每次循环迭代相同的随机数

2021-12-11 00:00:00 random for-loop c++

我有一个运行 15 次的 for 循环，每次迭代都使用 dh.setDoors().

I have a for loop that runs 15 times, with dh.setDoors() in every iteration.

setDoors 所做的是调用 srand(time(0))，然后每当需要随机数时它都会使用，例如 carSetter =rand()%3+1.或者，它可以使用 decider = rand()%2+1.

What setDoors does is call srand(time(0)), then whenever a random number is needed it'll use, for example, carSetter = rand()%3+1. Alternatively, it may use decider = rand()%2+1.

现在，通常decider和carSetter以不同的方式使用，但我怀疑有问题并打印出carSetter和decider 在每次迭代中.这是结果:

Now, normally decider and carSetter are used in a different ways, but I suspected a problem and made it print out carSetter and decider at every iteration. Here's what came out:

Door 1 has car
Decider is 2
Door 1 has car
Decider is 2
Door 1 has car
Decider is 2
Door 1 has car
Decider is 2
Door 1 has car
Decider is 2
etc...

当我多次运行它时，值1"和2"会发生变化，但在 15 次中仍然相同.

The values '1' and '2' change when I run it multiple times, but are still the same throughout the 15 times.

既然循环运行了 15 次不同的时间，难道 carSetter 和 decider 不应该每次迭代都打印出不同的随机数吗?

Since the loop is running 15 different times, shouldn't carSetter and decider print out a different random number every iteration?

当我没有 srand(time(0)) 时，它按预期工作，但是没有种子集，所以每次都是相同的随机"数字序列，所以它是可能是种子有问题?

When I don't have srand(time(0)), it works as expected, but there's no seed set, so it's the same sequence of "random" numbers each time, so it's probably a problem with the seed?

推荐答案

当你调用 srand(x) 时，那么 x 的值决定了伪序列在对 rand() 的后续调用中返回的随机数，完全取决于 x 的值.

When you call srand(x), then the value of x determines the sequence of pseudo-random numbers returned in following calls to rand(), depending entirely on the value of x.

当您处于循环中并在顶部调用 srand() 时:

When you're in a loop and call srand() at the top:

while (...) {
    srand(time(0));
    x = rand();
    y = rand();
}

然后根据time(0)返回的值生成相同随机数序列.由于计算机速度很快，而且您的循环可能会在不到一秒的时间内运行，time(0) 每次通过循环都会返回 相同 值.所以 x 和 y 每次迭代都是一样的.

then the same random number sequence is generated depending on the value that time(0) returns. Since computers are fast and your loop probably runs in less than a second, time(0) returns the same value each time through the loop. So x and y will be the same each iteration.

相反，您通常只需要在程序开始时调用 srand() 一次:

Instead, you only usually need to call srand() once at the start of your program:

srand(time(0));

while (...) {
    x = rand();
    y = rand();
}

在上面的例子中，x 和 y 每次循环都会有不同的值.

In the above case, x and y will have different values each time through the loop.

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