# Help, my 'case/when' code doesn't work **URL:** \ **Category:** ruby-talk\ **Created:** [4 December 2006 17:33 UTC](https://rubytalk.org/t/help-my-case-when-code-doesnt-work/33445 "2006-12-04T17:33:10Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Chuck\_Remes](https://avatars.discourse-cdn.com/v4/letter/c/3be4f8/32.png) [@Chuck\_Remes](https://rubytalk.org/u/Chuck_Remes)\ **Post date:** [4 December 2006 17:33 UTC](https://rubytalk.org/t/help-my-case-when-code-doesnt-work/33445/1 "2006-12-04T17:33:10Z")
I've been using ruby for several months now so imagine my surprise when I wrote something using the 'case' construct and I discovered that I didn't understand how to use it! What's worse is that even after looking through the Pickaxe and some code "in the wild" that uses 'case' I still don't see what I'm doing wrong. Here's the code in question: def case\_test(obj)   print "testing via case... "   case obj.class   when Array     puts "obj is a #{obj.class}"   when String     puts "obj is a #{obj.class}"   else     puts "obj is unknown: #{obj.class}"   end end def if\_test(obj)   print "testing via if... "   klass = obj.class   if klass == Array     puts "obj is a #{klass}"   elsif klass == String     puts "obj is a #{klass}"   else     puts "obj is unknown: #{obj.class}"   end end case\_test(Array.new) if\_test(Array.new) case\_test(String.new) if\_test(String.new) case\_test(Hash.new) if\_test(Hash.new) cremes$ ruby a.rb testing via case... obj is unknown: Array testing via if... obj is a Array testing via case... obj is unknown: String testing via if... obj is a String testing via case... obj is unknown: Hash testing via if... obj is unknown: Hash What am I doing wrong here? ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [4 December 2006 17:40 UTC](https://rubytalk.org/t/help-my-case-when-code-doesnt-work/33445/2 "2006-12-04T17:40:04Z")
> I've been using ruby for several months now so imagine my surprise when I wrote something using the 'case' construct and I discovered that I didn't understand how to use it! What's worse is that even after looking through the Pickaxe and some code "in the wild" that uses 'case' I still don't see what I'm doing wrong. > > Here's the code in question: > > def case\_test(obj) >   print "testing via case... " >   case obj.class Make that "case obj" instead. >   when Array >     puts "obj is a #{obj.class}" >   when String >     puts "obj is a #{obj.class}" >   else >     puts "obj is unknown: #{obj.class}" >   end > end > > def if\_test(obj) >   print "testing via if... " >   klass = obj.class >   if klass == Array >     puts "obj is a #{klass}" >   elsif klass == String >     puts "obj is a #{klass}" >   else >     puts "obj is unknown: #{obj.class}" >   end > end > > case\_test(Array.new) > if\_test(Array.new) > case\_test(String.new) > if\_test(String.new) > case\_test(Hash.new) > if\_test(Hash.new) > cremes$ ruby a.rb > testing via case... obj is unknown: Array > testing via if... obj is a Array > testing via case... obj is unknown: String > testing via if... obj is a String > testing via case... obj is unknown: Hash > testing via if... obj is unknown: Hash > > What am I doing wrong here?   robert > **···** > > On 04.12.2006 18:33, cremes.devlist@mac.com wrote: ---
**Author:** ![Stefano\_Crocco](https://avatars.discourse-cdn.com/v4/letter/s/5f9b8f/32.png) [@Stefano\_Crocco](https://rubytalk.org/u/Stefano_Crocco)\ **Post date:** [4 December 2006 17:44 UTC](https://rubytalk.org/t/help-my-case-when-code-doesnt-work/33445/3 "2006-12-04T17:44:06Z")
Alle 18:33, lunedì 4 dicembre 2006, cremes.devlist@mac.com ha scritto: > def case\_test(obj) >   print "testing via case... " >   case obj.class >   when Array >     puts "obj is a #{obj.class}" >   when String >     puts "obj is a #{obj.class}" >   else >     puts "obj is unknown: #{obj.class}" >   end > end Your code will call Array===obj.class. According to ri aClass===anObject returns true if anObject is an instance of aClass or of one of its descentents (which more or less means if anObject.is\_a?(aClass) returns true). Because of this, all your 'when' statements will fail, because obj.class is not an instance of Array or String. To do what you want, you should substitute   case obj.class with   case obj Stefano ---
**Author:** ![Chuck\_Remes](https://avatars.discourse-cdn.com/v4/letter/c/3be4f8/32.png) [@Chuck\_Remes](https://rubytalk.org/u/Chuck_Remes)\ **Post date:** [4 December 2006 17:57 UTC](https://rubytalk.org/t/help-my-case-when-code-doesnt-work/33445/4 "2006-12-04T17:57:37Z")
Sheesh! I \*knew\* it was something easy. I feel like a dope... cr > **···** > > On Monday, December 04, 2006, at 11:44AM, "Stefano Crocco" \ wrote: > > > Alle 18:33, lunedì 4 dicembre 2006, cremes.devlist@mac.com ha scritto: > > > > > def case\_test(obj) > > >   print "testing via case... " > > >   case obj.class > > >   when Array > > >     puts "obj is a #{obj.class}" > > >   when String > > >     puts "obj is a #{obj.class}" > > >   else > > >     puts "obj is unknown: #{obj.class}" > > >   end > > > end > > > > Your code will call Array===obj.class. According to ri aClass===anObject > > returns true if anObject is an instance of aClass or of one of its > > descentents (which more or less means if anObject.is\_a?(aClass) returns > > true). Because of this, all your 'when' statements will fail, because > > obj.class is not an instance of Array or String. To do what you want, you > > should substitute > > > > case obj.class > > > > with > > > > case obj ---
**Author:** ![Paul\_Lutus](https://avatars.discourse-cdn.com/v4/letter/p/2bfe46/32.png) [@Paul\_Lutus](https://rubytalk.org/u/Paul_Lutus)\ **Post date:** [4 December 2006 18:20 UTC](https://rubytalk.org/t/help-my-case-when-code-doesnt-work/33445/5 "2006-12-04T18:20:05Z")
/ ... > Sheesh! I \*knew\* it was something easy. I feel like a dope... Perhaps, but in point of fact, you are not a dope. You asked the right question and got the right answer. A person who can't ask any questions for fear of appearing to be a dope, really does end up a a dope, a self-fulfilling prophecy. Your self-fulfilling prophecy is that you will learn anything you want to, because you care more about learning things than about hiding what you don't know. Diagnosis: not a dope. > **···** > > cremes.devlist@mac.com wrote: > > -- > Paul Lutus > [http://www.arachnoid.com](http://www.arachnoid.com) ---
**Author:** ![HAL\_9000](https://avatars.discourse-cdn.com/v4/letter/h/90ced4/32.png) [@HAL\_9000](https://rubytalk.org/u/HAL_9000)\ **Post date:** [5 December 2006 00:28 UTC](https://rubytalk.org/t/help-my-case-when-code-doesnt-work/33445/6 "2006-12-05T00:28:51Z")
Don't feel like a dope. That is fairly subtle. Consider this code:    x = foo    y = foo    case x      when y        puts "yes"      else        puts "no"    end When would you ever expect this to print "no"? In most cases, it will print yes. If foo is a class, it will print no. Hal > **···** > > cremes.devlist@mac.com wrote: > > > Sheesh! I \*knew\* it was something easy. I feel like a dope...