10.2. Variables And Functions
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### 10.2. Variables And Functions

Now that we know that every Scheme statement is enclosed in parentheses, and that the function name/operator is listed first, we need to know how to create and use variables, and how to create and use functions. We'll start with the variables.

#### Declaring Variables

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) )

| Note
You'll have to put all of this on one line if you're using the console window. In general, however, you'll want to adopt a similar practice of indentation to help make your scripts more readable. We'll talk a bit more about this in the section on White Space.

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

#### 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.

#### 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.

#### 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.

#### 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:

            (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.

#### 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:

            (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.

- First, notice that the parameters don't have any "types" (that is, we didn't declare them as strings, or integers, etc.). Scheme is a type-less language. This is handy and allows for quicker script writing.

- Second, notice that we don't need to worry about how to "return" the result of our function -- the last statement is the value "returned" when calling this function. Type the function into the console, then try something like:

                  (AddXY (AddXY 5 6) 4)

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