Why does this code populate every array?

Question

I have this piece of code that creates an array of arrays ([[],[],[]]) and an iterator that attempts to populate the respective arrays with numbers (shown below)

array = Array.new(3,[])
10.times do
  array[0] << 2
  array[1] << 3
  array[2] << 4
end

When I execute this code, I expected to see this

[[2,2,2,2,etc....],[3,3,3,3,etc...],[4,4,4,4,4...etc]]

but instead I get this:

[[2,3,4,2,3,4,2,3,4....repeat],[2,3,4,2,3,4,2,3,4....repeat],[2,3,4,2,3,4,2,3,4....repeat]]

I've tried to walk through it with byebug and it makes no sense to me. What is going on here?

array.map(&:object_id) can be illuminating.

tadman
– tadman

2016-08-31 03:44:08 +00:00
Commented Aug 31, 2016 at 3:44 — tadman
– tadman, Commented Aug 31, 2016 at 3:44

Amadan · Accepted Answer · 2016-08-31 03:39:49Z

5

Array.new(3, []) is not equivalent to [[],[],[]]. It is equivalent to array = []; [array, array, array], an array of three references to the same array. After array[0] << 2, you have [[2], [2], [2]], because all three elements point at the same array.

What you want is Array.new(3) { [] }, with a block specifying the default; this way, a new [] is created for each element.

Even better, your whole code can be written as:

Array.new(3) { |i| Array.new(10, i + 2) }

answered Aug 31, 2016 at 3:39

Amadan

200k23 gold badges252 silver badges321 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Michael Kohl Over a year ago

Or even Array.new(3) { |i| [i + 2] * 10 } which is shorter to type and some people may find more intuitive.

Michael Kohl · Accepted Answer · 2016-08-31 03:42:23Z

1

The initializer you use is meant to be used with immutable objects, like creating an array of n times the same integer. For mutable objects use the block version. Here's the solution to your problem:

array = Array.new(3) { [] }

10.times do
  array[0] << 2
  array[1] << 3
  array[2] << 4
end

array[0]
#=> [2, 2, 2, 2, 2, 2, 2, 2, 2, 2]

answered Aug 31, 2016 at 3:42

Michael Kohl

66.9k14 gold badges143 silver badges161 bronze badges

Comments

Waqar Ul Aziz · Accepted Answer · 2016-08-31 03:47:42Z

1

Read the documentation

The second argument populates the array with references to the same object. Therefore, it is only recommended in cases when you need to instantiate arrays with natively immutable objects such as Symbols, numbers, true or false.

To create an array with separate objects a block can be passed instead. This method is safe to use with mutable objects such as hashes, strings or other arrays:

Array.new(4) { Hash.new } #=> [{}, {}, {}, {}]

This is also a quick way to build up multi-dimensional arrays:

empty_table = Array.new(3) { Array.new(3) }

Therefore, your code first line should be

array = Array.new(3) { Array.new(3) }

answered Aug 31, 2016 at 3:47

Waqar Ul Aziz

1972 silver badges9 bronze badges

Collectives™ on Stack Overflow

Why does this code populate every array?

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related