> Yes, that makes sense. x is nil, so I can't invoke method on it. But in
our case 'name' was not null,
First, let's put this code in a simple class, to test it:
class A
attr_accessor :name
def initialize(name)
@name = name
end
def process_name_ok
self.name = name.gsub(/[^A-Za-z]/, "")
end
def process_name_notok
name = name.gsub(/[^A-Za-z]/, "")
end
end
a = A.new("123Pawell")
p a.process_name_ok
# => "Pawell"
# Now, let's call the bad guy, to make sure we recreated the bug:
begin
p a.process_name_not_ok
rescue Exception => e
puts "Uups! #{e}"
end
# => Uups! undefined method `process_name_not_ok' for #<A:0x542f90
@name="Pawell">
--------
Ok, now let me play the part of the MRI (Matz Ruby Interpreter) executing
the bad code (find his words in the comments :-):
# ... I am given this line inside a method:
name = name.gsub(/[^A-Za-z]/, "")
# I see a 'name' on the left side; I remember the day when I was still young
# and I went with Matz to that Japanese garden, when he told me to first
# look for local variables, then for methods (I still remember that gentle
# afternoon, how times have changed! now everyone wants to do a better
# version of me... but I must be strong and accept it).
# Ok, as I don't have a local variable 'name' in this method, the user is
clearly
# creating one and assigning to it; ok, there! I put a local variable 'name'
in
# my symbol table for this method, and I will initialize it to nil, while
# waiting to see what the user will do on the right side.
# But first let's take a little break for everyone to think about it...
# All right, I am the MRI and I am back! let's look at the right side:
name.gsub(/[^A-Za-z]/, "")
# Aha, I see a 'name' on the right side; as Matz taught me, first I look for
# variables: do I have a 'name' variable? oh, yes, there is a 'name'
variable
# in the little symbol table for this method, and its value is nil. Let's
use it!
# Oh, my God! the user asks to run a gsub on a nil value... KABOOOMM!
-----
Does it make sense? :-)
Raul
On Thu, Sep 25, 2008 at 3:43 PM, Paweł Stawicki <[EMAIL PROTECTED]>wrote:
> >
> > First, let's do something very simple in irb:
> >
> > x = x + 1
> >
> > Do you see what happened? you are trying to assign a value to a variable
> > that you just created (on the left side) which is for the moment nil, so
> ...
> > does it make sense?
> >
> > And this is exactly what happens when you wrote.
> > name = name.gsub...
> > Do you see it now? (if not, don't be afraid to ask again; I am sure that
> > your questions are shared by many).
>
> Yes, that makes sense. x is nil, so I can't invoke method on it. But
> in our case name was not null, I was able to invoke gsub on it, it
> only depended on "self." on the left side, while method was invoked on
> the right side.
>
> >> If you could also explain how ruby can think "name" on the left side is
> >> method. It has "=" sign on the right, is it possible with methods?
> >
> > Ah... but the method would be not 'name' but 'name='. In the code that we
> > discussed, Ruby has to decide if you are creating a local variable 'name'
> or
> > calling the accessor name= (that for the reasons discussed you should
> write
> > 'self.name=').
>
> And space sign doesn't matter? There is no difference between "name="
> and "name ="? Well, if I can call
> object.name = "smth"
> and
> object.name="smth"
> and it is the same, I think there is no difference. Ah, I like to
> answer to my own questions :)
>
> > It is always a pleasure to hear your questions (they force me to think,
> > before answering). Do not hesitate if you have more,
>
> Thanks. I also like your responses very much. You are very helpful and
> it is very good to have such person in this group.
>
> Best regards
> --
> Pawel Stawicki
> http://pawelstawicki.blogspot.com
>
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