# Help, my 'case/when' code doesn't work **URL:** \ **Category:** Ruby\ **Created:** [December 4, 2006, 6:33pm UTC](https://www.ruby-forum.com/t/help-my-case-when-code-doesnt-work/78267 "2006-12-04T18:33:55Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![cremes](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e19b73/32.png) [@cremes](https://www.ruby-forum.com/u/cremes)\ **Post date:** [December 4, 2006, 6:33pm UTC](https://www.ruby-forum.com/t/help-my-case-when-code-doesnt-work/78267/1 "2006-12-04T18:33:55Z")
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:** ![cremes](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e19b73/32.png) [@cremes](https://www.ruby-forum.com/u/cremes)\ **Post date:** [December 4, 2006, 6:41pm UTC](https://www.ruby-forum.com/t/help-my-case-when-code-doesnt-work/78267/2 "2006-12-04T18:41:56Z")
On 04.12.2006 18:33, removed\_email\_address@domain.invalid wrote: > 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. > print "testing via if… " > case\_test(Array.new) > testing via case… obj is unknown: Hash > testing via if… obj is unknown: Hash > > What am I doing wrong here? ``` robert ``` ---
**Author:** ![cremes](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e19b73/32.png) [@cremes](https://www.ruby-forum.com/u/cremes)\ **Post date:** [December 4, 2006, 6:45pm UTC](https://www.ruby-forum.com/t/help-my-case-when-code-doesnt-work/78267/3 "2006-12-04T18:45:57Z")
Alle 18:33, lunedì 4 dicembre 2006, removed\_email\_address@domain.invalid ha scritto: > 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:** ![cremes](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e19b73/32.png) [@cremes](https://www.ruby-forum.com/u/cremes)\ **Post date:** [December 4, 2006, 6:58pm UTC](https://www.ruby-forum.com/t/help-my-case-when-code-doesnt-work/78267/4 "2006-12-04T18:58:00Z")
On Monday, December 04, 2006, at 11:44AM, “Stefano C.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > end > > with > > case obj Sheesh! I _knew_ it was something easy. I feel like a dope… cr ---
**Author:** ![cremes](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e19b73/32.png) [@cremes](https://www.ruby-forum.com/u/cremes)\ **Post date:** [December 5, 2006, 1:31am UTC](https://www.ruby-forum.com/t/help-my-case-when-code-doesnt-work/78267/5 "2006-12-05T01:31:43Z")
removed\_email\_address@domain.invalid wrote: > Sheesh! I _knew_ it was something easy. I feel like a dope… 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 ---
**Author:** ![cremes](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e19b73/32.png) [@cremes](https://www.ruby-forum.com/u/cremes)\ **Post date:** [December 4, 2006, 7:22pm UTC](https://www.ruby-forum.com/t/help-my-case-when-code-doesnt-work/78267/6 "2006-12-04T19:22:05Z")
removed\_email\_address@domain.invalid wrote: / … > 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.