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CS61A Lecture 10

CS61A Lecture 10. 2011-07-06 Colleen Lewis. Clicker poll . Where do you think you’re learning the most? I assume you’ll learn the most in lab & disc (so I won’t be offended) Lecture Lab Discussion Lab & Discussion Lecture ≈ Lab ≈ Discussion. TODAY. Make a calculator program

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CS61A Lecture 10

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  1. CS61A Lecture 10 2011-07-06 Colleen Lewis

  2. Clicker poll  Where do you think you’re learning the most? I assume you’ll learn the most in lab & disc (so I won’t be offended) Lecture Lab Discussion Lab & Discussion Lecture ≈ Lab ≈ Discussion

  3. TODAY • Make a calculator program • To better understand how the Scheme interpreter works • STEP 1: calc-apply • STEP 2: list versus quote (Scheme primitives) • STEP 3: read (Scheme primitive) • STEP 4: read-print loop • STEP 5: read-eval-print loop • STEP 6: calc-eval

  4. calc-apply DEMO

  5. calc-apply STk> (calc-apply '+ '(1 2 3)) 6 STk> (calc-apply '* '(2 4 3)) 24 STk> (calc-apply '/ '(10 2)) 5 STk> (calc-apply '- '(9 2 3 1)) 3

  6. (define (calc-apply fn-wd arg-list) (cond ((equal? fn-wd '+) (add-up-stuff-in arg-list)) ((equal? fn-wd '-) (subtract-stuff-in arg-list)) ((equal? fn-wd '*) (multiply-stuff-in arg-list)) ((equal? fn-wd '/) (divide-stuff-in arg-list)) (else (error "Calc: bad op: " fn-wd))))

  7. add-up-stuff-in (define (add-up-stuff-in lst) (accumulate + 0 lst)) STk> (accumulate + 0 '(1 2 4)) .. -> + with args = (4 0) .. <- + returns 4 .. -> + with args = (2 4) .. <- + returns 6 .. -> + with args = (1 6) .. <- + returns 7 7

  8. The two arguments to calc-apply are: A word and a sentence A word and a list A procedure and a sentence A procedure and a list Not sure…

  9. list versus quote

  10. list versus quote STk> '(1 2 +) (1 2 +) STk> (list 1 2 +) (1 2 #[closure arglist=args 7ff53de8])

  11. (read) Demo

  12. Demo (read) I typed this! After I hit return, Scheme printed this I didn’t have to quote words ' is really syntactic sugar for quote(a special form) STk> (read) 45 45 STk> (read) hello hello STk> (read) 'hello (quote hello)

  13. Demo (read) STk> (define a (read)) hello a STk> a hello STk>

  14. Demo (read) Not: #[closure arglist=args 7ff53de8] STk> (define b (read)) (+ 1 2) b STk> b (+ 1 2) STk> (car b) + STk> (list-ref b 1) 1

  15. Demo (read) Woah! read figured out it was a list within a list. STk> (define c (read)) (+ 3 (+ 1 2)) c STk> (list-ref c 2) (+ 1 2) STk> (car c) +

  16. Demo (read) read waits for me to put necessary close-parens STk> (define d (read)) (+ 3 ) d STk> d (+ 3)

  17. read Summary • Prompts user for input • NOT a function • Whatever the user types it returns • They can type words (without quotes) • They can type numbers • They can type lists • If it looks like a list it waits for you to put necessary close parentheses

  18. Read-Print Loop

  19. (read-print) display prints stuff Make the line above visible print prints stuff on a new line Waits for user input recursive call (infinite loop) (define (read-print) (display "type here: ") (flush) (print (read)) (read-print))

  20. I’m typing HERE not at STk> Infinite loop! STk> (read-print) type here: 4 4 type here: hi hi type here: (+ 1 2) (+ 1 2) type here: (+ (+ 3 4) 2) (+ (+ 3 4) 2) type here:

  21. Read-Eval-Print Loop

  22. (calc) demo (read-print) Was sort of silly (calc) actually does something STk> (calc) calc: 1 1 calc: (+ 2 3) 5 calc: (+ 2 (* 3 4)) 14

