CODEWITHSUNDEEP
rubyvariables
Bala
Bala

Reputation: 11234

Variable assignment using eval

Assigning a instance variable using eval works fine whereas the other doesn't. Trying to understand whats happening here. Any help is appreciated.

>> var = "a value"
=> "a value"

>> @v
=> nil

>> eval "@v = var"
=> "a value"

>> @v
=> "a value"

>> eval "var_new = var"
=> "a value"

>> var_new
NameError: undefined local variable or method `var_new' for main:Object
        from (irb):7
        from C:/Ruby193/bin/irb:12:in `<main>'

Upvotes: 3

Views: 1322

Answers (3)

Denis de Bernardy
Denis de Bernardy

Reputation: 78423

eval creates its own scope:

>> i = 1; local_variables.count
=> 2
>> eval "j = 1; local_variables.count"
=> 3
>> local_variables.count
=> 2

Upvotes: 3

Arup Rakshit
Arup Rakshit

Reputation: 118261

Do kind of introspection as below :

var = "a value"
eval "var_new = var,defined?(var_new)" #=> ["a value", "local-variable"]
defined?(var_new) #=>nil
defined?(var) #=>"local-variable"
defined?(temp) #=>nil

You can see var_new is known to only inside the eval. Outside of eval temp and var_new both are same.

Upvotes: 2

Patrick Oscity
Patrick Oscity

Reputation: 54674

eval simply has its own scope. You can access variables that were defined before but you will not get access to variables that are defined inside eval.

Scoping-wise, your example is similar to:

var = "a value"

1.times do # create new scope
  new_var = var
end

new_var
# NameError: undefined local variable or method `new_var' for main:Object

Upvotes: 3

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