Question

I am trying to learn Ruby, and want to implement the Python algorithms from the book "Programming Collective Intelligence" in Ruby.

In chapter 8 the author passed a method as a parameter which seems to work in Python but not in Ruby.

I am using the method:

def gaussian(dist, sigma=10.0)   foo end 

and want to call this using another method:

def weightedknn(data, vec1, k = 5, weightf = gaussian)   foo   weight = weightf(dist)   foo end 

But I got is an error:

ArgumentError: wrong number of arguments (0 for 1) 

Answer 1

You want a proc object:

gaussian = Proc.new do |dist, *args|   sigma = args.first || 10.0   ... end  def weightedknn(data, vec1, k = 5, weightf = gaussian)   ...   weight = weightf.call(dist)   ... end 

Just note that you can’t set a default argument in a block declaration like that. So you need to use a splat and setup the default in the proc code itself.

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Or, depending on your scope of all this, it may be easier to pass in a method name instead.

def weightedknn(data, vec1, k = 5, weightf = :gaussian)   ...   weight = self.send(weightf)   ... end 

In this case you are just calling a method that is defined on an object rather than passing in a complete chunk of code. Depending on how you structure this you may need replace self.send with object_that_has_the_these_math_methods.send

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Last but not least, you can hang a block off the method.

def weightedknn(data, vec1, k = 5)   ...   weight =      if block_given?       yield(dist)     else       gaussian.call(dist)     end   end   ... end  weightedknn(foo, bar) do |dist|   # square the dist   dist * dist end 

But it sounds like you would like more reusable chunks of code here.