From: dblack@...
Date: 2006-03-02T06:25:49+09:00
Subject: Re: case/when question

Hi --

On Thu, 2 Mar 2006, cremes.devlist@mac.com wrote:

>
> On Mar 1, 2006, at 3:13 PM, cremes.devlist@mac.com wrote:
>
>> I was intrigued by the post that _why put up a few days/weeks ago about 
>> using case/when for testing arrays [1]. I thought I'd give it a try but I 
>> can't make it work. What am I doing wrong?
>> 
>> irb(main):001:0> r1 = (0..12).to_a
>> => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
>> irb(main):002:0> r2 = (20..32).to_a
>> => [20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]
>> irb(main):003:0> def foo range
>> irb(main):004:1>  case range
>> irb(main):005:2> when *r1
>> irb(main):006:2>   puts "#{range} in r1"
>> irb(main):007:2> when *r2
>> irb(main):008:2>   puts "#{range} in r2"
>> irb(main):009:2> else
>> irb(main):010:2*   puts "#{range} NOT in any range"
>> irb(main):011:2> end
>> irb(main):012:1> end
>> => nil
>> irb(main):013:0> foo 3
>> NameError: undefined local variable or method `r1' for main:Object
>>        from (irb):5:in `foo'
>>        from (irb):13
>
> Responding to my own email...
>
> I figured it out about 5 minutes after sending. This is purely a scope 
> mistake on my part. Variables r1 and r2 are NOT in scope inside foo which is 
> exactly what ruby told me.
>
> The fix is to make r1 and r2 into class vars. Change r1 to @r1 and r2 to @r2 
> in that example and it works just fine.

Sort of :-)  Actually @r1 is an instance variable, not a class
variable.  It's true that if you do:

   @a = 1
   def meth
     puts @a
   end
   meth

you'll get "1", but only if you are writing code at the top level of
the program.  If you do this:

   class MyClass
     @a = 1
     def meth
       puts @a
      end
    end

    MyClass.new.meth

you won't because, the two @a's are different from each other.

Class variables (@@r1) will cross the method-definition divide, but
they're somewhat cumbersome....  The best thing is to create local
variables in the scope where you need them (the method definition).


David

-- 
David A. Black (dblack@wobblini.net)
Ruby Power and Light (http://www.rubypowerandlight.com)

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