Returning a list of indices where a certain object appears in a nested array

Question

I am trying to figure out how to form an array that collects every index of a particular object (in this case a single letter) where it appears in a nested set of arrays. For instance, using the array set below,

boggle_board = [["P", "P", "X", "A"], 
                ["V", "F", "S", "Z"],
                ["O", "P", "W", "N"],
                ["D", "H", "L", "E"]]

I would expect something like boggle_board.include?("P") to return a nested array of indices [[0,0][0,1],[2,1]]. Any ideas on how to do this?

Thiago Lewin · Accepted Answer · 2013-06-07 02:53:45Z

1

Nothing super-elegant comes to mind for me right now. This seems to work:

def indices_of(board, letter)
  indices = []

  board.each_with_index do |ar, i|
    ar.each_with_index do |s, j|
      indices.push([i, j]) if s == letter
    end
  end

  indices
end

boggle_board = [["P", "P", "X", "A"], 
                ["V", "F", "S", "Z"],
                ["O", "P", "W", "N"],
                ["D", "H", "L", "E"]]

indices_of(boggle_board, "P")
# => [[0, 0], [0, 1], [2, 1]]

edited Jun 7, 2013 at 2:53

Thiago Lewin

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answered Jun 7, 2013 at 2:47

Darshan Rivka Whittle

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2 Comments

Pavlov's_Dawg Over a year ago

Thank you! This is much nicer than the solution I came up with. Involved nested while loops...ugh.

Darshan Rivka Whittle Over a year ago

@tlewin Thanks for fixing that typo. ;)

Arup Rakshit · Accepted Answer · 2013-06-22 18:30:48Z

0

I would use Matrix#each_with_index.The below code is more Rubyistic:

require "matrix"
m = Matrix[["P", "P", "X", "A"], 
           ["V", "F", "S", "Z"],
           ["O", "P", "W", "N"],
           ["D", "H", "L", "E"]]
ar = []
m.each_with_index {|e, row, col| ar << [row,col] if e == "P"}
ar #=> [[0, 0], [0, 1], [2, 1]]

edited Jun 22, 2013 at 18:30

answered Jun 7, 2013 at 7:44

Arup Rakshit

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1 Comment

Arup Rakshit Over a year ago

@Pavlov's_Dawg you can take a look at mine also.. :) Ruby supports matrix kind of calculations. :)

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