From: Geert Fannes <Geert.Fannes@...>
Date: 2005-05-30T21:39:01+09:00
Subject: creating variable with eval

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Hello, what is the scope of a variable created inside an eval()
statement? To demonstrate what I mean, I created the following program:

=20

a=3D1

puts local_variables.sort.collect{|v|"1 #{v}: #{eval(v)}"}

eval("b=3D2")

puts local_variables.sort.collect{|v|"2 #{v}: #{eval(v)}"}

c=3Deval("b")

d=3Db

puts local_variables.sort.collect{|v|"3 #{v}: #{eval(v)}"}

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This gives as output:

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1 a: 1

1 c:

1 d:

2 a: 1

2 b: 2

2 c:

2 d:

tmp.rb:6: undefined local variable or method `b' for main:Object
(NameError)

=20

Apparently, Ruby creates immediately the variables a,c,d (so all the
variables that are on the left-hand-side of an assignment), even before
he actually processed the assignment.

After processing the line "eval("b=3D2")", Ruby also created the local
variable b, which is accessible via eval("b"), but not directly (Ruby
crashes on line 6, "d=3Db" because he does not know local variable b,
which is clearly NOT the case, since he did write out local variable b
correctly).

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Any idea how these things work in Ruby and why my above program does
not?

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If you are wondering why I need this: I want to develop a
"named-argument" system for ruby-methods. My plan is to pass in a hash
all the optional arguments and automatically create variables for all
the keys in this hash. This generic procedure to create variables from
the hash-keys, does not know the names for these variables, but the rest
of the method that uses the named arguments does:

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def
some_method(required_arg1,required_arg2,opt_arg_hash=3D{:opt_arg1=3D>nil,=
:op
t_arg2=3D>nil})

  #this generic method should create the variables opt_arg1 and opt_arg2

  process_opt_args(opt_arg_hash)

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  #here we should be able to use the variables opt_arg1 and opt_arg2
directly, without having

  #to write opt_arg_hash[:opt_arg1] and opt_arg_hash[:opt_arg2]

  puts(opt_arg1)

  puts(opt_arg2)

end

=20

Greetings,

Geert.


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