Added organise-duplicate-numbers-in-a-list.rb

Author: malachispencer

ruby/6kyu/organise-duplicate-numbers-in-a-list.rb

@@ -0,0 +1,24 @@
+=begin
+Codewars. 29/04/20. 'Organise duplicate numbers in a list'. 6kyu. Here we create 
+a method that takes an array of integers and returns an array of arrays where each 
+sub-array contains all duplicates of a particular number. The sub-arrays should be 
+in the same order as the first occurrence of the numbers they contain. Here is the 
+solution I developed to solve the challenge.
+1) We define our method duplicate_organiser_ms, which takes an array of integers
+ as an argument.
+2) We call the group_by method, which groups the collection by the results of
+ the block and returns a hash. By passing in the itself method as group_by's
+ argument, we return a hash where the keys are the number from the array and
+ the values are every incarnation of that number in the array i.e. the
+ duplicates. The hash keys are in order of first appearance of that key's
+ number in the array.
+3) Now that our duplicates are grouped together in the values of the hash, we
+ use the hash method values to return an array populated with the values of
+ the hash. This gives us an array of arrays where the duplicates are
+ grouped together and everything is in order of first appearance of the
+ number in the original array.
+=end
+
+def duplicate_organiser_ms(arr)
+ arr.group_by(&:itself).values
+end