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 Ruby user's guide Local variables  

A local variable has a name starting with a lower case letter or an underscore character (_).  Local variables do not, like globals and instance variables, have the value nil before initialization:

 ruby> $foo
    nil
 ruby> @foo
    nil
 ruby> foo
 ERR: (eval):1: undefined local variable or method `foo' for main(Object)

The first assignment you make to a local variable acts something like a declaration.  If you refer to an uninitialized local variable, the ruby interpreter thinks of it as an attempt to invoke a method of that name; hence the error message you see above.

Generally, the scope of a local variable is one of

 ruby> foo = 44; print foo, "\n"; defined? foo
 44
    "local-variable"
 ruby> loop{bar=45; print bar, "\n"; break}; defined? bar
 45
    nil

In the above, defined? is an operator which checks whether an identifier is defined.  It returns a description of the identifier if it is defined, or nil otherwise. As you see, bar's scope is local to the loop; when the loop exits, bar is undefined.

Procedure objects that live in the same scope share whatever local variables also belong to that scope.  Here, the local variable bar is shared by main and the procedure objects p1 and p2:

 ruby> bar=0
    0
 ruby> p1 = proc{|n| bar=n}
    #<Proc:0x8deb0>
 ruby> p2 = proc{bar}
    #<Proc:0x8dce8>
 ruby> p1.call(5)
    5
 ruby> bar
    5
 ruby> p2.call
    5

Note that the "bar=0" at the beginning cannot be omitted; that assignment ensures that the scope of bar will encompass p1 and p2.  Otherwise p1 and p2 would each end up with its own local variable bar, and calling p2 would have resulted in that "undefined local variable or method" error.

A powerful feature of procedure objects follows from their ability to be passed as arguments: shared local variables remain valid even when they are passed out of the original scope.

 ruby> def box
     |   contents = 15
     |   get = proc{contents}
     |   set = proc{|n| contents = n}
     |   return get, set
     | end
    nil
 ruby> reader, writer = box
    [#<Proc:0x40170fc0>, #<Proc:0x40170fac>] 
 ruby> reader.call
    15
 ruby> writer.call(2)
    2
 ruby> reader.call
    2

Ruby is particularly smart about scope.  It is evident in our example that the contents variable is being shared between the reader and writer.  But we can also manufacture multiple reader-writer pairs using box as defined above; each pair shares a contents variable, and the pairs do not interfere with each other.

 ruby> reader_1, writer_1 = box
    [#<Proc:0x40172820>, #<Proc:0x4017280c>]
 ruby> reader_2, writer_2 = box
    [#<Proc:0x40172668>, #<Proc:0x40172654>]
 ruby> writer_1.call(99)
    99
 ruby> reader_1.call
    99
 ruby> reader_2.call
    15

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