Structure & Interpretation of Computer ProgramsXiao Xiao

Ben Charrow

电脑程序的结构和编译 萧潇陈斌

Who We Are

Xiao XiaoMIT, Junior, Majoring in Computer Science

x_x@mit.edu

Ben CharrowMIT, Senior, Majoring in Computer Science

bcharrow@mit.edu

What We Want to Cover

The "Scheme" programming languageData Abstraction / 数据抽象Iterative and recursive processes / 重复和递归进程

Data structures / 数据结构Basic algorithms / 算法

Our Bible / 我们的圣经

电脑程序的 结构和编译

SICP (sick-pea)

Wizard Book /巫师书

Scheme

Language features Simple (syntax /语法 & specification /规格 ) Prefix notation /前缀表示法 Interpreted / 解释器 Tail-recursive /尾递归 Weakly typed /弱类型 Functional

Simplicity

The Scheme specification is 50 pages long

The Java specification is 684 pages long

Scheme is designed to be as simple and small as possible

Interpreters / 解释器

Characteristics Each line of code is only examined at runtime

/ 运行时间 Works on all computer architectures /计算机体系结构

Easy to fix problems Slower than compiled / 编译器 code

Scheme Keywords

• Keywords: Words that have a 'special meaning' to the scheme interpreter

• They are what allow you to make programs

Prefix Notation / 前缀表示法

The function (函数 ) always comes before its arguments

(+ 3 4) (- 5 3) (* 5 4)

7 2 20

Keyword Example: define

(define x y) Makes the value of x the value of y Does not work if y does not have a value

(define x 3)x3

Combining Ideas

(define x 12)

(define x 5)(define y x)(* x y)25

15 (+ x 3)

Defining ProceduresWant to define a function that squares

its argument Call the function 'square' Call the argument 'arg'

How do we write it in scheme?

Defining Procedures (cont.)

(define (square arg)

(* arg arg))

(square 4)

16

Weakly Typed

Unlike C++ and Java, scheme does not worry about types! Don't need to declare variables as 'int' or 'char' Makes generalizing code easier Be careful what you pass!

(define (add arg1 arg2)

(+ arg1 arg2))

(add 'a 'b)

Another Keyword: cond

cond Similar to the 'if' statement in C++ or Java If predicate is #t / true, then it evaluates the

statement If predicate is #f / false, then it moves on

(define (sign? arg)

(cond ((> 0 arg) ’negative)

((< 0 arg) ’positive)

(else 'zero)))

Recursion (next lecture)

A 'recursive procedure' calls itself

Factorial(n) = n! If n > 0, Factorial(n) = n*Factorial(n-1) otherwise factorial(n) = 1 Factorial is a recursive procedure