What does def <=>( b ) mean?

I'm checking out a class that has the following method definition:

Code:

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def <=>(b)

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I've never seen a

Code:

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<=>

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definition before. what does it mean?

Thanks
--
Jimmy Z

Hi.

This nifty little syntax is called the general comparison operator. Or more affectionately, the "spaceship" operator.

The Pragmatic Programmer's guide defines it as:

Quote:

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...the spaceship operator, <=> compares two values, returning -1, 0, or +1 depending on whether the first is less than, equal to, or greater than the second.

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The use of this operator is that you can teach ruby how to compare two objects with values that are not intuitively apparent. Suppose you have a Building object, and you want to define one object's superiority over another in terms of their height--without having to expose their instance variables. To set this up, you would first include the module "Comparable" in your class declaration and override it's <=> function:

Code:

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class Building
  include Comparable

  def <=> (other_building)
    self.height <=> other.height 
    #since "height" is presumably an integer, just call this method again
    #because integer objects already know how to compare themselves.
  end
 
  #or if you need a more custom operation...

  def <=> (other)
    if self.height < other.height-50
      -1
    elsif self.height > other.height+50
      1
    else
      0
    end
    # this would find two buildings equal if they were within 50 units of
    # each other in height. Otherwise it would return the proper result if
    #one is more or less than 50 units in height from the other.
  end


end


#then you use could this to compare any Building object using standard comparison operators.

if my_building > your_building
  puts "My building is taller"
end

---

You can rewrite the alphabet to put u and i together.

I try to use

Code:

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def +
# .... addition routine
end

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but it seems doesn't work. should I use alias

If you are defining your own + method it will need a parameter (the object you are adding).

Otherwise use +@.

Code:

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class Something
  def +@
    "hi"
  end
end

+Something.new  # => "hi"

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This is handy for DSLs.