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Turn Rows Into Columns With The Transpose Method

Today you are going to learn how to deal with grids in Ruby using the Ruby transpose method.

Imagine that you have a perfect grid, let’s say a 3×3 square, in the form of a multi-dimensional array.

And you want to take the rows & convert them into columns.

Why would you want to do that?

One use is for the classic game: tic-tac-toe.

You store your board as a grid. And to find a winning move you have to check the rows, columns & diagonals.

The problem is that if you are storing your grid as an array you only get direct access to the rows.

Columns The Hard Way

By “direct access” I mean that you can go over your array (with each, map, etc.) without having to use more array indexing than necessary.

Let’s see an example!

Here’s a grid:

grid = [
  [1,2,3],
  [4,5,6],
  [7,8,9]
]

Here’s a visual I made for you:

ruby transpose method

You could get the columns by referencing the indexes.

For example, the first column would be:

[grid[0][0], grid[1][0], grid[2][0]]

# [1, 4, 7]

But the first row is just this:

grid[0]

# [1, 2, 3]

How can we make working with columns as easy as working with rows?

Columns The Easy Way

The easy way to do this is by using the Array#transpose method.

Example:

columns = grid.transpose

Yes, that’s all you have to do!

Now you can get the first column like this:

columns[0]

# [1, 4, 7]

As you can see knowing a lot of methods can save you a lot of work 🙂

Tic-Tac-Toe

I’m not going to explain the whole thing. I just want to show you how this method can apply to a real project.

To win a game of tic-tac-toe you need to fill a row, a column or a diagonal.

Here’s the code for checking rows:

def check_rows
  @board.each { |row| return row.first if all_equal?(row) }
end

And here’s is the code for columns:

def check_columns
  @board.transpose.each { |row| return row.first if all_equal?(row) }
end

Notice how the only difference is the transpose method!

Here’s the all_equal? method:

def all_equal?(row)
   return if row.first == nil

   row.each_cons(2).all? { |x,y| x == y }
 end

You can learn more about that each_cons method by reading this post on Enumerable methods.

Summary

You have learned about the transpose method.

Given a perfect grid, transpose allows you to transform the rows into columns for easy access.

If you enjoyed this post you may want to consider buying a copy of my book Ruby Deep Dive.