sicp.htm

Structure and Interpretation of Computer Programs

Every language has:

• Primitive expressions
• Means of combination
• Means of abstraction

E.g.,

(- 136 214)

(define size 2)
(define radius 10)

To evaluate combinations we:

1. Evaluate the sub-expressions
2. Apply the procedure that is the value of the leftmost subexpression to the other arguments

This is recursive in nature.

(* (+ 2 (* 4 6))
   (+ 3 5 7))  

Compound procedures:

• Numbers and arithmetic are primitive data and procedures
• Nesting combinations provides a means of combining operations
• Definitions that associate names with values provide a limited means of abstraction

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

(square 21)
441

There are two ways of evaluating:

• Applicative order: evaluate the arguments and then apply
• Normal order: fully expand and then reduce

Lisp uses applicative-order evaluation.

Conditionals

(define (abs x)
  (cond ((> x 0) x)
        ((= x 0) 0)
        ((< x 0) (- x))))

(define (abs x)
  (cond ((< x 0) (- x))
        (else x)))