Setting ruby hash .default to a list [duplicate]

Question

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?

Answer 1

This is a very useful idiom:

(myhash[key] ||= []) << value 

It can even be nested:

((myhash[key1] ||= {})[key2] ||= []) << value 

The other way is to do:

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

But this has the significant side-effect that asking about a key will create it, which renders has_key? fairly useless, so I avoid this method.

Answer 2

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