Passing a method as a parameter in Ruby

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.

---

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

---

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.

The comments referring to blocks and Procs are correct in that they are more usual in Ruby. But you can pass a method if you want. You call method to get the method and .call to call it:

def weightedknn( data, vec1, k = 5, weightf = method(:gaussian) )
  ...
  weight = weightf.call( dist )
  ...
end

You can pass a method as parameter with method(:function) way. Below is a very simple example:

def double(a)
  return a * 2 
end
=> nil

def method_with_function_as_param( callback, number) 
  callback.call(number) 
end 
=> nil

method_with_function_as_param( method(:double) , 10 ) 
=> 20

The normal Ruby way to do this is to use a block.

So it would be something like:

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

And used like:

weightedknn(data, vec1) { |dist| gaussian( dist ) }

This pattern is used extensively in Ruby.