From: olczyk@... (Thaddeus L. Olczyk)
Date: 2001-12-24T01:37:35+09:00
Subject: [ruby-talk:29368] Re: a better way?

On Sun, 23 Dec 2001 02:08:09 GMT, HarryO <harryo@zipworld.com.au>
wrote:

>On Sun, 23 Dec 2001 12:46:10 +1100, David Alan Black wrote:
>
>
>>   class Temp
>>     @@save = {}
>>     def fahrenheit(c)
>>       result = @@save[self] and return result puts "Performing
>>       calculation...."
>>       @@save[self] = c * 9 / 5 + 32
>>     end
>>   end
>> 
>>   t = Temp.new
>> 
>>   puts t.fahrenheit(100) # "Performing calculation....\n212" puts
>>   t.fahrenheit(100) # "212"
>
>As usual, I'm probably missing something obvious here, but
>shouldn't the index used to access @@save be "c" rather than
>"self", in both cases?
You hagve to forgive Dave, he's an amateur programmer and thus misses
these things. Presumably self might work, if you were interested in
the caching the object's state. For example:

class Temp
  @@save={}
  def initialize(tIn)
    @c=tIn
  end
  def PrintTemp
    puts @c
  end
  def fahrenheit
    result = @@save[@c]
    if result==nil then
      puts "New Calculation"
      result=@c * 9 / 5 + 32
      @@save[@c]=result 
    end
    return result
  end
end


1.upto(10){
    |t|
    T=Temp.new(t)
    printf("Temp=")
    T.PrintTemp
    printf("In Farenheit %d\n",T.fahrenheit)
  }

1.upto(10){
    |t|
    T=Temp.new(t)
    printf("Temp=")
    T.PrintTemp
    printf("In Farenheit %d\n",T.fahrenheit)
  }

1.upto(10){
    |t|
    T=Temp.new(t)
    printf("Temp=")
    T.PrintTemp
    printf("In Farenheit %d\n",T.fahrenheit)
  }

there are a couple errors ( an initialization error, plus a error in
using integers ), forgive me ruby is still new to me.

In the method fahrenheit you could put self as the argument to
save then the second loop would print "New Calculation".

In other words, if you were going to calculate based on the object
state then self would work, but using the state as an index works
the more fficiently.

The technique described is called memoization, a technique that
I believe was invented in Lisp and mostly used in that language.
It has the disadvantage that it can only be used in functions that
have no side effects.

The one time I've seen numbers on memoziation, Kiczales, Lopes,
et al. first AOP project RG ( a image filtering system ), it preformed
more poorly in all aspects. It took more lines of code ( obviously),
took longer to execute, and it failed to save memory.