Ruby: what is the pitfall in this simple code excerpt that tests variable existence

I'm starting with Ruby, and while making some test samples, I've stumbled against an error in the code that I don't understand why it happens.

The code pretends to tests if a variable finn is defined?() and if it is defined, then it increments it. If it isn't defined, then it will define it with value 0 (zero). As the code threw an error, I started to decompose it in small pieces and run it, to better trace where the error was comming from. The code was run in IRB irb 0.9.5(05/04/13), using ruby 1.9.1p378

First I certify that the variable finn 开发者_开发知识库is not yet defined, and all is ok:

?> finn
NameError: undefined local variable or method `finn' for main:Object
        from (irb):134
        from /home/paulo/.rvm/rubies/ruby-1.9.1-p378/bin/irb:15:in `<main>'
>>

Then I certify that the following inline-condition executes as expected, and all is ok:

?> ((defined?(finn)) ? (finn+1):(0))
=> 0

And now comes the code that throws the error:

?> finn=((defined?(finn)) ? (finn+1):(0))
NoMethodError: undefined method `+' for nil:NilClass
        from (irb):143
        from /home/paulo/.rvm/rubies/ruby-1.9.1-p378/bin/irb:15:in `<main>'

I was expecting that the code would not throw any error, and that after executing the variable finn would be defined with a first value of 0 (zero). But instead, the code thows the error, and finn get defined but with a value of nil.

>> finn
=> nil

Where might the error come from?!? Why does the inline-condition work alone, but not when used for the finn assignment?

Any help apreciated :)

In this line,

finn=((defined?(finn)) ? (finn+1):(0))

When you write finn=, it declares finn, and sets it to nil until the assignment occurs. Therefore, defined?(finn) returns true , so it attempts to execute finn+1, which is equivalent to nil+1, which results in the NoMethodError you mentioned above.

You may be looking for this:

finn ||= 0
finn += 1

john is right. try:

finn = finn ? finn + 1 : 0