XML Reference Guide

Mar 14, 2003

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At this point, we have the basics of the application, so let's take things a step farther. Aside from storing data, the most common thing you'll do with an array is loop through it looking for something. Ruby provides a number of different iterators, which you can use for any structure that involves multiple items, such as an array, list, and so on.

Will start with the each iterator.

The `each` Iterator

Now it would, of course, be useful for us to be able to iterate through a list of objects. As this is a topic that comes up most frequently with arrays, the let's look to arrays to solve the problem.

Arrays have available to them the each iterator, which literally enables you to iterate through each item in a list. Let's take a look at how it works:

...
puts "The second question is: " 
puts survey.questions[1].questionText
puts "The third answer of the second question is: "
puts survey.questions[1].answers[2].answerText

sequelAnswers.each do |thisAnswer|
   puts thisAnswer.answerText
end

Let's take this apart bit by bit. We start with the array, from which we call the iterator. For each run through the iterator, we want to do something. For that, we are creating a block between the do keyword and the end keyword. Into the block, we are passing the current item for the iterator. We are passing it as thisAnswer, as indicated by the pipe symbols (|).

Each time we run through the block, we can output the answerText attribute of the thisAnswer object. If we execute the script, it looks like this:

Survey is called Retitled Survey
The second question is: 
What is your favorite movie series?
The third answer of the second question is: 
Halloween
Star Wars
Lord of the Rings
Halloween
None of the above

As it happens, we can also nest these blocks:

...
puts "The second question is: " + survey.questions[1].questionText
puts "The third answer of the second question is: " + survey.questions[1].answers[2].answerText

survey.questions.each do |thisQuestion|

   puts thisQuestion.questionText
   thisQuestion.answers.each do |thisAnswer|
      puts "   " + thisAnswer.answerText
   end

end

In this case, we iterate through each of the questions that have been added to the survey, and for each one we output the text, and then iterate through the answers. The result gives us a pretty complete look at the survey:

Survey is called Retitled Survey
The second question is: What is your favorite movie series?
The third answer of the second question is: Halloween
Do you like movie sequels?
   yes
   no
What is your favorite movie series?
   Star Wars
   Lord of the Rings
   Halloween
   None of the above

In fact, this would be useful as the string version for the survey itself:

class Survey
   attr_reader :title, :questions
   attr_writer :title

   def initialize (givenTitle)
      @title = givenTitle
      @questions = Array.new
   end
   
   def to_s
      stringToReturn = "Survey: " + @title
      @questions.each do |thisQuestion|
         stringToReturn = stringToReturn + "\n" + thisQuestion.questionText
         thisQuestion.answers.each do |thisAnswer|
              stringToReturn = stringToReturn + "\n   " + thisAnswer.answerText
         end
      end
      return stringToReturn

   end
   
   def addQuestion(question)
      @questions.push(question)
   end
   
end
...
puts "The second question is: " + survey.questions[1].questionText
puts "The third answer of the second question is: " + survey.questions[1].answers[2].answerText

puts survey

which gives us:

Survey is called Retitled Survey
The second question is: What is your favorite movie series?
The third answer of the second question is: Halloween
Survey: Retitled Survey
Do you like movie sequels?
   yes
   no
What is your favorite movie series?
   Star Wars
   Lord of the Rings
   Halloween
   None of the above

Persisting Objects

Now, I actually had an ulterior motive in creating such a complete string version of the survey, and that's the fact that we are now going to look at saving the object to a file and getting it back out again.

As it happens, saving an object to a file is actually quite straightforward:

...
puts "The second question is: " + survey.questions[1].questionText
puts "The third answer of the second question is: " + survey.questions[1].answers[2].answerText

puts "\n"
puts survey

File.open("savedSurvey", "w+") do |f|
  Marshal.dump(survey, f)
end

newlyLoadedSurvey = nil
File.open("savedSurvey") do |f|
  newlyLoadedSurvey = Marshal.load(f)
end

puts "\nThe new survey is: \n\n"
puts newlyLoadedSurvey

The first step is to get a writable reference to the new file in which we are going to store the object. Notice that as before, the object that is returned by the method, in this case open, is passed to the reference in the pipe symbols, in this case f.

From there, we can use the Marshal class's class method (think static), dump, to save a representation of the object to the file. To get it out again, we create a new object, reopen the file, and load the file's contents into it. Notice that we have not set the type for the newlyLoadedSurvey object; all of the information comes from the persisted data. We can then print the string representation of the object to the command line, as before:

Survey is called Retitled Survey
The second question is: What is your favorite movie series?
The third answer of the second question is: Halloween

Survey: Retitled Survey
Do you like movie sequels?
   yes
   no
What is your favorite movie series?
   Star Wars
   Lord of the Rings
   Halloween
   None of the above

The new survey is:

Survey: Retitled Survey
Do you like movie sequels?
   yes
   no
What is your favorite movie series?
   Star Wars
   Lord of the Rings
   Halloween
   None of the above

Taking Stock

Okay, let's recap. We currently have a Ruby application with three classes, two of which exist as arrays stored and other objects. We looked at basic tasks such as getting information from the command line and outputting it back to the command line, iterating through arrays, and persisting objects in files.

Next, we'll look at putting this Ruby application on Rails.

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