Generating Random Numbers in Ruby
Tags: ruby, beginners
Published: 2018-07-31 12:40:11

---
title: Generating Random Numbers in Ruby
description: Random numbers are useful for a variety of purposes such as in gaming, encryption and building simulations. In this article, we'll look at the various ways you can generate random numbers in Ruby.
cover_image: https://thepracticaldev.s3.amazonaws.com/i/kc6k135aluztmfnuwqol.jpg
tags: ruby, beginners
published: true
canonical_url: https://blog.appsignal.com/2018/07/31/generating-random-numbers-in-ruby.html
---

Random numbers are useful for a variety of purposes such as in gaming, encryption and building simulations.  Technically, computers cannot generate random numbers purely by computation. It is fundamentally impossible to produce truly random numbers on any deterministic device. The best you can hope for are pseudorandom numbers, a stream of numbers that appear as if they were generated randomly.

In this article, we'll look at the various ways you can generate random numbers in Ruby.

## Generating Random Numbers with Kernel#rand
To start off, let's generate random numbers with the [`rand`](http://ruby-doc.org/core-2.5.1/Kernel.html#method-i-rand) method. When the method is called with no arguments, it returns a float that is greater than or equal to 0.0 and less than 1.0.

```ruby
rand()
> 0.7308136972953823
```

To get an integer, you pass an integer to the function. The function will return a random integer value that is greater than or equal to 0 and less than the integer passed to the function. Each time the following is run, you will get a number that is between 0 and 7.

```ruby
rand(8)
> 5
```

For a random number within a particular range, pass the [Range](https://ruby-doc.org/core-2.2.0/Range.html) to `rand`.

The following uses an _inclusive_ Range to generate random numbers from a lower limit (`1`), up to (and including) the upper limit (`10`).

```ruby
rand(1..10)
> 6
```

The next example uses a _non-inclusive_ Range to generate random numbers from a lower limit, up to (but not including) the upper limit.

```ruby
rand(1...10)
> 9
```

The range can also be between floating point values.

```ruby
rand(1.5..3.0)
> 1.7494305393711571
```

You can also use negative range limits with `rand`.

```ruby
rand(-5..-1)
> -5
```

Passing in single negative numbers may give surprising results, as shown below.

```ruby
rand(-100)
> 94

rand(-0.5)
> 0.7692627344737486
```

This is because for an argument `n` passed into `rand`, `rand` returns random numbers from 0 up to (but not including) `n.to_i.abs`. For the above example `(-100).to_i.abs` is `100` and `(-0.5).to_i.abs` is `0`, thus the resulting random numbers.

Calling `rand(0)` is similar to calling `rand()`. You will get random numbers between 0.0 and 1.0 (not inclusive).

## Generating Reproducible Sequences with Kernel#srand

Before moving on to the next method of generating random numbers, let's first look at the `srand` function.

[`Kernel#srand`](http://ruby-doc.org/core-2.5.1/Kernel.html#method-i-srand) sets the seed for `Kernel#rand`. We can use it to generate repeatable sequences of random numbers between different runs of the program.

To understand what this means, we first need to understand how random numbers are generated.

<div class="header academy">
<h2>Generating "Random" Numbers from a Seed</h2>
<p>As stated earlier, computers don't generate truly random numbers purely from computation. What they do is generate a sequence of numbers that seem random. To do this, the computer starts with a seed number, which it runs through some algorithm and then spits out a seemingly random output.</p>
<p>The seed number is generated by the computer using a combination of different elements, e.g. timestamp, the process ID of the program, e.t.c. Because these elements vary for each request to generate a random number, the seed number will always be different, which would produce a different sequence of numbers, thus the resulting random number output. If you ran the algorithm with the same seed, then you would get the same sequence of numbers each time. This is what <code>Kernel#srand</code> allows us to do.</p>
</div>

`srand` is usually used in testing. It could be handy for testing code in your app that deals with randomness, with values that are random but still predictable enough to test. It could also help in isolating or reproducing bugs.

