There are some examples on the Mozilla Developer Network page:
/**
* Returns a random number between min (inclusive) and max (exclusive)
*/
function getRandomArbitrary(min, max) {
return Math.random() * (max - min) + min;
}
/**
* Returns a random integer between min (inclusive) and max (inclusive).
* The value is no lower than min (or the next integer greater than min
* if min isn't an integer) and no greater than max (or the next integer
* lower than max if max isn't an integer).
* Using Math.round() will give you a non-uniform distribution!
*/
function getRandomInt(min, max) {
min = Math.ceil(min);
max = Math.floor(max);
return Math.floor(Math.random() * (max - min + 1)) + min;
}
Here's the logic behind it. It's a simple rule of three:
Math.random() returns a Number between 0 (inclusive) and 1 (exclusive). So we have an interval like this:
[0 .................................... 1)
Now, we'd like a number between min (inclusive) and max (exclusive):
[0 .................................... 1)
[min .................................. max)
We can use the Math.random to get the correspondent in the [min, max) interval. But, first we should factor a little bit the problem by subtracting min from the second interval:
[0 .................................... 1)
[min - min ............................ max - min)
This gives:
[0 .................................... 1)
[0 .................................... max - min)
We may now apply Math.random and then calculate the correspondent. Let's choose a random number:
Math.random()
|
[0 .................................... 1)
[0 .................................... max - min)
|
x (what we need)
So, in order to find x, we would do:
x = Math.random() * (max - min);
Don't forget to add min back, so that we get a number in the [min, max) interval:
x = Math.random() * (max - min) + min;
That was the first function from MDN. The second one, returns an integer between min and max, both inclusive.
Now for getting integers, you could use round, ceil or floor.
You could use Math.round(Math.random() * (max - min)) + min, this however gives a non-even distribution. Both, min and max only have approximately half the chance to roll:
min...min+0.5...min+1...min+1.5 ... max-0.5....max
βββββ¬ββββββββββββββ¬ββββββββββββββ ... βββββββββββ¬βββ β Math.round()
min min+1 max
With max excluded from the interval, it has an even less chance to roll than min.
With Math.floor(Math.random() * (max - min +1)) + min you have a perfectly even distribution.
min... min+1... ... max-1... max.... (max+1 is excluded from interval)
βββββ¬βββββββββ¬ββββββββ ... ββββββ¬βββββββββ¬ββββ β Math.floor()
min min+1 max-1 max
You can't use ceil() and -1 in that equation because max now had a slightly less chance to roll, but you can roll the (unwanted) min-1 result too.
There are some examples on the Mozilla Developer Network page:
/**
* Returns a random number between min (inclusive) and max (exclusive)
*/
function getRandomArbitrary(min, max) {
return Math.random() * (max - min) + min;
}
/**
* Returns a random integer between min (inclusive) and max (inclusive).
* The value is no lower than min (or the next integer greater than min
* if min isn't an integer) and no greater than max (or the next integer
* lower than max if max isn't an integer).
* Using Math.round() will give you a non-uniform distribution!
*/
function getRandomInt(min, max) {
min = Math.ceil(min);
max = Math.floor(max);
return Math.floor(Math.random() * (max - min + 1)) + min;
}
Here's the logic behind it. It's a simple rule of three:
Math.random() returns a Number between 0 (inclusive) and 1 (exclusive). So we have an interval like this:
[0 .................................... 1)
Now, we'd like a number between min (inclusive) and max (exclusive):
[0 .................................... 1)
[min .................................. max)
We can use the Math.random to get the correspondent in the [min, max) interval. But, first we should factor a little bit the problem by subtracting min from the second interval:
[0 .................................... 1)
[min - min ............................ max - min)
This gives:
[0 .................................... 1)
[0 .................................... max - min)
We may now apply Math.random and then calculate the correspondent. Let's choose a random number:
Math.random()
|
[0 .................................... 1)
[0 .................................... max - min)
|
x (what we need)
So, in order to find x, we would do:
x = Math.random() * (max - min);
Don't forget to add min back, so that we get a number in the [min, max) interval:
x = Math.random() * (max - min) + min;
That was the first function from MDN. The second one, returns an integer between min and max, both inclusive.
Now for getting integers, you could use round, ceil or floor.
You could use Math.round(Math.random() * (max - min)) + min, this however gives a non-even distribution. Both, min and max only have approximately half the chance to roll:
min...min+0.5...min+1...min+1.5 ... max-0.5....max
βββββ¬ββββββββββββββ¬ββββββββββββββ ... βββββββββββ¬βββ β Math.round()
min min+1 max
With max excluded from the interval, it has an even less chance to roll than min.
With Math.floor(Math.random() * (max - min +1)) + min you have a perfectly even distribution.
min... min+1... ... max-1... max.... (max+1 is excluded from interval)
βββββ¬βββββββββ¬ββββββββ ... ββββββ¬βββββββββ¬ββββ β Math.floor()
min min+1 max-1 max
You can't use ceil() and -1 in that equation because max now had a slightly less chance to roll, but you can roll the (unwanted) min-1 result too.
var randomnumber = Math.floor(Math.random() * (maximum - minimum + 1)) + minimum;
Videos
Hi all,
I'm learning JS through FCC and it's going okay so far. I'm not a very mathematically inclined person, however, so I struggle with some concepts.
I'm struggling to understand how the following code works;
Math.floor(Math.random() * (max - min + 1)) + min
I tried writing out an example on paper and used 10 and 5 as max and min respectively.
Math.floor(Math.random() * (10 - 5 + 1)) + 5
wouldn't this mean that I can get numbers higher than the max though? Say I get a randomly generated 0.99; well 0.99 * (6) + 5 = 10.94 which is greater than my max of 10?
Am I doing this wrong?
function randomIntFromInterval(min, max) { // min and max included
return Math.floor(Math.random() * (max - min + 1) + min);
}
const rndInt = randomIntFromInterval(1, 6);
console.log(rndInt);
What it does "extra" is it allows random intervals that do not start with 1. So you can get a random number from 10 to 15 for example. Flexibility.
Important
The following code works only if the minimum value is `1`. It does not work for minimum values other than `1`.If you wanted to get a random integer between 1 (and only 1) and 6, you would calculate:
const rndInt = Math.floor(Math.random() * 6) + 1
console.log(rndInt)
Where:
- 1 is the start number
- 6 is the number of possible results (1 + start (6) - end (1))
I'm a complete newbie not just to JavaScript but to programming altogether. And I mean, a newbie, no previous experience, just about finishing the codecademy's JS track right now and reading a couple of books atm. Codecademy's great fun but I get the feeling that being hold by a virtual hand while writing itsy bits of code is not how this is going to work, so I've decided to fiddle away at my own, obviously incredibly basic, little projects.
And for one of them I need a dice roll function which I'll be able to use for multiple types of dice (you know d4, d6, d8 etc). I came up with this:
function diceRoll(min, max) {
return Math.floor(Math.random()* (max-min)) + min;
}It's working, but can anyone tell me if it's the best option? What other ways are there to generate random numbers within a range?