Better algorithm generating random numbers in JS

Question

My goal is to generate random integers between 1 and 100 in JavaScript.

I'm currently using this:

const random = Math.ceil(Math.random() * 100)
console.log(random)

But I saw a lot of places an alternate solution,

const random = Math.floor(Math.random() * 100 + 1)
console.log(random)

which produces the same results.

My question is that:

Why is the second code better (if better at all) than my first code?
Isn't it better to perform one operation rather than two (Math.floor() and +1)?

Thanks for your time and answers!

The first example produces numbers between 0 (inclusive) and 100 (inclusive), the second produces numbers between 1 (inclusive) and 100 (inclusive). They are not the same. — str, Mar 14 '19 at 21:17

score 3 · Accepted Answer · answered Mar 14 '19 at 21:21

3

There is one notable difference between these two

Math.ceil(Math.random() * 100)
Math.floor(Math.random() * 100 + 1)

The first has a theoretical possibility to produce 0 with a very tiny probability, the second not.

answered Mar 14 '19 at 21:21

trincot

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Nina Scholz · Answer 2 · 2019-03-14T21:34:49.957

1

Both produce nearly the same result. You could take a quantitative test and have a look to the count of the drawn numbers.

const
    getRandomCeil = () => Math.ceil(Math.random() * 100),       // 0 ... 100 0 is less likely
    getRandomFloor = () => Math.floor(Math.random() * 100 + 1); // 1 ... 100

var i,
    count = { ceil: {}, floor: {} };

for (i = 0; i < 1e7; i++) {
    value = getRandomCeil();
    count.ceil[value] = (count.ceil[value] || 0) + 1;
    value = getRandomFloor();
    count.floor[value] = (count.floor[value] || 0) + 1;
}
console.log(count);

.as-console-wrapper { max-height: 100% !important; top: 0; }

edited Mar 14 '19 at 21:34

answered Mar 14 '19 at 21:18

Nina Scholz

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You modified OP's formula. – PM 77-1 Mar 14 '19 at 21:21
What if `Math.random` returns 0? – Jonas Wilms Mar 14 '19 at 21:23
i did, but it is negligible. – Nina Scholz Mar 14 '19 at 21:23
@JonasWilms, right. but how is the probability of it? – Nina Scholz Mar 14 '19 at 21:27
@nina I guess roughly 1 to 2**63 (64 bit float, assuming a negative exponent sign) – Jonas Wilms Mar 14 '19 at 21:28
Regardless of how small the probability, they do not produce the same numbers. Getting or not getting 0 instead of a positive number is a very important distinction. – str Mar 14 '19 at 21:34
@NinaScholz As some people wrote, the first method can return zero, so they are not exactly the same, but thanks for the clear visualisation example! – FZs Mar 14 '19 at 21:38
@everybody I see it now! I was unable to find the difference on my own, but as you pointed out, I understand. Thanks! – FZs Mar 14 '19 at 21:40

Salman A · Answer 3 · 2019-03-15T08:55:25.213

Math.random produces number in the range [0, 1) which means 0 is inclusive and 1 is not. Looking at the two extreme cases:

// When Math.random() returns 0
Math.ceil(0 * 100)         // returns 0 since ceiling of 0 is 0
Math.floor(0 * 100 + 1)    // returns 1

// When Math.random() returns 0.999999...
Math.ceil(0.999999 * 100)  // returns 100
Math.floor(0.999999 + 1)   // returns 100

The ceil variant has the potential of returning 0 when random function returns exactly 0; although the probability is very~~, very~~ little.

Better algorithm generating random numbers in JS

3 Answers3

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