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 14.1.3 Defining and Setting Variables
 -------------------------------------
 
 To define a new variable, you use Scheme's `define' syntax like this:
 
      (define VARIABLE-NAME VALUE)
 
    This makes a new variable called VARIABLE-NAME and stores VALUE in
 it as the variable's initial value.  For example:
 
      ;; Make a variable `x' with initial numeric value 1.
      (define x 1)
 
      ;; Make a variable `organization' with an initial string value.
      (define organization "Free Software Foundation")
 
    (In Scheme, a semicolon marks the beginning of a comment that
 continues until the end of the line.  So the lines beginning `;;' are
 comments.)
 
    Changing the value of an already existing variable is very similar,
 except that `define' is replaced by the Scheme syntax `set!', like this:
 
      (set! VARIABLE-NAME NEW-VALUE)
 
    Remember that variables do not have fixed types, so NEW-VALUE may
 have a completely different type from whatever was previously stored in
 the location named by VARIABLE-NAME.  Both of the following examples
 are therefore correct.
 
      ;; Change the value of `x' to 5.
      (set! x 5)
 
      ;; Change the value of `organization' to the FSF's street number.
      (set! organization 545)
 
    In these examples, VALUE and NEW-VALUE are literal numeric or string
 values.  In general, however, VALUE and NEW-VALUE can be any Scheme
 expression.  Even though we have not yet covered the forms that Scheme
 expressions can take ( About Expressions), you can probably
 guess what the following `set!' example does...
 
      (set! x (+ x 1))
 
    (Note: this is not a complete description of `define' and `set!',
 because we need to introduce some other aspects of Scheme before the
 missing pieces can be filled in.  If, however, you are already familiar
 with the structure of Scheme, you may like to read about those missing
 pieces immediately by jumping ahead to the following references.
 
    *  Lambda Alternatives, to read about an alternative form of
      the `define' syntax that can be used when defining new procedures.
 
    *  Procedures with Setters, to read about an alternative form
      of the `set!' syntax that helps with changing a single value in
      the depths of a compound data structure.)
 
    *  Internal Definitions, to read about using `define' other
      than at top level in a Scheme program, including a discussion of
      when it works to use `define' rather than `set!' to change the
      value of an existing variable.
 

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