Making an object initialize to nil

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August 16th, 2007, 06:55 PM

dustpyle_x3

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Making an object initialize to nil

I'm doing an assignment from the Learning Ruby tutorial, and I'm having a bit of a problem. I have a student class that holds a first name, a last name, and an age; pretty generic. Now I figure that should an empty string be given for one of the names, or a value less than zero for the age, then the object should be nil, since there is no apparent logical way to just substitute the invalid values with some generic value. Thing is, I'm not sure how to initialize an object to nil. Here's my failed attempt:

Student class:

Ruby Code:

Original - Ruby Code
class Student attr_reader :f_name attr_reader :l_name attr_reader :age def initialize(f_name,l_name,age) if f_name.empty? \|\| f_name == nil \|\| l_name.empty? \|\| l_name == nil \|\| age < 0 return nil end @f_name = f_name @l_name = l_name @age = age end def full_name return f_name + ' ' + l_name end end

class Student
    attr_reader :f_name
    attr_reader :l_name
    attr_reader :age
    
    def initialize(f_name,l_name,age)
        if f_name.empty? || f_name == nil || l_name.empty? || l_name == nil || age < 0
            return nil
        end
        @f_name = f_name
        @l_name = l_name
        @age = age
    end
    
    def full_name
        return f_name + ' ' + l_name
    end
end

TestStudent class:

Ruby Code:

Original - Ruby Code

require 'test/unit'
require 'student'
class TestStudent < Test::Unit::TestCase
    def setup
        @obj1 = Student.new('Dustin','Pyle',17)
        @obj2 = Student.new('','Shulz',12)
        @obj3 = Student.new('Jamey','',17)
        @obj4 = Student.new('Benny','Rutledge',0)
    end
    
    def test_first_name
        assert_equal('Dustin',@obj1.f_name)
        assert_nil(@obj2)
    end
    
    def test_last_name
        assert_equal('Pyle',@obj1.l_name)
        assert_nil(@obj3)
    end
    
    def test_age
        assert_equal(17,@obj1.age)
        assert_nil(@obj4)
    end
    
    def test_full_name
        assert_equal('Dustin Pyle',@obj1.full_name)
        assert_nil(@obj2)
        assert_nil(@obj3)
        assert_nil(@obj4)
    end
end

The test objects are obviously not initializing to nil since I always get failures. How do I go about this?

August 17th, 2007, 06:42 AM

L7Sqr

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new is used to create an object, not to assign it a value. If you look at what Object.new returns it is not a 'value' as you suspect:

Code:

irb(main):002:0> class Test
irb(main):003:1> def initialize(val)
irb(main):004:2> nil if val < 0
irb(main):005:2> val
irb(main):006:2> end
irb(main):007:1> end
=> nil
irb(main):008:0> Test.new(42)
=> #<Test:0xb7f41a84>
irb(main):009:0>

So you have a couple of options, you can make a wrapper to the class and do your checks there

Code:

def wrapper(fname,lname,age)
   nil || Student.new(...) if not [fname,lname].include? nil and age > 0
end

Or a less elegant way is to have a valid 'flag' within the Student class itself

Code:

class Student
   def initialize(...)
      @valid = true if ...
      # ...
   end
end

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August 17th, 2007, 03:56 PM

dustpyle_x3

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Join Date: Nov 2006

Posts: 82

Time spent in forums: 2 Days 3 h 48 m 14 sec
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Ah, I see. Thanks for the help.

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