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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 (Note: 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.

   * REFFIXME, 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.

   * Note: Lambda Alternatives, to read about an alternative form of
     the `define' syntax that can be used when defining new procedures.

   * REFFIXME, 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.)