From: Todd Benson <caduceass@...>
Date: 2008-10-01T08:19:46+09:00
Subject: Re: Mode method for Array

On Tue, Sep 30, 2008 at 6:02 PM, Rob Biedenharn
<Rob@agileconsultingllc.com> wrote:
> On Sep 30, 2008, at 6:30 PM, Glenn wrote:
>>
>> Hi,
>>
>> I wrote 2 ways.  I don't know if either way is good or not.  Any feedback
>> is welcome.
>>
>> The first way makes a hash of the array, with the unique values in the
>> array as the keys, and the number of times the keys occur as the values.
>>  Then I create a new hash out of that first hash with the frequencies as the
>> keys of the hash and the elements that had that frequency as the values.
>>  Then I pick out the value of the highest key.
>>
>> The second way creates that frequency hash, then iterates over the hash
>> and creates an array with the elements that have the highest frequency.
>>
>> class Array
>>  def hash_of_frequency
>>   h = Hash.new(0)
>>   each_with_index do |e, i|
>>     e = e.to_f if e != nil
>>     h[e] = h[e] += 1
>>   end
>>   h
>>  end
>
> You never use i, so just use each (and loose the second block parameter)
>
> h[e] = h[e] += 1 ???
> Did you mean just:  h[e] += 1
>
>>  def get_mode
>>   h = hash_of_frequency.frequency_in_key
>>   h[h.keys.max].sort
>>  end
>> end
>>
>> class Hash
>>  def frequency_in_key
>>   h = Hash.new { |k, v| k[v] = [] }
>>   each { |k, v| h[v] << k if k != nil }
>>   h
>>  end
>>
>>  def get_mode
>>   a = []
>>   max_value = values.max
>>   each { |k, v| a << k if v == max_value }
>>   a
>>  end
>> end
>>
>>
>> [3, 1, 1, 55, 55].hash_of_frequency.get_mode.inspect ## returns [1.0,
>> 55.0]
>> [3, 1, 1, 55, 55].get_mode.inspect ## returns[1.0, 55.0]
>
> If you don't bother with the .to_f on the elements, you could define this
> over ANY Enumerable with a variation on what TaQ posted; something like:
>
> module Enumerable
>  def mode
>    alist = inject(Hash.new(0)) {|h,e| h[e]+=1;h}.sort_by {|(e,c)| -c}
>    alist.select {|(e,c)| c == alist[0][1]}.map{|(e,c)| e}
>  end
> end
>
> irb> [3,1,1,55,55].mode
> => [55, 1]
> irb> ['dog','dog','cat','bird',5,5,42].mode
> => [5, "dog"]
>
> -Rob

I was thinking similarly...

module Enumerable
  def mode
    h, max = inject(Hash.new(0)) {|h, i| h[i] += 1; h}, h.values.max
    h.select {|k, v| v == max}.transpose.first
  end
end

p [3, 1, 55, 1, 55].mode

=> [55, 1]

Sort afterward if you want.

Todd