To define a new variable, you use Scheme's
define syntax like
(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
Changing the value of an already existing variable is very similar,
define is replaced by the Scheme syntax
(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 (see About Expressions), you
can probably guess what the following
set! example does...
(set! x (+ x 1))
(Note: this is not a complete description of
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
definesyntax that can be used when defining new procedures.
set!syntax that helps with changing a single value in the depths of a compound data structure.)
defineother than at top level in a Scheme program, including a discussion of when it works to use
set!to change the value of an existing variable.