Skip to content
master
Go to file
Code

Latest commit

 

Git stats

Files

Permalink
Failed to load latest commit information.
Type
Name
Latest commit message
Commit time
lib
 
 
 
 
 
 
 
 
 
 
 
 
 
 

README.md

WalkerMethod

Walker's Alias Method is an O(1) algorithm for selecting elements from an array given a weighted distribution.

For example, let's say you want to return :win 60% of the time, :lose 20% of the time, and :tie 20% of the time. You could fill an array with 6 examples of :win and 2 examples each of :lose and :tie, then pick a random element. You could also use ranges, picking a random number between 0.0 and 1.0 and returning :win when the number is below 0.6, :lose if the number is below 0.8, and :tie otherwise. But, these algorithms are still O(n). You can do better by using a heap or binary search tree, but Walker's Alias Method is better still, with a constant runtime once an O(n) pre-computation phase has completed. For this example, you could do:

selector = WalkerMethod.new([:win, :lose, :tie], [60, 20, 20])
selector.random

This implementation is a port of http://code.activestate.com/recipes/576564-walkers-alias-method-for-random-objects-with-diffe/ from Python to Ruby. There is also a Node.js implementation.

Speed

It's FAST and simple. It samples from a 50,000 word frequency dictionary 10,000 times in 1.2 milliseconds.

Installation

Add this line to your application's Gemfile:

gem 'walker_method'

And then execute:

$ bundle

Or install it yourself as:

$ gem install walker_method

Usage

# Sampling from an English word frequency dictionary care of http://invokeit.wordpress.com/frequency-word-lists

words = ["you", "the", "i", "to", "a", "and", "it", "of", "that", "in", "is", "me", "what", "this", "for", "my", "on", "your", "we", "have", "do", "no", "don't", "are", "be"]
freqs = [4621939, 3957465, 3476773, 2873389, 2551033, 1775393, 1693042, 1531878, 1323823, 1295198, 1242191, 1208959, 1071825, 961194, 898671, 877684, 867296, 834953, 819499, 812625, 799991, 788200, 764177, 743194, 743014]

selector = WalkerMethod.new(words, weights)
selector.random
# => "and"

Contributing

  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

About

A Ruby implementation of Walker's Alias Method for quickly sampling from an array with a given probability distribution

Resources

License

Releases

No releases published

Packages

No packages published

Languages

You can’t perform that action at this time.