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Variables

One defines variables with the define function:

    (define x 13)

The above expression creates a variable named x in the current scope and initializes it to the value 13. If the initializer is missing:

    (define y)

an uninitialized variable error is generated:

    EXCEPTION: uninitializedVariable
    variable y is uninitialized

Functions

A function definition is another way to create a variable. There are two ways to define a function. The first is through a regular variable definition, where the initializer is a lambda expression:

    (define square (lambda (x) (* x x)))

    (square 3)
    -> 9

    (inspect square)
    -> square is <function anonymous(x)>

The above expression defines a function that returns the square of its argument and emphasizes that the name of a function is simply a variable that is bound to a lambda (i.e. an anonymous function). The second method uses a special syntax:

    (define (square x) (* x x))
    
    (inspect square)
    -> square is <function square(x)>

In either case, a variable is created and bound to some entity that knows how to compute values.

Scopes, Environments and Objects

When one defines a variable, the variable name and value are inserted into the current scope. In actuality, the name-value pair is stored in a table called an environment. For the predefined variable this, its value is the current scope or environment. For example, consider the following interaction:

    (define n 10)

    (ppTable this)
    -> <object 8393>
                 __label  : environment
               __context  : <environment 4495>
                 __level  : 0
           __constructor  : nil
                    this  : <environment 8393>
                       n  : 10

The pp function will print out the list of variables and their values for the given environment. Among other information stored in the current environment, we see an entry for n and its value is indeed 10.

The Scam object system is based upon environments. We will learn about objects in a later chapter.

Defining Variables Programatically

The function addSymbol is used to define variables on the fly. For example, to define a variable named x in the current scope and to initialized it to 13, one might use the following expression:

    (addSymbol 'x 13 this)

You can also define functions this way:

    (addSymbol 'square (lambda (x) (* x x)) this)

Since addSymbol evaluates all its arguments, the first argument can be any expression that resolves to a symbol, the second argument can be any expression that resolves to an appropriate value, and the third argument can be any expression that resolves to an environment or object.

Variable naming

Unlike many languages, Scam is quite liberal in regards to legal variable names. A variable can't begin with any of the these characters: 0123456789;,`'"() nor whitespace and cannot contain any of these characters: ;,`'"() nor whitespace. Typically, variable names start with a letter or underscore, but they do not have to. This flexibility allows Scam programmers to easily define new functions that have appropriate names. Here is a function that increments the value of its argument:

    (define (+1 n) (+ n 1))

While Scam lets you name variables in wild ways:

    (define $#1_2!3iiiiii@ 7)

you should temper your creativity if it gets out of hand. While the name $#1_2!3iiiiii@ is a perfectly good variable name from Scam's point of view, it is a particularly poor name from the point of making your Scam programs readable by you and others. It is important that your variable names reflect their purpose.

lusth@cs.ua.edu


Lists, Strings, and Arrays Top AssignmentVariables Contents