I stored the class of a variable in a variable and later checked in a switch-case. Instead of going into the Fixnum case, it goes into ELSE. c=23.class # => Fixnum c == Fixnum # => true case c; when Fixnum; puts "YES"; else; puts "NO"; end # => prints NO # after much head-scratching tried this: case c; when Fixnum; puts "YES"; when Class; puts "CLASS"; else; puts "NO"; end # prints CLASS --- As a result of this I had to store the class as a string (to_s), so the case could work. Could someone explain what I am missing here. Thx.
on 2009-01-07 06:58
on 2009-01-07 08:57
From: Ruby Rabbit [mailto:firstname.lastname@example.org] # c=23.class # => Fixnum # c == Fixnum # => true # case c; when Fixnum; puts "YES"; else; puts "NO"; end # # # => prints NO # # after much head-scratching tried this: # # case c; when Fixnum; puts "YES"; when Class; puts "CLASS"; else; puts # "NO"; end # # prints CLASS case uses ===; thus on your case, it should be easier... eg try, > c=23 => 23 > case c; when Fixnum; puts "YES"; else; puts "NO"; end YES => nil > c="hello" => "hello" > case c; when Fixnum; puts "YES"; when String; puts "YES STRING!"; else; puts "NO"; end YES STRING! => nil
on 2009-01-07 08:59
Alle Wednesday 07 January 2009, Ruby Rabbit ha scritto: > > Could someone explain what I am missing here. > Thx. The case expression uses the === operator of the objects in the "when" statements to decide which one needs to be executed. This means that the expression case c when Fixnum then puts "YES" when Class then puts "CLASS" else puts "NO" end is (more or less) equivalent to the following if expression: if Fixnum === c then puts "YES" elsif Class === c then puts "CLASS" else puts "NO" end Now, the documentation for Class#=== (since both Fixnum and Class are object of class Class) states that it returns true if an instance of the class or of one of its descendents and false otherwise. In other words, cls === obj returns true if obj.is_a?(cls) returns true and false otherwise. In your case, the object you're testing (c) is the class of 23, that is Fixnum. So, when executing the case expression, ruby does something like this: Fixnum === c # returns false because c.is_a?(Fixnum), that is # Fixnum.is_a?(Fixnum) is false Class === c # returns true because c.is_a?(Class), that is # Fixnum.is_a?(Class) is true puts "CLASS" To get what you want, you have two possibilities: 1) use "case 23" instead of "case c". This way, it will work as you expected, because 23 is an instance of class Fixnum, so Fixnum === 23 returns true 2) use an if expression instead of a case expression: if c.is_a?(Fixnum) then #... elsif c.is_a?(Class) then #... else #... end I hope this helps Stefano
on 2009-01-07 09:09
Stefano Crocco wrote: > Alle Wednesday 07 January 2009, Ruby Rabbit ha scritto: >> >> Could someone explain what I am missing here. >> Thx. > > Stefano Thanks. Actually both replies side step the issue I have at hand. I have an incoming variable, I store its class, the variable is converted to a string. When the class returns the variable, it converts back. def set_buffer val @datatype = val.class @buffer = val.to_s end def get_buffer case @datatype when Fixnum return @buffer.to_i etc end Its like the above. So, instead of saying @datatype = val.class.to_s and later checking for a string, i was trying to store the class and check against it. Thanks for clearing up the confusion.
on 2009-01-07 09:43
Ruby Rabbit wrote: > > Could someone explain what I am missing here. > Thx. > Case statements do not use the equality operator (==), they use the "relationship operator" (===), which is typically not commutative. Also, the value in the "when" part is the left-hand value (not the right-hand value, as you might expect). Futhermore, Class objects (like Fixnum) use Module's definition of "===", which returns true if the right-hand side is an instance of the module or one of its descendents. Therefore: irb(main):001:0> c = 32.class => Fixnum irb(main):002:0> Fixnum === c => false irb(main):003:0> Fixnum === Fixnum => false irb(main):004:0> Fixnum === 23 => true -Justin