javascript 随机数，Math.floor(...) vs Math.ceil(...)
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Random Number, Math.floor(...) vs Math.ceil(...)
提问by doplumi
I've seen a lot of code where random numbers are generated like
我见过很多生成随机数的代码，例如
// random integers in the interval [1, 10]
Math.floor(Math.random()*10 + 1)
Anyway, I feel like I'm missing something. Why don't people use the more succint way
无论如何，我觉得我错过了一些东西。为什么人们不使用更简洁的方式
Math.ceil(Math.random()*10);
?
?
I tried to test the randomness and it seems true so far.
我试图测试随机性，到目前为止似乎是正确的。
In fact, the subsequent code
其实后面的代码
// will generate random integers from 1 to 4
var frequencies = [ 0, 0, 0, 0, 0 ]; // not using the first place
var randomNumber;
for ( var i = 0; i < 1*1000*1000; ++i ) {
randomNumber = Math.ceil(Math.random()*4);
frequencies[randomNumber]++;
}
for ( var i = 1; i <= 4; ++i ) {
console.log(i +": "+ frequencies[i]);
}
prints out
打印出来
1: 250103
2: 250161
3: 250163
4: 249573
What am I missing?
我错过了什么？
Quick OT: Is there a more succint way to declare and initialize frequencies? I mean like frequencies[5]?= { 0 };from C++...
Quick OT：有没有更简洁的方法来声明和初始化频率？我的意思是像frequencies[5]?= { 0 };来自 C++ ......
回答by Fabrizio Calderan
as stated in MDN referenceabout Math.random()
如在所述MDN参考关于Math.random()
Returns a floating-point, pseudo-random number in the range [0, 1) that is, from 0 (inclusive) up to but not including 1 (exclusive), which you can then scale to your desired range.
返回 [0, 1) 范围内的浮点伪随机数，即从 0（含）到但不包括 1（不含），然后您可以将其缩放到所需的范围。
Since Math.random can return 0, then Math.ceil(Math.random()*10)could also return 0and that value is out of your [1..10]range.
由于 Math.random 可以返回0，那么Math.ceil(Math.random()*10)也可以返回0并且该值超出了您的[1..10]范围。
About your second question, see Most efficient way to create a zero filled JavaScript array?
关于您的第二个问题，请参阅创建零填充 JavaScript 数组的最有效方法？
回答by Ja?ck
Math.floor()is preferred here because of the range of Math.random().
Math.floor()由于 的范围，这里是首选Math.random()。
For instance, Math.random() * 10gives a range of [0, 10). Using Math.floor()you will neverget to the value of 10, whereas Math.ceil()maygive 0.
例如，Math.random() * 10给出一个范围[0, 10)。使用Math.floor()你永远不会得到 的价值10，而Math.ceil()可能会给0。
回答by d'alar'cop
random integers in the interval [1, 10]:
区间 [1, 10] 中的随机整数：
Math.floor(Math.random()*10 + 1)
random integers in the interval [0, 10]:
区间 [0, 10] 中的随机整数：
Math.ceil(Math.random()*10);
Just depends what you need.
只看你需要什么。
