Passing a method as a parameter in Ruby

I am trying to mess around a little bit with Ruby. Therefor I try to implement the algorithms (given in Python) from the book "Programming Collective Intelligence" Ruby.

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

I have here the method

def gaussian(dist, sigma=10.0)
  foo
end

and want to call this with another method

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

All I got is an error

ArgumentError: wrong number of arguments (0 for 1)

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 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.

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:

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

This pattern is used extensively in Ruby.

You can use the & operator on the Method instance of your method to convert the method to a block.

Example:

def foo(arg)
  p arg
end

def bar(&block)
  p 'bar'
  block.call('foo')
end

bar(&method(:foo))

More details at http://weblog.raganwald.com/2008/06/what-does-do-when-used-as-unary.html

Passing a method as a parameter in Ruby

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