Check hash if key exists; if not, create array and append value

Question

Example as follows:

if house['windows'][floor_1]
  house['windows'][floor_1] << north_side
else
  house['windows'][floor_1] = [north_side]
end

Best way to check for existing key?

Possible duplicate of [Creating a Hash with values as arrays and default value as empty array](https://stackoverflow.com/questions/30367487/creating-a-hash-with-values-as-arrays-and-default-value-as-empty-array) — user513951, Feb 16 '19 at 15:31
One common way of doing this (which some claim is faster than using a default proc): `h = {}; h[:a] = (h[:a] || []) << 1 #=> [1]; h[:a] = (h[:a] || []) << 2 #=> [1, 2]`. — Cary Swoveland, Feb 16 '19 at 18:46

user513951 · Answer 1 · 2019-02-16T22:32:46.123

4

The fact that house['windows'] is an element in a hash already is a bit of a red herring, so I will use windows as a variable referencing a hash.

Set up a default value for the windows hash, so that any non-preexisting key is assigned an array value:

windows = Hash.new {|hash, key| hash[key] = [] }

Now you can append (<<) to new hash elements automatically.

windows['floor_1'] << 'north_side'

windows # => {"floor_1"=>["north_side"]}

For your specific case, replace windows with house['windows'].

EDIT

As pointed out in the comments, this behavior can be added to an already-instantiated hash:

windows.default_proc = proc {|hash, key| hash[key] = [] }

edited Feb 16 '19 at 22:32

answered Feb 16 '19 at 15:26

user513951

1

What to do if `windows` is external data and we can't create it? – mechnicov Feb 16 '19 at 16:02
1

@mechnicov Then you can set the default proc with [`#default_proc`](https://ruby-doc.org/core-2.6.1/Hash.html#method-i-default_proc-3D). `windows.default_proc = proc { |hash, key| hash[key] = [] }` – 3limin4t0r Feb 16 '19 at 16:12
@JohanWentholt, good idea. I asked this question so that user513951 improves the answer. – mechnicov Feb 16 '19 at 16:18

arieljuod · Answer 2 · 2021-08-27T21:31:27.093

2

I would do something like:

house['windows'][floor_1] ||= []
house['windows'][floor_1] << north_side

edited Aug 27 '21 at 21:31

answered Feb 16 '19 at 23:27

arieljuod

iGian · Answer 3 · 2019-02-16T16:12:37.610

Given your Hash, I imagine:

house = { windows: { floor_0: ['f0'] } }

You can check the existence of a key using Hash#has_key?

house[:windows].has_key? :floor_1 #=> false

So you can create it:

house[:windows].merge!({floor_1: []}) unless house[:windows].has_key? :floor_1

Better if you define a defalt value using for example Hash#default_proc=:

house[:windows].default_proc = proc { |h, k| h[k] = [] }

So you can

house[:windows][:floor_3] << 'f3'

house #=> {:windows=>{:floor_0=>["f0"], :floor_1=>[], :floor_3=>["f3"]}}

3 Answers3