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 cannot be sure whether you are referencing a bogus variable; you might, for example, have misspelled a method name. Hence the rather nonspecific 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; puts foo; defined?(foo)
44
   "local-variable"
ruby> loop{bar=45; puts bar; 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=nil
   nil
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=nil" at the beginning cannot be omitted; it 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 an "undefined local variable or method" error. We could have said bar=0 instead, but using nil is a courtesy to others who will read your code later. It indicates fairly clearly that you are only establishing scope, because the value being assigned is not intended to be meaningful.

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 = nil
    |   get = proc{contents}
    |   set = proc{|n| contents = n}
    |   return get, set
    | end
   nil
ruby> reader, writer = box
   [#<Proc:0x40170fc0>, #<Proc:0x40170fac>]
ruby> reader.call
   nil
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  # nothing is in this box yet
   nil

This kind of programming could be considered a perverse little object-oriented framework. The box method acts something like a class, with get and set serving as methods (except those aren't really the method names, which could vary with each box instance) and contents being the lone instance variable. Of course, using ruby's legitimate class framework leads to much more readable code.

Copyright (c) 2005-2014 Mark Slagell

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