Below we use `srand` to set the seed and then call `rand` first to produce a couple of individual random numbers and then to produce a couple sequences of random numbers.

```ruby
srand(777)

rand()
> 0.152663734901322

rand()
> 0.3023566097075212

10.times.map { rand(10) }
> [7, 1, 7, 4, 7, 9, 8, 7, 2, 0]

10.times.map { rand(10) }
> [1, 2, 4, 5, 7, 1, 7, 2, 2, 7]
```

If you run `srand` again with the same seed and make the same calls we made previously, you will see that we get the same random numbers.

```ruby
srand(777)

rand()
> 0.152663734901322

rand()
> 0.3023566097075212

10.times.map { rand(10) }
> [7, 1, 7, 4, 7, 9, 8, 7, 2, 0]

10.times.map { rand(10) }
> [1, 2, 4, 5, 7, 1, 7, 2, 2, 7]
```

## Generating Random Numbers with Random
You can also generate random numbers with the [Random](http://ruby-doc.org/core-2.1.3/Random.html) class.

The class method `rand` provides the base functionality of `Kernel#rand` along with better handling of floating point values.

```ruby
Random.rand(1...10)
> 5
```

Unlike `Kernel#rand`, if `Random.rand` is given a negative or `0` argument, it raises an [ArgumentError](http://ruby-doc.org/core-2.1.3/ArgumentError.html).

## Generating Random Numbers Based on Normal Distribution
In the real world, many things tend to follow a [Normal Distribution](https://en.wikipedia.org/wiki/Normal_distribution). If you have a range of values that something falls under, rarely do you get an equal distribution of all the values. Mostly, a majority of the data tends to fall within a smaller range, with a smaller percentage falling within the larger range. Let's take an adult man's height as an example. The shortest height recorded is 54.6 cm (21.5 inches) while the tallest is 267 cm (8'9"). If you want to generate data to simulate the height of men in a population, you might not want to use `rand` with these limits. You don't want the probability of getting an 8'9" man to be the same as getting a 6' man, because the latter is more common.

Other examples of things that follow a Normal Distribution are:

- Errors in measurements
- Blood pressure
- Test scores
- Weight of an adult man/woman

 To generate better random numbers for such use cases, you can use the [`rubystats`](https://rubygems.org/gems/rubystats) gem.

```sh
$ gem install rubystats
```

```ruby
require 'rubystats'

adult_male_height = Rubystats::NormalDistribution.new(178, 10)
sample = 50.times.map { adult_male_height.rng.round(1) }

> [183.2, 169.5, 189.7, 171.9, 176.0, 179.3, 189.3, 175.3, 188.3, 190.0, 185.5, 182.8, 187.2, 191.6, 185.4, 178.4, 187.1, 183.3, 189.6, 179.7, 172.7, 174.4, 153.8, 197.4, 176.0, 174.6, 181.1, 182.0, 204.7, 185.2, 175.9, 167.7, 160.6, 170.5, 169.3, 160.6, 165.6, 166.4, 182.6, 179.7, 183.1, 171.9, 185.4, 175.4, 179.7, 176.9, 160.6, 173.8, 181.9, 190.2]
 ```

In the above, we pass the average height for men (178cm) and a standard deviation of 10cm to `NormalDistribution.new`, before generating 50 values that fall in this normal distribution. If you are curious about the math, [this article](http://www.usablestats.com/lessons/normal) may interest you.

## Random roundup

That brings us to the end of this discussion. We covered a few different ways of creating 'random' numbers in Ruby, with `rand`, `srand`, `Random` and `Rubystats`. We also briefly touched on how 'random' numbers are created and looked at the reason why deterministic devices cannot create real random numbers. We hope you found some of this interesting. If you have any comments or questions on what we covered, please reach out to us [@AppSignal](https://twitter.com/appsignal). You can also send us your requests for topics you want covered.

*This post is written by guest author [Joyce Echessa](http://echessa.com). Joyce is a full stack developer, and loves words and literature.*