random number from min to max

一、从 min 到 max 的随机数

内建函数 Math.random() 会创建一个在 0 到 1 之间（不包括 1）的随机数.
编写一个 random(min, max) 函数,to generate an in min 到 max 之间的随机浮点数（不包括 max)）.
We need to set the interval 0…1 中的所有值“映射”为范围在 min 到 max 中的值.

This can be done in two stages：

1. 如果我们将 0…1 的随机数乘以 max-min,Then the range of random numbers will be from 0…1 增加到 0..max-min.
2. 现在,If we combine random numbers with min 相加,Then the range of random numbers will be min 到 max.

function random(min, max) {
    
  return min + Math.random() * (max - min);
}

二、从 min 到 max 的随机整数

创建一个函数 randomInteger(min, max),The function generates a range in min 到 max 中的随机整数,包括 min 和 max.

在 min…max All numbers in the range must occur with the same probability.

Simple but wrong solution

The simplest but wrong solution is to generate a range in min 到 max 的值,and take the value after rounding it：

function randomInteger(min, max) {
    
  let rand = min + Math.random() * (max - min);
  return Math.round(rand);
}

This function can work,但不正确.Get edge values min 和 max is twice as likely as the other values.
If you run the above example multiple times,You will see easily 2 出现的频率最高.
发生这种情况是因为 Math.round() 从范围 1…3 中获得随机数,And round up as shown below：

values from 1    ... to 1.4999999999  become 1
values from 1.5  ... to 2.4999999999  become 2
values from 2.5  ... to 2.9999999999  become 3

现在我们可以清楚地看到 1 的值比 2 twice less.和 3 一样.

正确的解决方案

There are many correct solutions to this problem.One of them is to adjust the boundaries of the range of values.To ensure the same value range,We can generate from 0.5 到 3.5 的值,This adds the desired probability to the boundaries of the range of values：

function randomInteger(min, max) {
    
  // 现在范围是从 (min-0.5) 到 (max+0.5)
  let rand = min - 0.5 + Math.random() * (max - min + 1);
  return Math.round(rand);
}

另一种方法是使用 Math.floor to take the range from min 到 max+1 的随机数：

function randomInteger(min, max) {
    
  // here rand is from min to (max+1)
  let rand = min + Math.random() * (max + 1 - min);
  return Math.floor(rand);
}

All intervals are now mapped this way：

values from 1  ... to 1.9999999999  become 1
values from 2  ... to 2.9999999999  become 2
values from 3  ... to 3.9999999999  become 3

All intervals are the same length,So that the final can be evenly distributed.