Using Symbols as Hash Keys

2017-11-03 09:05:06

Credit: Ben Giddings

Problem

When using a hash, you want the slight optimization you can get by using symbols as keys instead of strings.

Solution

Whenever you would otherwise use a quoted string, use a symbol instead. A symbol can be created by either using a colon in front of a word, like :keyname, or by transforming a string to a symbol using String#intern.

people = Hash.new people[:nickname] = 'Matz' people[:language] = 'Japanese' people['last name'.intern] = 'Matsumoto' people[:nickname] # => "Matz" people['nickname'.intern] # => "Matz"

Discussion

While 'name' and 'name' appear exactly identical, they're actually different. Each time you create a quoted string in Ruby, you create a unique object. You can see this by looking at the object_id method.

'name'.object_id # => -605973716 'name'.object_id # => -605976356 'name'.object_id # => -605978996

By comparison, each instance of a symbol refers to a single object.

:name.object_id # => 878862 :name.object_id # => 878862 'name'.intern.object_id # => 878862 'name'.intern.object_id # => 878862

Using symbols instead of strings saves memory and time. It saves memory because there's only one symbol instance, instead of many string instances. If you have many hashes that contain the same keys, the memory savings adds up.

Using symbols as hash keys is faster because the hash value of a symbol is simply its object ID. If you use strings in a hash, Ruby must calculate the hash value of a string each time it's used as a hash key.

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