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 Ruby user's guideLocal 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

In the next example, 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.

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

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
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