DZone Snippets is a public source code repository. Easily build up your personal collection of code snippets, categorize them with tags / keywords, and share them with the world

# Weighted Random Methods For Array

class Array # Chooses a random array element from the receiver based on the weights # provided. If _weights_ is nil, then each element is weighed equally. # # [1,2,3].random #=> 2 # [1,2,3].random #=> 1 # [1,2,3].random #=> 3 # # If _weights_ is an array, then each element of the receiver gets its # weight from the corresponding element of _weights_. Notice that it # favors the element with the highest weight. # # [1,2,3].random([1,4,1]) #=> 2 # [1,2,3].random([1,4,1]) #=> 1 # [1,2,3].random([1,4,1]) #=> 2 # [1,2,3].random([1,4,1]) #=> 2 # [1,2,3].random([1,4,1]) #=> 3 # # If _weights_ is a symbol, the weight array is constructed by calling # the appropriate method on each array element in turn. Notice that # it favors the longer word when using :length. # # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "hippopotamus" # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "dog" # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "hippopotamus" # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "hippopotamus" # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "cat" def random(weights=nil) return random(map {|n| n.send(weights)}) if weights.is_a? Symbol weights ||= Array.new(length, 1.0) total = weights.inject(0.0) {|t,w| t+w} point = rand * total zip(weights).each do |n,w| return n if w >= point point -= w end end # Generates a permutation of the receiver based on _weights_ as in # Array#random. Notice that it favors the element with the highest # weight. # # [1,2,3].randomize #=> [2,1,3] # [1,2,3].randomize #=> [1,3,2] # [1,2,3].randomize([1,4,1]) #=> [2,1,3] # [1,2,3].randomize([1,4,1]) #=> [2,3,1] # [1,2,3].randomize([1,4,1]) #=> [1,2,3] # [1,2,3].randomize([1,4,1]) #=> [2,3,1] # [1,2,3].randomize([1,4,1]) #=> [3,2,1] # [1,2,3].randomize([1,4,1]) #=> [2,1,3] def randomize(weights=nil) return randomize(map {|n| n.send(weights)}) if weights.is_a? Symbol weights = weights.nil? ? Array.new(length, 1.0) : weights.dup # pick out elements until there are none left list, result = self.dup, [] until list.empty? # pick an element result << list.random(weights) # remove the element from the temporary list and its weight weights.delete_at(list.index(result.last)) list.delete result.last end result end end

Intended for use in selecting random elements from an array based on weight. Not optimized!

## Comments

## Snippets Manager replied on Thu, 2009/10/15 - 3:44am

`point = rand * total`

to`point = Kernel.rand * total`

to avoid conflicting with the built-in Array.rand method.## Snippets Manager replied on Wed, 2008/01/02 - 3:00pm

`require 'test/unit' require 'rand_array' class TestRandArray < Test::Unit::TestCase def test_randomness arr = ['a','b','c'] weights = [1,2,7] counts = {} counts['a'] = 0 counts['b'] = 0 counts['c'] = 0 1000000.times do r = arr.random(weights) counts[r] += 1 end a_exp = 100000 b_exp = 200000 c_exp = 700000 assert_in_delta(a_exp, counts['a'], 0.01*a_exp) assert_in_delta(b_exp, counts['b'], 0.01*b_exp) assert_in_delta(c_exp, counts['c'], 0.01*c_exp) end end`

## Snippets Manager replied on Mon, 2012/05/07 - 2:16pm