在同一个程序中调用srand（）两次 [英] Calling srand() twice in the same program

问题描述

为什么是当我调用srand（）在2非常不同的点，它导致数字不是随机的？一旦我删除其中的一个，它会恢复正常。

why is it when i call srand() at 2 very different points it cause numbers to not be random? Once i remove one of them it goes back to normal.

推荐答案

这取决于你如何调用它。 srand（）的目的是为 rand（）使用的伪随机数生成器。因此，当你调用 srand（i）时，它会将 rand（）初始化为一个固定的序列， c $ c> i 。所以当你用相同的种子重新种子，你开始得到相同的序列。

It depends on how you call it. The purpose of srand() is to seed the pseudo-random number generator used by rand(). So when you call srand(i), it will initialise rand() to a fixed sequence which depends on i. So when you re-seed with the same seed, you start getting the same sequence.

最常见的用例是种子生成器只有一次， 随机值（例如惯用的 time（NULL））。

The most common use case is to seed the generator just once, and with a suitable "random" value (such as the idiomatic time(NULL)). This guarantees makes it likely that you'll get different sequences of pseudo-random numbers in different program executions.

但是，有时候你可能会使用不同的程序来执行不同的伪随机数字序列。

However, occasionally you might want to make the pseudo-random sequence "replayable." Imagine you're testing several sorting algorithms on random data. To get fair comparisons, you should test each algorithm on the exact same data - so you'll re-seed the generator with the same seed before each run.

在其他词语：如果你想要的数字只是伪随机，种子一次，并尽可能随机的值。如果你想要一些控制&可重播性，必要的种子。

In other words: if you want the numbers simply pseudo-random, seed once, and with a value as random as possible. If you want some control & replayability, seed as necessary.

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