What does this array syntax mean in Ruby?

Question

I just came across this code:

@board=Array.new(7){Array.new(7)}

I've never seen this syntax for an array in ruby and I couldn't find much on it after a search. I don't really get what's going on with the curly braces here. I was hoping someone could just give me a brief explanation. Thanks!

Read the documentation here: ruby-doc.org/core-2.2.0/Array.html#method-c-new — Wand Maker
– Wand Maker, Commented Jul 1, 2016 at 14:44
I understand in the documentation when they use an example with a hash. I'm assuming this just makes a 2-d array, but I guess I just don't understand why. — Harry B.
– Harry B., Commented Jul 1, 2016 at 14:47
It clear says ...In the last form, an array of the given size is created. Each element in this array is created by passing the element’s index to the given block and storing the return value. - what it means is that when you access any index of @board, it will be an array of 7 elements - yes its a way to create a 2D array. — Wand Maker
– Wand Maker, Commented Jul 1, 2016 at 14:51
Your question is unclear. Which part of the syntax exactly do you not understand? There are only 5 syntactic constructs in the code you posted: message sends, variable dereferences, integer literals, a block, and an assignment. Which of the 5 are you asking about? — Jörg W Mittag
– Jörg W Mittag, Commented Jul 3, 2016 at 21:45

SoAwesomeMan · Accepted Answer · 2016-07-01 14:53:10Z

3

The block-syntax of new allows you to initialize the individual array elements, optionally based on the index number. In your case, the index is not used, but all 7 array elements are initialized with a nested array of also 7 elements, so you get a 7x7 "matrix".

To illustrate:

$ irb
irb(main):001:0> Array.new(7)
=> [nil, nil, nil, nil, nil, nil, nil]

$ irb
irb(main):001:0> require 'pp'
=> true
irb(main):002:0> pp Array.new(7) {Array.new(7)}
[[nil, nil, nil, nil, nil, nil, nil],
 [nil, nil, nil, nil, nil, nil, nil],
 [nil, nil, nil, nil, nil, nil, nil],
 [nil, nil, nil, nil, nil, nil, nil],
 [nil, nil, nil, nil, nil, nil, nil],
 [nil, nil, nil, nil, nil, nil, nil],
 [nil, nil, nil, nil, nil, nil, nil]]

edited Jul 1, 2016 at 14:53

SoAwesomeMan

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answered Jul 1, 2016 at 14:48

sschuberth

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2 Comments

Harry B. Over a year ago

Okay, that makes more sense to me now. I didn't really understand what was going on under the hood I guess. Thanks!

SoAwesomeMan Over a year ago

@HarryB. Consider accepting this answer, unless you're waiting for more answers: stackoverflow.com/help/someone-answers

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