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11.3.2.2
What Is A Local Variable?
You’ll notice that we wrote the summation (+ a b) within the parens of the let* expression, not after it.
This is because the let* statement defines an area in your script in which the declared variables are usable; if you type the (+ a
b) statement after the (let* ...) statement, you’ll get an error, because the declared variables are only valid within the context of
the let* statement; they are what programmers call local variables.
b) statement after the (let* ...) statement, you’ll get an error, because the declared variables are only valid within the context of
the let* statement; they are what programmers call local variables.
11.3.2.3
The General Syntax Of
let*
The general form of a let* statement is:
(let* ( variables )
expressions )
where variables are declared within parens, e.g., (a 2), and expressions are any valid Scheme expressions. Remember that the
variables declared here are only valid within the let* statement -- they’re local variables.
variables declared here are only valid within the let* statement -- they’re local variables.
11.3.2.4
White Space
Previously, we mentioned the fact that you’ll probably want to use indentation to help clarify and organize your scripts. This
is a good policy to adopt, and is not a problem in Scheme -- white space is ignored by the Scheme interpreter, and can thus be
liberally applied to help clarify and organize the code within a script. However, if you’re working in Script-Fu’s Console window,
you’ll have to enter an entire expression on one line; that is, everything between the opening and closing parens of an expression
must come on one line in the Script-Fu Console window.
is a good policy to adopt, and is not a problem in Scheme -- white space is ignored by the Scheme interpreter, and can thus be
liberally applied to help clarify and organize the code within a script. However, if you’re working in Script-Fu’s Console window,
you’ll have to enter an entire expression on one line; that is, everything between the opening and closing parens of an expression
must come on one line in the Script-Fu Console window.
11.3.2.5
Assigning A New Value To A Variable
Once you’ve initialized a variable, you might need to change its value later on in the script. Use the set! statement to change the
variable’s value:
variable’s value:
(let* ( (theNum 10) ) (set! theNum (+ theNum \
theNum)) )
Try to guess what the above statement will do, then go ahead and enter it in the Script-Fu Console window.
Note
The ‘\’ indicates that there is no line break. Ignore it (don’t type it in your Script-Fu console and don’t hit Enter), just
continue with the next line.
The ‘\’ indicates that there is no line break. Ignore it (don’t type it in your Script-Fu console and don’t hit Enter), just
continue with the next line.
11.3.2.6
Functions
Now that you’ve got the hang of variables, let’s get to work with some functions. You declare a function with the following
syntax:
syntax:
(define
(
name
param-list
)
expressions
)
where
name
is the name assigned to this function,
param-list
is a space-delimited list of parameter names, and
expressions
is a series of expressions that the function executes when it’s called. For example: