Java:生成具有对数分布的随机数 [英] Java: Generating a random numbers with a logarithmic distribution

问题描述

我正在尝试生成具有对数分布的随机数.

I am attempting to generate a random numbers with a logarithmic distribution.

n = 1出现在一半时间，n = 2出现在四分之一的时间，n = 3出现在八分之一的时间等的情况.

Where n=1 occurs half of the time, n=2 occurs a quarter of the time, n=3 occurs an eighth of the time, etc.

    int maxN = 5;
    int t = 1 << (maxN); // 2^maxN
    int n = maxN -
            ((int) (Math.log((Math.random() * t))
            / Math.log(2))); // maxN - log2(1..maxN)
    System.out.println("n=" + n);

在大多数情况下，我会得到所需的结果，但是每隔一段时间，我会得到一个比maxN大的n值.

Most of the time, I am getting the result I need, however once every so often, I get a value of n that is larger than maxN.

为什么会这样?我的看法是，Math.random()的最大值是1.0;
因此(Math.random() * t))的最大值为t;
由于t = 2 ^ maxN，因此log2(t)的最大值为maxN；

Why is this so? The way I see it, the max value of Math.random() is 1.0;
therefore the max value of (Math.random() * t)) is t;
therefore the max value of log2(t) is maxN, since t = 2^maxN;

我的逻辑在哪里偏离了轨道?

Where has my logic gone off track?

谢谢

推荐答案

小于1.0的对数为负.当生成的随机数小于1.0时，表达式((int) (Math.log(Math.random() * t) / Math.log(2)))为负数，因此maxN - (the negative number)大于maxN.

logarithm of numbers less than 1.0 is negative. When the random number generated is such that it is less than 1.0, the expression ((int) (Math.log(Math.random() * t) / Math.log(2))) is a negative number and hence maxN - (the negative number) is more than maxN.

下面的表达式应该给出正确的分布.

The following expression should give correct distribution.

n = Math.floor(Math.log((Math.random() * t) + 1)/Math.log(2))

请注意:

0.0 <= Math.random() <= 1.0
0.0 <= Math.random() * t <= t
1.0 <= (Math.random() * t) + 1 <= t + 1.0
0.0 <= Math.log((Math.random() * t) + 1) <= Math.log(t + 1.0)
0.0 <= Math.log((Math.random() * t) + 1)/Math.log(2) <= Math.log(t + 1.0)/Math.log(2)

Since t = 2^maxN,
Math.log(t + 1.0)/Math.log(2) is slightly larger than maxN.

So do a Math.floor and you get the correct result:
0.0 <= Math.floor(Math.log((Math.random() * t) + 1)/Math.log(2)) <= maxN

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