SICP lec1a: # Lisp overview

origin link sicp 1

Black box(module)

(* x (+ a b))

First scene:
Every parentheses is a container called black box,
we can take a and b as to varible, like number, electric signal, whatever, we add them, then mul or expand them x times.

tips

primitive elements
+ : operator
17.4 : number

means of combination
(+ 3 17.4 5)

means of abstration
It’s a tree, but we write it as a plain text formation.

define

varible:

(define a (* 5 5))
// the above a will be like 5 * 5(expression could be varible)

function:

(define (square x) (* x x))
(square (square (square 5)))

sweet:

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

this seems like every define is a process.

condition:

(if (< x 100) (display "lower than 100"))

Recursive defination (Heron square root)

Hreon : get the square root of x

1. make a guess

2. improve guess by get the average of guess and x/guess

3. good-enough? done : improve guess

tips: good-enough? how close between x/guess
(< abs (- (square guess) x) .001)

---

Analyze as a tree:

(root 2)

(try 1 2)

(try (improve 1 2) 2)

(try 1.5 2)

---

Important note:

(define a (* 5 5))
(define (d) (* 5 5))

output:

a -> 25
d -> d procedure
(d) -> 25
(a) -> error