131 lines
4.3 KiB
Markdown
131 lines
4.3 KiB
Markdown
---
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title: Random Method
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---
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## Random Method
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The JavaScript `Math.random()` method is an excellent built-in method for producing random numbers. When `Math.random()` is executed, it returns a random number that can be anywhere between 0 and 1. The 0 is included and 1 is excluded.
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### Generating a random floating point number between 0 and 1
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The `Math.random()` method will return a floating point (decimal) number greater than or equal to 0 and less than (but never equal to) 1. In other words `0 <= x < 1`. For example:
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```JavaScript
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console.log(Math.random());
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// 0.7069207248635578
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console.log(Math.random());
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// 0.765046694794209
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console.log(Math.random());
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// 0.14069121642698246
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```
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(Of course, the numbers returned will be different every time. This will be assumed for all following examples - different results will happen on each pass.)
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To get a random number between a larger range multiply the result of `Math.random()` by a number.
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### Generating a random floating point number between 0 and a specified max
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Usually you do not need random numbers between 0 and 1 - you need larger numbers or even integers.
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For example, if you want a random floating point number between 0 and 10, you could use:
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```JavaScript
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var x = Math.random()*10;
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console.log(x);
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// 4.133793901445541
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//In function form
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function randomFloatZeroToMax(max){
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return Math.random()*max;
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}
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```
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### Generating a random floating point number within a range
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If you need a random floating point number that ranges between two specific numbers, you could do something like this:
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```JavaScript
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var min = 83.1;
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var max = 193.36;
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var x = Math.random()*(max - min)+min;
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console.log(x);
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// 126.94014012699063
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//In function form
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function randomFloatInRange(min, max){
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return Math.random()*(max - min)+min;
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}
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```
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### Generating a random integer between 0 and a max
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Often you need integers. To do this you will have to use some other methods from the `Math` object, `Math.floor()` (rounds down to the nearest integer) and `Math.ceil()` (rounds up to the nearest integer).
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For example, if you need to select randomly from an array of 10 elements, you would need a random number between 0 and 9 inclusive (remember that arrays are zero indexed).
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```JavaScript
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var x = Math.floor(Math.random()*10);
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console.log(x);
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// 7
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//In function form
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function randomIntZeroToMax(max){
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return Math.floor(Math.random()*max);
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}
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```
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(Remember that `Math.random()` will never return exactly 1, so `Math.random()*10` will never return exactly 10. This means that after rounding down, the result will always be 9 or less.)
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### Generating a random integer between 1 and a max
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If you need a random number with the minimum number being 1 (for example picking a random day in January) you could use the `Math.ceil()` method.
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```JavaScript
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var x = Math.ceil(Math.random()*31);
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console.log(x);
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// 23
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//In function form
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function randomIntOneToMax(max){
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return Math.ceil(Math.random()*max);
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}
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```
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Another way would have been to use the previous function (using `Math.floor()`) and add 1 to it:
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```JavaScript
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var x = Math.floor(Math.random()*31)+1;
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console.log(x);
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// 17
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//In function form
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function randomIntInRange(min, max){
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return Math.floor(Math.random()*(max-1))+1;
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}
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```
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### Generating a random integer within a range
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Lastly, occasionally you need a random integer between two specific integers. For example, if you are trying to pick raffle tickets and you know the numbers of the lowest and largest number:
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```JavaScript
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var min = 1718;
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var max = 3429;
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var x = Math.floor(Math.random()*(max-min+1)+min);
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console.log(x);
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//2509
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//In function form
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function randomIntInRange(min, max){
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return Math.floor(Math.random()*(max - min)+min);
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}
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```
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### How random is Math.random()?
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It may be pointed out that the number returned by `Math.random()` is a pseudo-random number as no computer can generate a truly random number, that exhibits randomness over all scales and over all sizes of data sets. However, the pseudo-random number generated by `Math.random()` is usually sufficient for the needs of nearly any program you may write. The not-truly-randomness only becomes apparent in astronomically large number sets or when uncommonly precise decimals are needed.
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### More Information:
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- Documentation: <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random' target='_blank' rel='nofollow'>MDN</a>
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