2d array, pushing to one array shows up in all arrays?

Question

I'm new to ruby so I'm clearly misunderstanding something. I intended to create an array of size 2, where each element is itself an array, then push items to one or the other sub-array:

#!/usr/bin/env ruby
arr = Array.new(2, Array.new)

puts 'default, no entries:'
arr.each_with_index { |a, i| puts 'arr[' + i.to_s + '] = ' + a.to_s }
puts ''

puts 'adding "kitty" to arr[0]:'
arr[0].push('kitty') # add an element to the sub-array at arr[0]
arr.each_with_index { |a, i| puts 'arr[' + i.to_s + '] = ' + a.to_s }
puts ''

puts 'adding "doggy" to arr[1]:'
arr[1].push('doggy') # add an element to the sub-array at arr[1]
arr.each_with_index { |a, i| puts 'arr[' + i.to_s + '] = ' + a.to_s }

output:

default, no entries:
arr[0] = []
arr[1] = []

adding "kitty" to arr[0]:
arr[0] = ["kitty"]
arr[1] = ["kitty"]

adding "doggy" to arr[1]:
arr[0] = ["kitty", "doggy"]
arr[1] = ["kitty", "doggy"]

I would expect arr[0].push() to add the element at arr[0][0], is that wrong?

Dylan Markow · Accepted Answer · 2014-01-10 18:18:43Z

4

arr = Array.new(2, Array.new) assigns the same array to both of the new objects.

If you use a block form instead, you'll get two separate arrays as you expect:

arr = Array.new(2) { Array.new }

answered Jan 10, 2014 at 18:18

Dylan Markow

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1 Comment

Patrick Oscity Over a year ago

You can observe this with arr.map(&:object_id) #=> [70190130252680, 70190130252680]

Matt · Accepted Answer · 2014-01-10 18:18:02Z

0

It seems that you're creating an array, where each element is a reference to the one same array object:

arr = Array.new(2, Array.new)
#=> [[], []]
arr[0].push(1)
#=> [1]
arr
#=> [[1], [1]]

Instead, do

arr = [[],[]]
#=> [[], []]
arr[0].push(1)
#=> [1]
arr
#=> [[1], []]

answered Jan 10, 2014 at 18:18

Matt

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