How to change value of an element in two dimensional array in ruby?

Question

I have created a two dimensional array and the whole 2D array is populated with 9 like this.

matrix = Array.new(5,(Array.new(5,9)))

Next I am printing the whole array

puts "#{matrix}" # => [[9, 9, 9, 9, 9], [9, 9, 9, 9, 9], [9, 9, 9, 9, 9], [9, 9, 9, 9, 9], [9, 9, 9, 9, 9]]

Next I am assigning 1 to the [0][0] position.

matrix[0][0] = 1

Then I am again printing the matrix

puts "#{matrix}" # => [[1, 9, 9, 9, 9], [1, 9, 9, 9, 9], [1, 9, 9, 9, 9], [1, 9, 9, 9, 9], [1, 9, 9, 9, 9]]

So, here is the case! Why every row is being affected by this assignment. Shouldn't it only change the value of [0][0] position. I am using ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-linux].

The clue is in the output. See what matrix.each {|a| puts a.object_id } returns. — Sagar Pandya
– Sagar Pandya, Commented Aug 11, 2016 at 14:17
matrix = Array.new(5, Array.new(5,9)) (your extra parens are not needed) is the same as arr = Array.new(5,9); matrix = Array.new(5, arr). Got it? — Cary Swoveland
– Cary Swoveland, Commented Aug 11, 2016 at 17:11

Ursus · Accepted Answer · 2016-08-11 14:13:58Z

7

Basically, you are using the same array reference for every subarray. Do it like this

matrix = Array.new(5) { Array.new(5, 9) }

answered Aug 11, 2016 at 14:13

Ursus

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1 Comment

Parnab Sanyal Over a year ago

ya I have understood what's happening with my declaration. Your declaration worked as charm!

Martin · Accepted Answer · 2016-08-11 14:18:25Z

1

The problem is that you are not creating 5 different arrays:

matrix = Array.new(5,(Array.new(5,9)))

this code is creating a new array which is then used five times. So, when you set the cell of the first array to 0, you actually set them all to 0.

To fix this, you need to create individual array, for example this way:

matrix = []

5.times do 
  matrix.push(Array.new(5,9))
end

Then the code will work the way you expect:

   matrix[0][0] = 5
   puts matrix #  [[5, 9, 9, 9, 9], [9, 9, 9, 9, 9], [9, 9, 9, 9, 9], [9, 9, 9, 9, 9], [9, 9, 9, 9, 9]]

answered Aug 11, 2016 at 14:18

Martin

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1 Comment

Cary Swoveland Over a year ago

That works but the idiomatic way to create matrix is as given by @ursus.

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