将元素推送到红宝石哈希中的数组上

Question

In Ruby, to create a hash of arrays and push elements onto those arrays, I've seen two idioms. I'd like to know which one people prefer, and why. (Disclosure: I have my own opinion, but I want to make sure I'm not missing something obvious.)

Approach 1: use Hash's fancy initializer:

ht = Hash.new {|h,k| h[k]=[]}
ht["cats"] << "Jellicle"
ht["cats"] << "Mr. Mistoffelees"

This approach creates an empty array when you access ht with a key that doesn't yet exist.

Approach 2: simple initializer, fancy accessor:

ht = {}
(ht["cats"] ||= []) << "Jellicle"
(ht["cats"] ||= []) << "Mr. Mistoffelees"

Do people have an opinion on which one is better (or situations where one is preferred over the other)?

Answer 1

Sometimes the hash is initially filled with data and later on it is only used to retrieve data. In those cases I prefer the first possibility, because the default proc can be "emptied" (in Ruby 1.9).

ht = Hash.new {|h,k| h[k]=[]}
ht["cats"] << "Jellicle"
ht["cats"] << "Mr. Mistoffelees"
ht["dogs"]
p ht
#=> {"cats"=>["Jellicle", "Mr. Mistoffelees"], "dogs"=>[]}

ht.default_proc = proc{}
ht["parrots"] #nil
p ht
#=> {"cats"=>["Jellicle", "Mr. Mistoffelees"], "dogs"=>[]} No parrots!

Answer 2

If you know in advance the number and the name for each key, then you can use the first option. Or even a simpler one

ht = { "cats" => [] }

Otherwise, if you don't want (need) to preinitialize the hash, the second option is a good choice.

Answer 3

In the OP, I said I had my own opinion. Here it is.

Although the "fancy initializer" approach is elegant, it can lead to some really unexpected behavior -- specifically generating keys when you don't expect it -- and there's no way to know this by looking at the hash table.

Consider the following:

>> ht1 = Hash.new {|h,k| h[k]=[]}
>> ht2 = {}
>> ht1["cats"] << "Jellicle"
=> ["Jellicle"]
>> (ht2["cats"] ||= []) << "Jellicle"
=> ["Jellicle"]

so far so good -- ht1 and ht2 are identical. but:

>> ht1["dogs"] ? "got dogs" : "no dogs"
=> "got dogs"
>> ht2["dogs"] ? "got dogs" : "no dogs"
=> "no dogs"

Note that simply accessing ht1[some_key] changes the state of the hash table , ie it create a new entry. You might argue that the end user should always use has_key?() to test for the presence of a hash entry -- and you'd be right -- but the above usage is an accepted idiom. For the hash table to automagically create an entry would be an unexpected side effect, so you should be careful if the hash table is ever exposed to the end user. (Note, however, that steenslag's answer shows how you can turn this off.)

Answer 4

Personally, I prefer:

ht = Hash.new {|h,k| h[k]=[]}
ht["cats"] << "Jellicle"
ht["cats"] << "Mr. Mistoffelees"

It's a matter of maintenance and self-documentation to me. The hash initialization dirty-work occurs once. From then on, all assignments to the arrays in the hash occurs in a standard manner, which will result in your code looking normal.

The alternate method:

ht = {}
(ht["cats"] ||= []) << "Jellicle"
(ht["cats"] ||= []) << "Mr. Mistoffelees"

starts off looking normal, but each assignment has what I call "visual noise". We have to mentally decode what ||= [] means every time something is pushed onto an array in the hash. It's mentally fatiguing, and if you're doing it throughout the code, would lead to less elegant looking code and a constant reassessment of what the heck the code is doing at THAT spot.