# Scheme Programming/Abstractions with Data

## Contents

### Introduction to Complex Numbers[edit]

In order to show how abstractions with data can be built, we're going to go through making a complex number package. A complex number is one that has 2 parts, a real part, and an imaginary part. They are often written in one of two ways, in rectangular form:

And in polar form:

Now, we can can do all of the usual arithmetic with complex numbers, addition, subtraction, multiplication and division. There are simple formulae for this;

Addition:

Subtraction:

Multiplication:

Division:

Note how multiplication and division are best expressed in polar form, while addition and subtraction are best expressed in rectangular form. This raises an interesting question: How does one best go about computing these? Do we have one internal representation? If so, which do we chose? There are a large amount of questions. These can be answered by trying to implement a new type of data; the complex number type.

### Creating our Generic 'Typed' Variable[edit]

Firstly, we shall create a generic 'Typed' variable:

(define typed-variable (lambda (type value) (cons 'Typed (list type value)) ) )

We now need a way to tell if a given variable has a type:

(define typed? (lambda (var) (and (list? var) (= 'Typed (car var))) ) )

Now, we've introduced two important concepts here, a 'Predicate' and a 'Constructor'. The first is a construct to find if some data is of the correct form, and the second is a procedure that builds our data structure for us.

We must have a way of extracting our data (in this case, the type) from this structure, a way of 'selecting' it:

(define type-of (lambda (var) (if (typed? var) (car (cdr var) ) ) )

### Creating our Complex Number Data Type[edit]

#### Building our Constructors[edit]

Using this typed value, we can go on to form a more detailed data structure for out complex number:

(define complex-rect (lambda (a b) (typed-variable 'Rect-Complex (list a b)) ) )

Now let's continue, and create a `complex-polar`

:

(define complex-polar (lambda (r thet) (typed-variable 'Polar-Complex (list r thet)) ) )

(define complex (lambda (type first-var second-var) (if (equal? 'type Polar) (cons (complex-polar first-var second-var) (complex-rect (sqrt (+ (expt first-var 2) (expt second-var 2) ) ) 0 ) ) ;; Change second half to be the calculated values. (cons (complex-polar 0 0) (complex-rect first-var second-var)) ) ) )

#### Building our Predicates[edit]

We have our constructors, now we need our predicates:

(define is-complex? (lambda (var) (and (typed? (car var)) (or (= 'Rect-Complex (type-of (car var))) (= 'Polar-Complex (type-of (car var))) ) ) ) )

Now we can define our arithmetic in terms of these procedures.