From: "David A. Black" <dblack@...>
Date: 2007-03-27T21:03:04+09:00
Subject: Re: Question about if in Ruby

Hi --

On 3/26/07, Rick DeNatale <rick.denatale@gmail.com> wrote:
> 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.

It's actually the 'a' in 'p a' that raises the error, not the one in
the assignment:

  true if a = 1              # no error
  b = 1; p b if a = 1     # no error


David

-- 
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
   (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)