Tip #17 - Struct Your Stuff!

Mon May 12 17:31:38 -0700 2008

Ruby is a really dynamic language, and you can do a lot of cool things, one of them is a Struct (Structure) that allows you to make throw away objects that you can call methods on….

How do you use them? Simple!

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Person = Struct.new(:name, :age, :job)
bob = Person.new("John", 21, "Postman")
bob.name #=> "John"
bob.age #=> 21
bob.job #=> "Postman"

“But wait!” you say, “That looks like a class!”, well, it sort of is, and you could do the same with:

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class Person
  attr_accessor :name, :job, :age
end

bob = Person.new
bob.name = "John"
bob.age = 21
bob.job = "Postman"

bob.name #=> "John"
bob.age #=> 21
bob.job #=> "Postman"

But it is more work :)

What is a struct useful for? Well, turning a return array of arrays, into a return array of objects. This is good if you are, say, parsing a delimited file.

Say you had comma delimited file of people, you only needed to handle this once to import the data into your real Ruby on Rails Person ActiveRecord class, but you needed to do some work on the imported data before you created the Person in the database permanently, this would be perfect for a Struct.

Now, I am ignoring the CSV and FasterCSV libraries here, to keep things simple.

Lets say the file looked like this:

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# PeopleFile.csv
BOB SMITH,21,FARMER,SYDNEY AUSTRALIA
JOHN JAMES,43,TECH,AUCKLAND NEW ZEALAND
SAM SMITHERS,99,TEACHER,BRISBANE AUSTRALIA
JESSIE JAMES,14,SCHOOL KID,ADELAIDE AUSTRALIA

Now, we can see that we have a list of people, the first field is the first and last name, the second is the age, the third is the job title and the last is the location.

Lets define a structure for this:



TempPerson = Struct.new(:first_name, :last_name, :age, :job, :city, :country)

Ok, good. Now, we go ahead and split the file up:

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people = Array.new
File.read('PeopleFile.csv').each_line do |line|
  attrs = line.split(",")
  name = attrs[0].split(" ", 2)
  location = attrs[3].split(" ", 2)
  people << TempPerson.new(name[0],
                           name[1],
                           attrs[2],
                           attrs[3],
                           location[0],
                           location[1])
end

At the end of this, we have an array called people that is full of TempPerson objects, each that can be accessed by #name, #age etc.

We could then get a list of all the countries by:



people.map { |p| p.country }

Which is a lot more readable than:



people.map { |p| p[3].split(" ", 2)[1] }

And do all sorts of other goodness with this fully fledged object!

Then when you are finally ready to save this into your database:

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people.each do |p|
  Person.create(:first_name => p.first_name,
                :last_name  => p.last_name,
                :age        => p.age,
                :job        => p.job,
                :city       => p.city,
                :country    => p.country)
end

You can obviously do a lot more with Structs, this is just a start and I like my tips to be short and sweet… anyone else got some good uses for them?

blogLater

Mikel

  1. Ali Rizvi Says:
    Tue May 13 00:47:03 -0700 2008

    Did you mean Person.new instead of MyStruct.new in your second code excerpt?

    Keep up the good work! Ali

  2. mikel Says:
    Tue May 13 05:27:21 -0700 2008

    Ali, thanks, yes you were right, good catch, I fixed the example.

  3. codesnik Says:
    Tue Jun 10 07:06:27 -0700 2008

    ~> irb >> Person = Struct.new(:name, :age, :job) => Person >> bob = Person.new(:name => “John”, ?> :age => 21, ?> :job => “Postman”) => #21, :name=>”John”, :job=>”Postman”}, age=nil, job=nil> >> bob.name #=> “John” => {:age=>21, :name=>”John”, :job=>”Postman”} >> bob.age #=> 21 => nil >> bob.job #=> “Postman” => nil >>

    what’s wrong?

  4. mikel Says:
    Tue Jun 10 18:01:49 -0700 2008

    Codesink, the problem is that somehow, between me writing the code and me putting it on the blog I must have had a mental lapse…

    Structs don’t get initialized with hashes, each item has to get initialized like a method.

    Very sorry about that, I have corrected the code above. You need to do:

    Person(“John”, 21, “Postman”)

    Regards

    Mikel

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