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When to use Struct instead of Hash in Ruby?

Posted by: admin November 30, 2017 Leave a comment

Questions:

I don’t have much programming experience. But, to me, Struct seems somewhat similar to Hash.

  • What can Struct do well?
  • Is there anything Struct can do, that Hash cannot do?

After googling, the concept of Struct is important in C, but I don’t know much about C.

Answers:

Structs differ from using hashmaps in the following ways (in addition to how the code looks):

  • A struct has a fixed set of attributes, while you add new keys to a hash.
  • Calling an attribute that does not exist on an instance of a struct will cause a NoMethodError, while getting the value for a non-existing key from a hash will just return nil.
  • Two instances of different structs will never be equal even if the structs have the same attributes and the instances have the same values (i.e. Struct.new(:x).new(42) == Struct.new(:x).new(42) is false, whereas Foo = Struct.new(:x); Foo.new(42)==Foo.new(42) is true).
  • The to_a method for structs returns an array of values, while to_a on a hash gets you an array of key-value-pairs (where “pair” means “two-element array”)
  • If Foo = Struct.new(:x, :y, :z) you can do Foo.new(1,2,3) to create an instance of Foo without having to spell out the attribute names.

So to answer the question: When you want to model objects with a known set of attributes, use structs. When you want to model arbitrary use hashmaps (e.g. counting how often each word occurs in a string or mapping nicknames to full names etc. are definitely not jobs for a struct, while modeling a person with a name, an age and an address would be a perfect fit for Person = Struct.new(name, age, address)).

As a sidenote: C structs have little to nothing to do with ruby structs, so don’t let yourself get confused by that.

Questions:
Answers:

I know this question was almost well-answered, but surprisingly nobody has talked about one of the biggest differences and the real benefits of Struct. And I guess that’s why somebody is still asking.

I understand the differences, but what’s the real advantage to using a Struct over a Hash, when a Hash can do the same thing, and is simpler to deal with? Seems like Structs are kind of superfluous.

Struct is faster.

require 'benchmark'

Benchmark.bm 10 do |bench|
  bench.report "Hash: " do
    50_000_000.times do { name: "John Smith", age: 45 } end
  end

  bench.report "Struct: " do
    klass = Struct.new(:name, :age)
    50_000_000.times do klass.new("John Smith", 45) end
  end

end

# ruby 2.2.2p95 (2015-04-13 revision 50295) [x64-mingw32].
#                 user     system      total        real
# Hash:       22.340000   0.016000  22.356000 ( 24.260674)
# Struct:     12.979000   0.000000  12.979000 ( 14.095455)

# ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-darwin11.0]
# 
#                  user     system      total        real
# Hash:       31.980000   0.060000  32.040000 ( 32.039914)
# Struct:     16.880000   0.010000  16.890000 ( 16.886061)

Questions:
Answers:

From the Struct documentation:

A Struct is a convenient way to bundle a number of attributes together, using accessor methods, without having to write an explicit class.

On the other hand, a Hash:

A Hash is a collection of key-value pairs. It is similar to an Array, except that indexing is done via arbitrary keys of any object type, not an integer index. The order in which you traverse a hash by either key or value may seem arbitrary, and will generally not be in the insertion order.

The main difference is how you access your data.

ruby-1.9.1-p378 > Point = Struct.new(:x, :y)
 => Point 
ruby-1.9.1-p378 > p = Point.new(4,5)
 => #<struct Point x=4, y=5> 
ruby-1.9.1-p378 > p.x
 => 4 
ruby-1.9.1-p378 > p.y
 => 5 
ruby-1.9.1-p378 > p = {:x => 4, :y => 5}
 => {:x=>4, :y=>5} 
ruby-1.9.1-p378 > p.x
NoMethodError: undefined method `x' for {:x=>4, :y=>5}:Hash
    from (irb):7
    from /Users/mr/.rvm/rubies/ruby-1.9.1-p378/bin/irb:17:in `<main>'
ruby-1.9.1-p378 > p[:x]
 => 4 
ruby-1.9.1-p378 > p[:y]
 => 5 

In short, you would make a new Struct when you want a class that’s a “plain old data” structure (optionally with the intent of extending it with more methods), and you would use a Hash when you don’t need a formal type at all.

Questions:
Answers:

One more main difference is you can add behavior methods to a Struct.

 Customer = Struct.new(:name, :address) do

  def greeting; "Hello #{name}!" ; end

end

Customer.new("Dave", "123 Main").greeting  # => "Hello Dave!"

Questions:
Answers:

If you’re just going to encapsulate the data, then a Hash (or an Array of Hashes) are fine. If you’re planning to have the data manipulate or interact with other data, then a Struct can open some interesting possibilities:

Point = Struct.new(:x, :y)
point_a = Point.new(0,0)
point_b = Point.new(2,3)

class Point
  def distance_to another_point
    Math.sqrt((self.x - another_point.x)**2 + (self.y - another_point.y)**2)
  end
end

puts point_a.distance_to point_b