CSC151 2007S, Class 10: Conditionals

Admin:
* Due: HW 5
* Homework 6 is now available.
* EC: Thursday's 4:30 talk on An Online Campus Map.
* EC: Tommy swims this weekend (all day Friday to Sunday)
* The next reading is on recursion.  Note that the reading is 
  complex enough that we'll spend two days on it.
  * Read -> Lab+Discussion -> Reread -> Continue lab

Overview:
* Preliminary Notes on Conditionals.
  * An example: Converting between numeric grades and letter grades
* Lab.
* Reflections.

/Lessons from the Reading/

* You can write if "statements" 
  (if TEST CONSEQUENT ALTERNATE)
* Meaninga
  * Evaluate the test
  * If the value of test is #f, evaluate the alternate and return
    its value
  * If the value of the test is not #f, evaluate the conseqeunt
    and return its value
* (if (< a b) a b) - Gives you the smaller of a and b
* (if (>= grade 60) 'pass 'fail) - If grade is >= 60, gives the
  symbol pass, o/w, gives the symbol 'fai

Problem: Convert letter grade (A, B, C, D, F) to a number grade
(define letter->number
  (lambda (grade)
    (if (equal? grade 'A) 96
        (if (equal? grade 'B) 86
	    (if (equal? grade 'C) 76
                (if (equal? grade 'D) 66
		    50))))))

(define number->letter
  (lambda (grade)
    (if (<= 90 grade 100) 'A

An alternate, teh cond

(cond
  (TEST1 EXP1)
  (TEST2 EXP2)
  ...
  (TESTn EXPn)
  (else ALTERNATE))

* Meaning:
  Evaluate TEST1.  If it holds (not false), use EXP1
  Otherwise, evaluate TEST2. ...

---


(define letter->number
  (lambda (grade)
    (if (equal? grade 'A) 96
        (if (equal? grade 'B) 86
            (if (equal? grade 'C) 76
                (if (equal? grade 'D) 66
                    50))))))

(define number->letter
  (lambda (grade)
    (if (<= 90 grade) 'A
        (if (<= 80 grade) 'B
            (if (<= 70 grade) 'C
                'D)))))
(define number->letter
  (lambda (grade)
    (cond
      ((<= 90 grade) 'A)
      ((<= 80 grade) 'B)
      ((<= 70 grade) 'C)
      (else 'D))))

(define valid-date?
  (lambda (month day)
    (or (and (= month 2) (<= 0 day 28))
        (and (or (= month 9) (= month 4) (= month 6) (= month 11))
             (<= 0 day 30))