6.090 IAP 05 Lecture 8

tags

payroll
  professor
  gradstudent - overhead on gradstudents

(define (make-professor name salary)
  (list name salary))

(define (professor-name prof)
  (first prof))

(define (professor-salary prof)
  (second prof))


(define (make-gradstudent name salary)
  (list name salary))

(define (gradstudent-name grad)
  (first grad))

(define (gradstudent-salary grad)
  (second grad))

(define (total-cost people-list)
  (if (null? list)
      0
      (+ ....?
         (total-cost (cdr people-list)))))

uh oh
could use (professor-salary (car people-list))
or        (gradstudent-salary (car people-list))
or        (second (car people-list))

all bad.  for different reasons (type analysis)

What if graduate student salarys cost more due to an overhead MIT
charges (70%).  Now problem is unsolvable.

Need a professor? and gradstudent?

modify abstractions:

(define (make-professor name salary)
  (list "professor" name salary))

(define (professor-name prof)
  (second prof))

(define (professor-salary prof)
  (third prof))


(define (make-gradstudent name salary)
  (list "gradstudent" name salary))

(define (gradstudent-name grad)
  (second grad))

(define (gradstudent-salary grad)
  (third grad))


now can write and include a professor? in the professor abstraction:

(define (professor? x)
  (and (pair? x) (string=? (car x) "professor")))

caveat: x must be a list with a string as the first element; ways to
fix, not relevant for example.

(define (gradstudent? x)
  (and (pair? x) (string=? (car x) "gradstudent")))

called tagging.

now we can write the procedure:

(define (total-cost people-list)
  (if (null? people-list)
      0
      (if (professor? (car people-list))
          (+ (professor-salary (car people-list))
             (total-cost (cdr people-list)))
	  (if (gradstudent? (car people-list))
              (+ (* 1.7 (gradstudent-salary (car people-list)))
                 (total-cost (cdr people-list)))
              (error "non professor or student in people-list")))))


Association lists

(assoc key alist)

(del-assoc key alist)

add association? cons

picture of alist


Trees

(define (make-node val left right)
  (list "node" val left right))

(define (node? x)
  (and (pair? x) (string=? (car x) "node")))

(define (node-val node)
  (second node))
(define (node-left node)
  (third node))
(define (node-right node)
  (fourth node))

(define (leaf? x)
  (not (node? x)))

;ordering property


(define (tree-contains? tree val)
  (if (leaf? tree)
      (if (= val tree)
          val
          #f)
      (if (< val (node-val tree))
          (tree-contains? (node-left tree) val)
          (tree-contains? (node-right tree) val))))

(define (sum-tree tree)
  (if (leaf? tree)
      tree
      (+ (sum-tree (node-left tree))
         (sum-tree (node-right tree)))))

;insert-list elem 

(define (insert-list elem lst)
  (if (null? lst)
      (list elem)
      (if (< elem (car lst))
          (cons elem lst)
          (cons (car lst) (insert-list elem (cdr lst))))))

(define (avg v1 v2)
  (/ (+ v1 v2) 2))

(define (insert-tree elem tree)
  (if (leaf? tree)
      (if (= elem tree)
          tree
          (if (< elem tree)
              (make-node (avg tree elem) elem tree)
	      (make-node (avg tree elem) tree elem)))
      (if (< elem (node-val tree))
          (make-node (node-val tree)
                     (insert-tree elem (node-left tree))
                     (node-right tree))
          (make-node (node-val tree)
		     (node-left tree)
                     (insert-tree elem (node-right tree))))))