frequency of objects in an array using Ruby

Question

If i had a list of balls each of which has a color property. how can i cleanly get the list of balls with the most frequent color.

[m1,m2,m3,m4]

say,

        m1.color = blue
        m2.color = blue
        m3.color = red
        m4.color = blue

[m1,m2,m4] is the list of balls with the most frequent color

My Approach is to do:

[m1,m2,m3,m4].group_by{|ball| ball.color}.each do |samecolor|
  my_items = samecolor.count
end

where count is defined as

class Array
  def count
  k =Hash.new(0)
  self.each{|x|k[x]+=1}
  k
  end
end

my_items will be a hash of frequencies foreach same color group. My implementation could be buggy and i feel there must be a better and more smarter way. any ideas please?

Answer 1

Your code isn't bad, but it is inefficient. If I were you I would seek a solution that iterates through your array only once, like this:

balls = [m1, m2, m3, m4]
most_idx = nil

groups = balls.inject({}) do |hsh, ball|
  hsh[ball.color] = [] if hsh[ball.color].nil?
  hsh[ball.color] << ball

  most_idx = ball.color if hsh[most_idx].nil? || hsh[ball.color].size > hsh[most_idx].size 
  hsh
end

groups[most_idx] # => [m1,m2,m4]

This does basically the same thing as group_by, but at the same time it counts up the groups and keeps a record of which group is largest (most_idx).

Answer 2

How about:

color,balls = [m1,m2,m3,m4].group_by { |b| b.color }.max_by(&:size)

Answer 3

myhash = {}

mylist.each do |ball|
  if myhash[ball.color]
    myhash[ball.color] += 1
  else
    myhash[ball.color] = 1    
  end
end

puts myhash.sort{|a,b| b[1] <=> a[1]}

Answer 4

You found group_by but missed max_by

max_color, max_balls = [m1,m2,m3,m4].group_by {|b| b.color}.max_by {|color, balls| balls.length}

Answer 5

Here's how I'd do it. The basic idea uses inject to accumulate the values into a hash, and comes from "12 - Building a Histogram" in "The Ruby Cookbook".

#!/usr/bin/env ruby

class M
  attr_reader :color
  def initialize(c)
    @color = c
  end
end

m1 = M.new('blue')
m2 = M.new('blue')
m3 = M.new('red')
m4 = M.new('blue')

hash = [m1.color, m2.color, m3.color, m4.color].inject(Hash.new(0)){ |h, x| h[x] += 1; h } # => {"blue"=>3, "red"=>1}
hash = [m1, m2, m3, m4].inject(Hash.new(0)){ |h, x| h[x.color] += 1; h } # => {"blue"=>3, "red"=>1}

There are two different ways to do it, depending on how much knowledge you want the inject() to know about your objects.