Overview

• We do some sample quiz questions.
• We gather preliminary questions and arrange them.
• I attempt answers.
• More Q&A, as appropriate.

Sample Quiz Questions

Topics

• map (o and section)
• homogeneous lists
• drawings-as-values
• conditionals
• GIMP tools

Types of Questions

• What is the value of this expression?
• Write a procedure to __.
• Document.

What is the value of each of these expressions?

    (iota 10)
    (append (iota 5) (iota 5))
    (reverse (append (iota 5) (iota 5)))
    (reverse (reverse (iota 3)))
    (list 1 2 3)
    (make-list 5 "hello")
    (reverse (append (make-list 2 'b) (make-list 3 'a)))
    (map * (list 1 2 3) (list 4 5 6))
    (map - (list 1 2 3) (list 4 5 6))
    (map / (list 1 2 3) (list 4 5 6))

    ; The following is a group
    (define f (lambda (x y) (+ (* 2 x) (* 3 y))))
    (map f (iota 3) (reverse (iota 3)))

    ; The following is a group;  Sketch the result
    (define canvas (image-new 200 200))
    (image-select-ellipse! canvas REPLACE 10 20 30 40)
    (context-set-fgcolor! "red")
    (image-fill! canvas)

    ; More "simple" ones
    (map (section / 1 <>) (list-drop (iota 5) 1))
    (map (o (section / 1 <>) (section + <> 1)) (iota 4))

Write a procedure to ...

    Draw a moon! (using GIMP tools; useful procedures are ...)
    Given a list of integers, double each of othem

            (define double-list
              (lambda (lst)
                (map (section * <> 2) lst)))

    Given two integers, x and y, return 
      The word "smaller" if x < y
      The word "equal", if x = y
      The word "greater", if x > y

            (define compare
              (lambda (x y)
                (if (< x y)
                    "smaller"
                    (if (= x y)
                        "equal"
                        "greater"))))

            (define compare
              (lambda (x y)
                (cond
                  [(< x y)
                   "smaller"]
                  [(= x y) 
                   "equal"]
                  [(> x y)
                   "greater"]
                  [else
                   "confusing"])))

Questions

Why would you have multiple consequents in a cond?

    (define draw-something
      (lambda (image thing)
        (cond
          [(equal? thing "moon")
           (image-select-ellipse! image REPLACE 20 30 40 50)
           (image-select-ellipse! image SUBTRACT 30 30 40 50)
           (context-set-fgcolor! "green")
           (image-fill! image)]
          [(equal? thing "football")
           (image-select-ellipse! image REPLACE 20 30 40 50)
           (image-select-ellipse! image INTERSECT 30 30 40 50)
           (context-set-fgcolor! "brown")
           (image-fill! image)]
          [else
           (display "You idiot, I don't know how to draw ")
           (display thing)
           (newline)
           (display "Try again")
           (newline)])))

What are the differences between cond, if, when?

If you have multiple tests, cond is generally easier to read. If you have only test, if or when are probably better.

If you have multiple things to do, if only allows one consequent and one alternate; cond and when allow multiple consequents.

Can you give an example of when?

    ; Only selects the ellipse when it's within the box
    (define image-select-ellipse-new!
      (lambda (image left top width height)
        (when (and (> left 0) (> top 0)
                   (< left (image-width image))
                   (< top (image-height image)))
           (image-select-ellipse! image REPLACE left top width height))))