From: "Daniel Schüle" <uval@...>
Date: 2006-01-04T08:52:58+09:00
Subject: block and method local variables

hello all,

consider follow code

irb(main):528:0* def abc
irb(main):529:1> ppp=111
irb(main):530:1> puts ppp
irb(main):531:1> yield
irb(main):532:1> puts ppp
irb(main):533:1> ppp
irb(main):534:1> end
=> nil
irb(main):536:0> abc {}
111
111
=> 111
irb(main):537:0> abc { puts ppp }
111
NameError: undefined local variable or method `ppp' for main:Object
         from (irb):537
         from (irb):537:in `abc'
         from (irb):537
         from :0
irb(main):538:0>
irb(main):539:0*
irb(main):540:0* ppp = "OOO"
=> "OOO"
irb(main):541:0> abc { puts ppp }
111
OOO
111
=> 111
irb(main):542:0>

I am right assuming that all "local" variables in the method abc
are invisible in the block?

I tried a little more

irb(main):544:0* def cba
irb(main):545:1> ppp = 222
irb(main):546:1> puts ppp
irb(main):547:1> yield ppp
irb(main):548:1> puts ppp
irb(main):549:1> ppp
irb(main):550:1> end
=> nil
irb(main):551:0> cba {}
222
222
=> 222
irb(main):552:0> cba {|p|}
222
222
=> 222
irb(main):553:0> cba {|p| p = 333}
222
222
=> 222
irb(main):554:0> cba {|p| puts p; p = 333}
222
222
222
=> 222
irb(main):555:0>

if explicitely passed to block ppp is now visible there.
but seems to be imposible to change the "method local" variable
ppp in this case.

the following *does* work

irb(main):557:0* def xxx
irb(main):558:1> ppp = [1]
irb(main):559:1> p ppp
irb(main):560:1> yield ppp
irb(main):561:1> p ppp
irb(main):562:1> ppp
irb(main):563:1> end
=> nil
irb(main):564:0> xxx {}
[1]
[1]
=> [1]
irb(main):565:0> xxx {|x| }
[1]
[1]
=> [1]
irb(main):566:0> xxx {|x| x[0]=2 }
[1]
[2]
=> [2]
irb(main):567:0>

and I kind of understand why it works

if I change line 566 to
irb(main):566:0> xxx {|x| x=[2] }
than it would not work

the reason I came accross this is following
I was reading

"
Parameters to a block may be existing local variables; if so, the new 
value of the variable will be retained after the block completes. This 
may lead to unexpected behavior, but there is also a performance gain to 
be had by using variables that already exist
"
and in my understanding all variables defined in a method
are "local" (C++ background)

If not local what are they considered to be then?

Regards, Daniel