# Study Notes (P3-P17)

Source: Internet
Author: User

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IDE: http://racket-lang.org/

Program Basic Design Elements

Basic Expressions are used to represent the simplest individual.

The composite elements can be constructed from a simple structure.

Abstract METHODS rename composite objects and operate them as units

General process definition form

General format of the definition process: (define (<Name> <formal paremeters>) <body>)

For example, define a square process.

`(Define (square X) (* X )) `

The square process can be used as the basic structure to form other processes, for example, to obtain the fourth power of a number.

` (Define (double-square X) (square X ))) `
Conditional expressions

The general expression of the condition: (cond (<P1> <E1>) (<P2> <E2>)... (<PN> <en> ))

For example, calculate the absolute value of a number.

`(Define (abs x) (cond (> x 0) X) (= x 0) 0) (<x 0) (-X )))) `

In addition, you can use the if expression, which is applicable to analysis in only two cases. evaluate an absolute value of a number, either less than 0 to return its opposite number, or return its own.

` (Define (f x) (if (<x 0) (-x) X )) `
Practice 1.6

It seems that there is no problem according to the conventional thinking. I also thought about it for a while, and finally I found out why. in the original scheme, the cond expression executes each condition branch. if each line of Cond is added with a "break" similar to C #, this problem should be solved.

Exercise 1.7 combined with exercise 1.8
``` (define (SQRT-begin guessvalue X) (Let (improved-guess (improve guessvalue X) (if (close-enough? Guessvalue improved-guess) improved-guess (SQRT-begin improved-guess X) (define (close-enough? A B) (LET (ratio (/a B) (and (

Ratio 0.999) (define (cube X) (* X) (define (sqrts X) (SQRT-begin 1.0 X) (define (improve guessvalue X) (average (/X (* guessvalue )) (* 2 guessvalue) (define (average a B) (/(+ a B) 3)
```
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