Alex Rivera | Logout

Setting ruby hash .default to a list

Asked 2008-10-10T10:28:53.080
43

I thought I understood what the default method does to a hash...

Give a default value for a key if it doesn't exist:

irb(main):001:0> a = {}
=> {}
irb(main):002:0> a.default = 4
=> 4
irb(main):003:0> a[8]
=> 4
irb(main):004:0> a[9] += 1
=> 5
irb(main):005:0> a
=> {9=>5}

All good.

But if I set the default to be a empty list, or empty hash, I don't understand it's behaviour at all....

irb(main):001:0> a = {}
=> {}
irb(main):002:0> a.default = []
=> []
irb(main):003:0> a[8] << 9
=> [9]                          # great!
irb(main):004:0> a
=> {}                           # ?! would have expected {8=>[9]}
irb(main):005:0> a[8]
=> [9]                          # awesome!
irb(main):006:0> a[9]
=> [9]                          # unawesome! shouldn't this be [] ??

I was hoping/expecting the same behaviour as if I had used the ||= operator...

irb(main):001:0> a = {}
=> {}
irb(main):002:0> a[8] ||= []
=> []
irb(main):003:0> a[8] << 9
=> [9]
irb(main):004:0> a
=> {8=>[9]}
irb(main):005:0> a[9]
=> nil

Can anyone explain what is going on?

ruby hashmap
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2 Answers

51

Hash.default is used to set the default value returned when you query a key that doesn't exist. An entry in the collection is not created for you, just because queried it.

Also, the value you set default to is an instance of an object (an Array in your case), so when this is returned, it can be manipulated.

a = {}
a.default = []     # set default to a new empty Array
a[8] << 9          # a[8] doesn't exist, so the Array instance is returned, and 9 appended to it
a.default          # => [9]
a[9]               # a[9] doesn't exist, so default is returned
answered 2008-10-10T10:48:54.817
34

I think this is the behavior you are looking for. This will automatically initialize any new keys in the Hash to an array:

irb(main):001:0> h = Hash.new{|h, k| h[k] = []}
=> {}
irb(main):002:0> h[1] << "ABC"
=> ["ABC"]
irb(main):003:0> h[3]
=> []
irb(main):004:0> h
=> {1=>["ABC"], 3=>[]}
answered 2008-10-11T16:39:26.407

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