  23. (calc) demo – it doesn’t have variables or “real” functions calc: + *** Error: Calc: bad expression: + Current eval stack: STk> (calc) calc: x *** Error: Calc: bad expression: x Current eval stack:

  24. (read-print) (define (read-print) (display "type here: ") (flush) (print (read)) (read-print))

  25. (calc) read-eval-print loop (define (calc) (display "calc: ") (flush) (print (calc-eval (read))) (calc))

  26. Representing Math 2 1 2 + 1 Children Translating to Scheme (+ 1 2) car: + cdr: (1 2)

  27. Representing Math in Scheme 5 4 + * 2 (+ (* 2 4) 5) car: + cdr: ((* 2 4) 5)

  28. Representing Math in Scheme How many open parens? A) 1 B) 2 C) 3 D) 4 E) 5 5 4 1 + * + 2 3 (+ (* 3 (+ 2 1) 4) 5) car: + cdr:((* 3 (+ 2 1) 4) 5)

  29. Remember the(calc)read-eval-print loop? (define (calc) (display "calc: ") (flush) (print (calc-eval (read))) (calc))

  30. calc-evalbase case STk> (calc) calc: 1 1 (define (calc-eval exp) (cond ((number? exp) exp) ((list? exp) _____________ (else (error “Calc: bad exp”))))

  31. Remembercalc-apply? STk> (calc-apply '+ '(1 2 3)) 6 STk> (calc-apply '* '(2 4 3)) 24 STk> (calc-apply '/ '(10 2)) 5 STk> (calc-apply '- '(9 2 3 1)) 3

  32. Remember map? STk> (map square ‘(1 2 3)) (1 4 9)

  33. calc-eval 2 + 1 ‘+ ‘(1 2) STk> (calc) calc: (+ 1 2) 3 (define (calc-eval exp) (cond ((number? exp) exp) ((list? exp) (calc-apply (car exp) (map calc-eval (cdr exp)))) (else (error “Calc: bad exp”))))

  34. calc-eval 5 4 + * 2 ‘+ ‘(8 5) STk> (calc) calc: (+ (* 2 4) 5) 40 (define (calc-eval exp) (cond ((number? exp) exp) ((list? exp) (calc-apply (car exp) (map calc-eval (cdr exp)))) (else (error “Calc: bad exp”))))

  35. (calc-eval exp)Works for bigger trees! 4 + 5 2 * 2 4 + * 2 1 3 + * + 3 3 (calc-apply (car exp) (map calc-eval (cdr exp))))

  36. deep-map

  37. Remember map? Meet deep-map STk> (map square ‘(1 2 3)) (1 4 9) STk> (deep-map square ‘(1 2 3)) (1 4 9) STk> (deep-map square ‘((3.4)(5 6))) ((9 . 16) (25 36)) STk> (deep-map square 3) 9 STk> (deep-map square ‘()) ()

  38. deep-map base cases STk> (deep-map square 3) 9 STk> (deep-map square ‘()) () (define (deep-map fn arg) (cond ((null? arg) '()) ((pair? arg) _____________ (else (fn arg))))

  39. Draw ‘((3 . 4) (5 6)) How many pairs? A) 1 B) 2 C) 3 D) 4 E) 5

  40. Draw ‘((3 . 4) (5 6)) car car car car car cdr cdr cdr cdr cdr 4 5 3 6 SOLUTION

  41. (deep-map sq ‘((3 . 4) (5 6)) car car car car car car cdr cdr cdr cdr cdr cdr 4 5 3 6 (cons (deep-map fn (car arg)) (deep-map fn (cdr arg))) ? ?

  42. deep-map solution (define (deep-map fn arg) (cond ((null? arg) '()) ((pair? arg) (cons (deep-map fn (car arg)) (deep-map fn (cdr arg)))) (else (fn arg))))

  43. map (define (map fn seq) (if (null? seq) ‘() (cons (fn (car seq)) (map fn (cdr seq)))))

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