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Convert Rows to Columns with Ruby's Transpose Method

meaganewaller profile image Meagan Waller Originally published at meaganwaller.com ・2 min read

The first time I used the transpose method, I was solving a particular problem. I was creating a tic-tac-toe game during my software apprenticeship. I had split up the game into a few classes. I had a class called Board that represented the tic-tac-toe board and was responsible for holding the board's state after each move. The Board class was initialized with an array called spaces to represent the board's spaces; the default was an array with nine spaces [0, 1, 2, 3, 4, 5, 6, 7, 8]. I had a method that would return the rows named rows.

def rows
  [@spaces[0..2],
   @spaces[3..5],
   @spaces[6..8]]
end
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I also needed a method to give me the columns. I could have written something like this.

def columns
  [@spaces[0], @spaces[3], @spaces[6],
   @spaces[1], @spaces[4], @spaces[7],
   @spaces[2], @spaces[5], @spaces[8]]
end
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But instead, my method took advantage of Ruby's transpose method.

def columns
  rows.transpose
end
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Transpose turned my rows into columns, and this was a fair use case for this because I was able to create a method that held all the possible winning solutions for a game. You win by getting your marker in 3 consecutive spaces in tic-tac-toe, either across a row, a column or diagonally.

def winning_solutions
  winning_solutions = []
  rows.map { |row| winning_solutions << row }
  columns.map { |col| winning_solutions << col }
  winning_solutions << diagonal_ltr
  winning_solutions << diagonal_rtl
end

def diagonal_ltr
  [@spaces[0], @spaces[4], @spaces[8]]
end

def diagonal_rtl
  [@spaces[2], @spaces[4], @spaces[6]]
end
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(Disclaimer for the above code: I wrote this code only months into my software journey, I would do things differently now.)

Transposing is perfect when you have a grid and want to transform the rows into columns for easy access.

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