Overview

Repeat
  Repeat
    You ask questions
    I attempt answers
  Until we run out of time or you run out of questions
  Repeat
    I ask questions
    You attempt answers
    I provide followup comments
  Until we run out of time or you start asking questions again
Until we run out of time

What's on the quiz tomorrow?

• Questions on drawings.
• Questions on procedures.
• For example, "sketch or describe the image that results from the following expressions"
• For example, "write a procedure to do _"

Are all quizzes on paper?

• Yes.
• Allows you to think about ideas, and not particular syntax details

Tell me more about what you expect to see in postconditions.

• An attempt to formalize exactly what we want the procedure to do.

• Consider "smaller-right-neighbor"

  ;;; Procedure:
  ;;;   smaller-right-neighbor
  ;;; Parameters:
  ;;;   drawing, a drawing
  ;;; Purpose:
  ;;;   Create a smaller drawing, to the right of the original
  ;;; Produces:
  ;;;   smaller, a drawing
  ;;; Preconditions:
  ;;;   [No additional]
  ;;; Postconditions:
  ;;;   (drawing-width smaller) = (* .75 (drawing-width drawing))
  ;;;   (drawing-height smaller) = (* .75 (drawing-height drawing))
  ;;;   (drawing-left smaller) = (drawing-right drawing)
  ;;;   (drawing-top smaller) = (drawing-top drawing)
  

• Details:

  • How much smaller? (We could also put this in the purpose)
  • What does "to the right of" mean?
    • How far to the right?
    • What's the vertical alignment?

What does drawing-group do?

• Given two (or more) drawings, create a compound drawing that we can then scale, or shift, or recolor, or group again or ....

Can you explain the following documentation?

;;; Procedure:
;;;   circle
;;; Parameters:
;;;   x, a real number
;;;   y, a real number
;;;   r, a positive real number
;;;   color, a color
;;; Purpose:
;;;   Creates a drawing of a circle of radius r, centered at (x,y).
;;; Produces:
;;;   drawing, a drawing
;;; Preconditions:
;;;   [No additional]
;;; Postconditions:
;;;   drawing is an ellipse.  That is (drawing-ellipse? drawing) holds.
;;;   (drawing-left drawing) = (- x r)
;;;   (drawing-top drawing) = (- y r)
;;;   (drawing-width drawing) = (* 2 r)
;;;   (drawing-height drawing) = (* 2 r)
;;;   (drawing-width drawing) = (drawing-height drawing)
;;;   (drawing-color drawing) = color

• The answer was drawn on the board. Sorry.

Can you give an example of something that would require preconditions?

;;; Procedure:
;;;   drawing->image
;;; Parameters:
;;;   drawing, a drawing
;;;   width, an integer
;;;   height, an integer
;;; Purpose:
;;;   Render the portion of the drawing that falls between (0,0) and
;;;   (width,height) on an image whose size is width-by-height.
;;; Produces:
;;;   rendering, an image
;;; Preconditions:
;;;   width and height are positive
;;;   Some part of the drawing falls between (0,0) and (width,height)
;;; Postconditions:
;;;   image contains a visual representation of the portion of the
;;;     drawing in the range (0,0) to (width,height)
;;;   (image-width rendering) = width
;;;   (image-height rendering) = height
;;;   rendering is not yet displayed on the screen

Can you discuss the relationship of scaling and shifting?

• Shifting moves the center (and everything else)
• Scaling scales all aspects of the drawing
  • Width
  • Height
  • Left edge
  • Top edge
• Many people expect that when you scale something, you scale only the width and height, the center/position should stay the same.
• Issue: If we do this, the relationship between separate drawings changes.

Followup question: It feels like you are scaling the image, not the drawing.

• It depends on how you think about drawings.
• You also need a point to scale around.
• I think of drawings as being on an infinite plane, and I always scale around 0. (So you have to, too.)

Sample quiz question: What do you get from each of the following?

(define circle1 (hshift-drawing 100 (vshift-drawing 100 (scale-drawing 100 (recolor-drawing "red" drawing-unit-circle)))))

(define circle2
  (recolor-drawing 
   "green"
   (hshift-drawing 
    1 
    circle1)))

(define circle3 
  (hshift-drawing 
   100
   (vshift-drawing 
    100
    (scale-drawing 
     100
     (recolor-drawing 
      "blue" 
      (hshift-drawing 1 drawing-unit-circle))))))

• Note that circle2 and circle3, although they have similar definitions, appear at very different places.
• Let's look at circle 3
  • drawing-unit-circle: black, diameter 1, center 0,0
  • (hshift-drawing 1 ...): black, diameter 1, center 1,0
  • (recolor-drawing "blue"...): blue, diameter 1, center 1,0
  • (scale-drawing 100 ...): blue, diameter 100, center 100,0
  • (vshift-drawing 100 ...): blue, diameter 100, center 100,100
  • (hshift-drawing 100 ...): blue, diameter 100, center 200,100