XML Reference Guide

Mar 14, 2003

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So far we've created two simple classes and their objects, and looked at getter and setter methods. We've also touched on input and output, requesting input from and printing strings to the command line.

Now let's up the ante a little bit.

Printing Objects

So far, we've just printed simple strings. But what happens if we have other types of data to print, such as numbers or objects? For example, let's create an array:

...
puts "Survey is called #{survey.title}"

a = Answer.new("yes")
b = Answer.new("no") 

answers = [a, b]

puts "So far we have " + answers.length + " answers."

Here I've created two answers, and added them to an array called answers. This array is like the arrays you work with than any other language, in that in addition to adding and removing items, you can also find out how many items it has, in this case by getting the length. There is, however, a problem here:

Survey is called Retitled Survey
demo.rb:40:in `+': cannot convert Fixnum into String (TypeError)
        from demo.rb:40

Ruby does not like the fact that we are attempting to put a number into a string. Unlike some other languages, it does not implicitly perform this conversion. So instead, we'll have to do it ourselves. Fortunately, numbers are also objects, so we have access to the to_s method:

...
puts "Survey is called #{survey.title}"

a = Answer.new("yes")
b = Answer.new("no")

answers = [a, b]

puts "So far we have " + answers.length.to_s + " answers."

Now we can run it:

Survey is called Retitled Survey
So far we have 2 answers.

What if we try to print other objects? For example, we can print the survey:

...
puts survey

This prints the object all right, but it's not very pretty:

Survey is called Retitled Survey
So far we have 2 answers.
#<Survey:0x2aa6f08>

But is there anything we can do about this? Yes, there is. All classes are extended from a single root object type, and it is from this ultimate superclass that they get their implementation of to_s, which is how they are output to the commandline. As in other object-oriented languages, however, we can override that method with our own implementation:

 class Survey
   attr_reader :title
   attr_writer :title

   def initialize (givenTitle)
      @title = givenTitle
   end
   
   def to_s
      return "Survey: " + @title
   end
   
end
...

In fact, we can do pretty much anything with this method, such as adding other information, recording how many times it's been printed, and so on. It is a method like any other.

Completing the Structure

The only piece we have left to add is the Question class:

...
   def to_s
      return "Survey: " + @title
   end
end

class Question
   attr_reader :questionText, :answers
   attr_writer :questionText
   def initialize (theQuestion, theAnswers)
      @questionText = theQuestion
      @answers = theAnswers
   end
end

class Answer
   attr_reader :answerText, :responses
...
a = Answer.new("yes")
b = Answer.new("no")

answers = [a, b]

firstQuestion = Question.new("Do you like movie sequels?", answers)

a = Answer.new("Star Wars")
b = Answer.new("Lord of the Rings")
c = Answer.new("Halloween")
d = Answer.new("None of the above")

sequelAnswers = [a, b, c, d]

secondQuestion = Question.new("What is your favorite movie series?", sequelAnswers)

Starting with the Question definition, we have a class with two attributes: the actual text of the question, and an array of answers. Moving down to the actual script, we see that we are creating two arrays of Answer objects, and then creating Question objects that accept them as parameters.

You may be wondering about the reuse of the a and b variables. We can do this because in Ruby, all variables are actually references. When we changed the references to point to new objects, it was the new objects that were added to the array.

Now we need to add the questions to the actual survey:

class Survey
   attr_reader :title
   attr_writer :title

   def initialize (givenTitle)
      @title = givenTitle
      @questions = Array.new

   end
   
   def to_s
      return "Survey: " + @title
   end
   
   def addQuestion(question)
      @questions.push(question)
   end
   
end

class Question
   attr_reader :questionText, :answers
...
secondQuestion = Question.new("What is your favorite movie series?", sequelAnswers)

survey.addQuestion(firstQuestion)
survey.addQuestion(secondQuestion)

Starting at the bottom, we can use the addQuestion method to add the question to the survey, and within that method definition, we see the standard push method so common in languages dealing with arrays. This method adds the item to the array. Ruby also has support for the shift and pop methods, which remove items from the front and back of the array, respectively.

Of course, in order for Ruby to know that this method is available, it needs to know that @questions is an array, so we'll declare it in the constructor.

But how do we know this is really working? We can check things by requesting specific items:

...
survey.addQuestion(firstQuestion)
survey.addQuestion(secondQuestion)

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

In the first case, we are requesting the second item in the @questions attribute. (Ruby arrays are zero-based.) Once we have a reference to that object, which is, remember, a Question object, we can request its questionText attribute.

In the second case, we request a reference to that same Question object, but then we are requesting a reference to the third item in its answers array. We can then retrieve that object's the answerText attribute.

Running the script 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

So now we have the basic structure for our application, and we know to how to create and manipulate arrays and their individual items. Next, we'll look at automating things a bit more using arrays.

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