问题描述
我正在使用 rand()
生成 0 或 1(rand() % 2
).我正在使用当前时间 (srand(time(NULL))
) 为其播种.
I am using rand()
to generate either 0 or 1 (rand() % 2
). I am seeding it using the current time (srand(time(NULL))
).
经过多次调试后,我意识到 rand()
永远不会连续 16 次或更多次返回偶数(奇数).
After much debugging, I realised that rand()
never returns an even (odd) number 16 or more times in a row.
这是一个已知问题吗?C有没有更好的PRNG?
Is this a known issue? Is there a better PRNG that comes with C?
我使用 Visual Studio 2010 在 Windows 7 上运行.
I am running on Windows 7 using Visual Studio 2010.
推荐答案
不要使用 rand()%2
,试试 rand()>(RAND_MAX/2)
>.你只能假设 rand()
在区间 [0, RAND_MAX]
上是一致的.
Instead of using rand()%2
, try rand()>(RAND_MAX/2)
. You can only assume rand()
to be uniform on the interval [0, RAND_MAX]
.
这是 Shahbaz 在评论中提出的建议,我在发布此答案后才注意到.
This was suggested by Shahbaz in the comments, which I only noticed after I posted this answer.
ArjunShankar 在我之前的措辞中叫我:rand() 仅在区间 [0, RAND_MAX] 上指定为统一的"
ArjunShankar called me out on my previous wording: "rand() is only specified to be uniform on the interval [0, RAND_MAX]"
来自 C99 标准:
rand 函数计算范围为 0 到RAND_MAX.
从技术上讲,未指定均匀性(或等分布),但它是用于实现常用 PRNG(例如 Mersenne Twister)的事实上的标准.这是为了允许程序员轻松创建具有非均匀分布的自定义 PRNG.如果没有此属性,程序员将被迫从头开始实现自定义 PRNG.
Technically, uniformity (or equidistributed) is not specified, but is the de-facto standard used for implementations of commonly used PRNG's (e.g. Mersenne Twister). This is to allow a programmer to easily create a custom PRNG with a non-uniform distribution. Without this property, a programmer is forced to implement a custom PRNG from scratch.
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