Although there are a couple of different methods for declaring variables, the preferred method is to use the let* construct. If you're familiar with other programming languages, this construct is equivalent to defining a list of local variables and a scope in which they're active. As an example, to declare two variables, a and b, initialized to 1 and 2, respectively, you'd write:

        (let*
           (
              (a 1)
              (b 2)
           )
           (+ a b)
        )
      

or, as one line:

(let* ( (a 1) (b 2) ) (+ a b) )

This declares two local variables, a and b, initializes them, then prints the sum of the two variables.

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.

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.

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.

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:

        (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.

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:

        (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:

(define (AddXY inX inY) (+ inX inY) )

AddXY is the function's name and inX and inY are the variables. This function takes its two parameters and adds them together.

If you've programmed in other imperative languages (like C/C++, Java, Pascal, etc.), you might notice that a couple of things are absent in this function definition when compared to other programming languages.