on each iteration?

int randomSet(int temp[])
{
  while (temp[0] != 3 && temp[1] != 3)
         {
            temp[0] =rand()%4;
            temp[1] =rand()%4;
         }
}

the main function is

int main()
{
    srand(time(0));
    int temp[2];
    int x =0,y =0;
    for (int i = 0; i < 5; i++) {
        randomSet(temp);
    }
    return 0;
}

Answer 1

You are not initializing temp before passing it to randomSet() , so its contents are indeterminate , causing while (temp[0] != 3 && temp[1] != 3) to have undefined behavior .

I suggest you change randomSet() to use a do..while loop instead so that you populate temp 1 time before you then start analyzing its content, eg:

int randomSet(int temp[])
{
    do
    {
        temp[0] = rand() % 4;
        temp[1] = rand() % 4;
    }
    while (temp[0] != 3 && temp[1] != 3);
}

Answer 2

You can easily instrument the code and see what goes on. As already pointed out, the key necessary change is that you must determine two random values on entry to the function before checking whether either (or both) of them is 3. That is easily achieved with a do { … } while (…); loop.

Despite suggestions to the contrary, the loop condition in the question is OK, though it needs to be converted from a while loop. The loop continues while neither element is 3; as soon as one or the other is 3 (or both are 3, but it only checks the first element, temp[0] ), the loop stops.

For example (file rand19.c ):

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

static void randomSet(int temp[])
{
    do
    {
        temp[0] = rand() % 4;
        temp[1] = rand() % 4;
        printf("--  %d  %d\n", temp[0], temp[1]);
    } while (temp[0] != 3 && temp[1] != 3);
}

int main(void)
{
    srand(time(0));
    int temp[2];
    for (int i = 0; i < 5; i++)
    {
        randomSet(temp);
        printf("%d:  %d  %d\n", i, temp[0], temp[1]);
    }
    return 0;
}

Example runs:

$ rand19
--  2  0
--  1  3
0:  1  3
--  0  3
1:  0  3
--  0  3
2:  0  3
--  2  2
--  0  1
--  3  0
3:  3  0
--  2  0
--  2  0
--  1  0
--  1  0
--  2  3
4:  2  3
$ sleep 60
$ rand19
--  3  3
0:  3  3
--  1  3
1:  1  3
--  1  0
--  3  1
2:  3  1
--  3  2
3:  3  2
--  0  2
--  1  1
--  1  2
--  0  1
--  1  2
--  2  1
--  3  1
4:  3  1
$

Answer 3

int randomSet(int temp[])
{
  while (1)
         {
            x = rand()%4;
            y = rand()%4;
            if (x < 3 && y < 3) 
            {
               temp[0] = x;
               temp[1] = y;
            }
            else
               break;
         }
}