From: Rick DeNatale <rick.denatale@...>
Date: 2007-03-27T07:10:30+09:00
Subject: Re: Question about if in Ruby

On 3/24/07, Vince H&K <vince.hetk@gmail.com> wrote:
> Trans wrote:
> > There is one other difference.
> >
> >   if a = 1
> >     p a
> >   end
> >
> >   p a if a = 1
> >
> > The later does not assign the a.
>
>   This is not exactly true (at least on ruby 1.8.6):
>
> irb(main):001:0> if a = 1
> irb(main):002:1> p a
> irb(main):003:1> end
> 1
> => nil
> irb(main):004:0> p b if b = 2
> NameError: undefined local variable or method `b' for main:Object
>         from (irb):4
>         from :0
>
>   It doesn't create the variable b in the same scope as the one of the
> instruction. However:
>
> irb(main):007:0> a = 1
> => 1
> irb(main):008:0> p a if a = 2
> 2
> => nil
> irb(main):009:0> a
> => 2
>
>   It does assign it if it already exists in the current scope. Works as
> well in ruby (it is not an irb artifact).

In fact it DOES assign:
rick@frodo:/public/rubysource/ruby1.8.5$ irb
irb(main):001:0> a
NameError: undefined local variable or method `a' for main:Object
        from (irb):1
irb(main):002:0> p a if a = 1
(irb):2: warning: found = in conditional, should be ==
NameError: undefined local variable or method `a' for main:Object
        from (irb):2
irb(main):003:0> a
=> 1


I think that it's more accurate to say that the difference between

if a = 1
  ...
end

and

... if a = 1

is that the latter checks for the existence of a and raises a
NameError after the assignment if the variable didn't exist before.

Is this a bug?  Maybe.  It is kind of an edge case, notice the warning
indicating that you really should be using a comparison instead of an
assignment.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/