Ruby hash use key value in default value

Question

I have the following code to create an array to object hash:

tp = TupleProfile.new(98, 99)
keyDict = Hash[Array[98,99] => tp]
keyDict[[98,99]].addLatency(0.45)
puts keyDict[[98,99]].getAvg()

This works, but I'd like to be able to call addLatency without checking for an existing hash value:

keyDict[[100,98]].addLatency(0.45) #throws error right now

So I want to create a default value that varies based on the key, something like:

keyDict = Hash.new(TupleProfile.new(theKey[0], theKey[1]))

Where theKey is some sort of special directive. Is there any reasonably clean way to do this, or am I better off checking each time or making a wrapper class for the hash?

Zach Kemp · Accepted Answer · 2012-11-15 20:31:14Z

3

Try the Hash.new block notation:

keyDict = Hash.new {|hash,key| hash[key] = TupleProfile.new(*key) }

Using the standard parameter notation (Hash.new(xyz)) will really only instantiate a single TupleProfile object for the hash; this way there will be one for each individual key.

edited Nov 15, 2012 at 20:31

answered Nov 15, 2012 at 20:26

Zach Kemp

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Comments

morri · Accepted Answer · 2012-11-15 20:59:55Z

1

If I understand your question, I think you might be able to use a default procedure. The code in the default procedure will get run if you ask for a key that doesn't exist. Here is an example using a tuple key:

class Test
  def initialize(a,b); @a = a; @b = b; end
  attr_accessor :a, :b
end

keyDict = {}

keyDict.default_proc = proc do |hash, (key1, key2)|
  hash[[key1, key2]] = Test.new(key1, key2)
end


keyDict[[99,200]]
 => #<Test:0x007f9681ad2720 @a=99, @b=200> 

keyDict[[99,200]].a
 => 99

answered Nov 15, 2012 at 20:59

morri

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2 Comments

jtpereyda Over a year ago

I'm getting this error on the default_proc line: undefined method `default_proc=' for {}:Hash (NoMethodError)

Zach Kemp Over a year ago

default_proc= was added in Ruby 1.9. Are you on 1.8?

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