From: Justin Collins <justincollins@...>
Date: 2008-01-23T06:25:20+09:00
Subject: Re: why must I initialize this variable?

matt neuburg wrote:
> Here's a simple variable initialization / scope question. In the
> following code:
>
> h = {:test => "cool"}
> k = :test
> result = nil
> puts "#{result}" if (result = h[k])
>
> ...if I omit the third line (initializing "result"), I get an error.
> Why? Intuitively I would have expected the second part of the 4th line
> to create / initialize "result".
>
> Another way of understanding my confusion is to notice that this *does*
> work:
>
> h = {:test => "cool"}
> k = :test
> if (result = h[k]) then puts "#{result}" end
>
> So evidently I'm asking about the difference between the last line of
> the first example and the last line of the second example. Intuitively I
> would have expected these lines to be absolutely equivalent, but clearly
> they are not. I'd like to understand the difference rigorously. Thx! m


I _believe_ this is because the left-hand side is parsed first, as Ruby 
works left to right.

You will notice that the variable _does_ get initialized:

irb(main):001:0> puts "#{hi}" if false
=> nil
irb(main):002:0> puts "#{hi}" if hi = "hello"
(irb):2: warning: found = in conditional, should be ==
NameError: undefined local variable or method `hi' for main:Object
        from (irb):2
        from :0
irb(main):003:0> hi
=> "hello"

But at the time (before evaluating the conditional clause) the variable 
does not exist. The code which is run when the conditional succeeds is 
what was parsed prior to evaluating the conditional.

At least, that is my understanding. Of course, best not to use 
assignment as a conditional as it leads to confusion.

-Justin