From: Rick DeNatale <rick.denatale@...>
Date: 2007-03-28T01:39:19+09:00
Subject: Re: Question about if in Ruby

On 3/27/07, David A. Black <dblack@wobblini.net> wrote:
> 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

True of course.

What's interesting is that the if a=1 doesn't create the variable,
since it does have to be executed first.

I wouldn't recommend actually writing code like this, but it does seem
a bit inconsitent.

A quirk of the ruby parser? eval?
-- 
Rick DeNatale

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