ruby - Why am I having trouble creating a custom array method that returns a hash with indices of matching array values? -

i'm trying monkey patch array class method that, when passed [1, 3, 4, 3, 0, 3, 1], return hash { 1 => [0, 6], 3 => [1, 3, 5] }, key number matching to, , value array indices of matches.

here's code have far. can't tell why it's returning {1=>[0, 2, 3, 1, 2, 0], 3=>[0, 2, 3, 1, 2, 0], 0=>[0, 2, 3, 1, 2, 0]}:

class array    def dups      matches = {}     matches_index = []      self.each |i|       self[(i_index + 1)..-1].each_with_index |j, j_index|         matches_index << j_index if self[i] == self[j]       end       matches[i] = matches_index     end      matches.keep_if { |key, value| value.length > 1 }   end  end 

to improve on stas s excellent solution:

class array   def dups     (self.each_index.group_by {|i| self[i]}).keep_if{|k, v| v.size > 1}   end end 

which results in array of only duplicates.