[ruby-core:72107] [Ruby trunk - Feature #11815] [Open] Proposal for method `Array#difference`

[cary@swoveland.com]

Issue #11815 has been reported by Cary Swoveland.

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Feature #11815: Proposal for method `Array#difference`
https://bugs.ruby-lang.org/issues/11815

* Author: Cary Swoveland
* Status: Open
* Priority: Normal
* Assignee: 
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I propose that a method `Array#difference` be added to the Ruby core. It is similar to [Array#-](http://ruby-doc.org/core-2.2.0/Array.html#method-i-2D) but for each element of the (array) argument it remove only one matching element from the receiver. For example:

    a = [1,2,3,4,3,2,2,4]
    b = [2,3,4,4,4]

    a     -      b     #=> [1]
    c = a.difference b #=> [1, 3, 2, 2] 

As you see, `a` contains three `2`'s and `b` `1`, so the first `3-1 #=> 2` `2`'s in `a` have been removed from `a` in constructing `c`. When `b` contains as least as many instances of an element as does `a`, `c` contains no instances of that element. 

It could be implemented as follows:

     class Array
       def difference(other)
         dup.tap do |cpy|
           other.each do |e|
             ndx = cpy.index(e)
             cpy.delete_at(ndx) if ndx
            end
          end
        end
      end

Here are a few examples of its use:

*Identify an array's non-unique elements*

      a = [1,3,2,4,3,4]
      u = a.uniq          #=> [1, 2, 3, 4]
      u - a.difference(u) #=> [1, 2]

*Determine if two words of the same size are anagrams of each other*

      w1, w2 = "stop", "pots"
      w1.chars.difference(w2.chars).empty?
        #=> true

*Identify a maximal number of 1-1 matches between the elements of two arrays and return an array of all elements from both arrays that were not matched*

      a = [1, 2, 4, 2, 1, 7, 4, 2, 9] 
      b = [4, 7, 3, 2, 2, 7] 
      a.difference(b).concat(b.difference(a))
        # => [1, 1, 4, 2, 9, 3, 7] 
  
To remove elements from `a` starting at the end (rather the beginning) of `a`:

    a = [1,2,3,4,3,2,2,4]
    b = [2,3,4,4,4]

    a.reverse.difference(b).reverse #=> [1,2,3,2]

`Array#difference!` could be defined in the obvious way.



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