Will tell you what Ruby method(s) you want given the input and output Now works with arguments!

havewant.rb

class HaveWant
  
  attr_reader :answers
  
  def initialize(*have, want)
    @have = have[0]
    @args = have[1..-1]
    @want = want
    
    find_matches
  end

  private
  
  def find_matches
    
    @answers = @have.methods.map do |method|
      @have.dup.method(method)
    end.select do |method|
      begin
        method.call(*@args).eql? @want
      rescue
        false
      end
    end.map(&:name)
    
  end
end

havewant = HaveWant.new("hello", "HELLO")

puts havewant.answers

# upcase
# swapcase
# upcase!
# swapcase!

havewant = HaveWant.new([3,3,3,4,5], 5)

puts havewant.answers

# last
# pop
# length
# size
# count
# max

# and sometimes
# sample

# you can also pass arguments between what you have and what you want
# 
havewant = HaveWant.new([3,3,3,4,5], 2..3, [3,4])

puts havewant.answers

# []
# values_at
# slice
# slice!

puts HaveWant.new("foo", "bar", "foobar").answers

# +
# concat
# <<

You can use def initialize(have, *args, want) instead: http://ideone.com/V0pVF0