Ruby: using a hash to access instance methods

Question

I'm working on a problem in Chris Pine's Learn to Program book. I've defined a class Pet, with some instance methods. Outside of the class definition, I'm trying to build a method that will take a string and an instance of the Pet class and run the appropriate instance method.

def dispatch(command, pet)
  dispatches = {'feed'       => pet.feed,
                'walk'       => pet.walk,
                'put to bed' => pet.putToBed,
                'rock'       => pet.rock,
                'toss'       => pet.toss}
  dispatches[command]
end

When the dispatch method runs, however, it executes all the instance methods that appear in the hash, not just the one corresponding to command. They execute in the order they appear in the code, and before even reaching the dispatches[command] line.

What am I doing wrong here?

give the class body also.. so that we can run

Arup Rakshit
– Arup Rakshit

2013-09-21 18:21:31 +00:00
Commented Sep 21, 2013 at 18:21 — Arup Rakshit
– Arup Rakshit, Commented Sep 21, 2013 at 18:21

Sergio Tulentsev · Accepted Answer · 2013-09-21 18:55:02Z

3

Yes, that is correct behaviour.

{'feed'       => pet.feed}

The line above means "call method pet.feed and use its return value for assigning to key 'feed'". You have to use lambdas (or similar) to create chunks of code that can be invoked later. Something like this:

def dispatch(command, pet)
  dispatches = {'feed'       => proc { pet.feed },
                'put to bed' => proc { pet.put_to_bed }}
  dispatches[command].call
end

edited Sep 21, 2013 at 18:55

answered Sep 21, 2013 at 18:22

Sergio Tulentsev

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9 Comments

Arup Rakshit Over a year ago

I don't understand,why there is a need of lamda.

ivan Over a year ago

Ahhh, I see. I was pretty sure it was calling each method as the hash was being populated, but forgot exactly why. Variables and objects, as usual :) Thanks!

Arup Rakshit Over a year ago

@SergioTulentsev I don't understand,why need lambda ? Without lambda it works..:)

Neil Slater Over a year ago

@Arup Rakshit - it works either way in terms of the value returned, but with the lambda it only calls the desired method of Pet. This might be important if the methods did anything other than return a value.

Cary Swoveland Over a year ago

@ArupRakshit If class Pet contains def feed; print "feed "; end and def put_to_bed; puts "put_to_bed"; end, the above procs are changed to pet.feed and pet.put_to_bed and .call is removed, then dispatch('put_to_bed', Pet.new) => feed put_to_bed. I'm sure you know that, so I suspect there may be a misunderstanding about what OP was saying.

|

spickermann · Accepted Answer · 2013-09-21 18:29:17Z

1

While creation of the hash all methods are evaluated. I advise to read about the use of the method send (http://www.ruby-doc.org/core-2.0.0/Object.html#method-i-send) If you use send you will end with something like:

def dispatch(command, pet)
  pet.send(command)
end

Note that you will need to adjust the name of the method putToBed.

edited Sep 21, 2013 at 18:29

answered Sep 21, 2013 at 18:23

spickermann

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3 Comments

Sergio Tulentsev Over a year ago

I'm afraid, send won't do for OP. If you review sample code, you'll see that he is trying to use string "put to bed" to call a method. That string is no valid method name. Apparently, this is part of some kind of human-friendly interface.

spickermann Over a year ago

It would be easy to transfer "put to bed" into "put_to_bed" or "putToBed" (what btw is a bad method name), I think...

Sergio Tulentsev Over a year ago

But what about "go to sleep" => pet.put_to_bed? How do you transform that? :) That's why it's good to have mappings. So that you don't leak your implementation details to the user.

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