Overview

• Preliminaries
  • Admin.
  • Preparation for quiz / confusing topics / etc.
  • Quiz
• Lab

Admin

• Warning! Friday the 13th falls on a Friday this month. (Today even.)
• The outline and what we're doing don't match very well today. We've rearranged the course, but I like leaving my old notes around.
• If you're finding this difficult, know that you're not alone - this stuff is challenging.
• I don't think folks are taking enough advantage of the mentor sessions. If you're having difficulty in the class and not going, please drop me a note to explain why.
• Reading for Monday: Programming the GIMP Tools
• EC Opportunities:
  • Submit a short paragraph giving reactions/thoughts/etc. (I don't need a summary)
  • CS Table NEXT Friday (Pair programming)
  • Soccer 1:30 pm Saturday against people from a very flat state
  • Other?

Preparation for Quiz (and other discussion items)

I have a bit of code to show you that I hope resolves some recent questions. Then I'll give you ten minutes for additonal questions.

#lang racket
(require gigls/unsafe)

;;; File:
;;;   sample.rkt
;;; Author:
;;;   Samuel A. Rebelsky
;;; Summary:
;;    A quick sample to illustrate a variety of points about drawings.

;;; Procedure:
;;;   bigred
;;; Parameters:
;;;   drawing, a drawing
;;; Purpose:
;;;   To confuse students
;;; Produces:
;;;   A drawing
;;; Props:
;;;   Taken from a clicker question designed by SR and JD
(define bigred
  (lambda (drawing)
    (drawing-recolor drawing "red")
    (drawing-scale drawing 10)))

;;; Procedure:
;;;   preview
;;; Parameters:
;;;   d, a drawing
;;; Purpose:
;;;   preview the drawing in a new window
;;; Produces:
;;;   img, the id of the preview image
;;; Preconditions:
;;;   (drawing-right d) >= 1
;;;   (drawing-bottom d) >= 1
;;; Postconditions:
;;;   img contains a rendering of d
(define preview
  (lambda (d)
    (image-show (drawing->image d (drawing-right d) (drawing-bottom d)))))

;;; Value
;;;   d1, a sample drawing
;;; Type:
;;;   drawing
;;; Contents:
;;;   A black, 50x50 circle centered at (25,25)
(define d1
  (drawing-hshift 
   (drawing-vshift
    (drawing-scale drawing-unit-circle 50)
    25)
   25))

;;; Value:
;;;   d2
;;; Type:
;;;   drawing
;;; Contents:
;;;   A purple 50x50 circle, centered at (75,75)
(define d2
  (drawing-hshift 
   (drawing-vshift
    (drawing-scale 
     (drawing-recolor drawing-unit-circle "purple")
     50)
    75)
   75))

;;; Value:
;;;   d3
;;; Type:
;;;   drawing
;;; Contents:
;;;   A green rectangle
(define d3
  (drawing-hshift
   (drawing-vshift
    (drawing-scale
     (drawing-recolor
      drawing-unit-square
      "green")
     30)
    20)
   80))

;;; Value:
;;;   scene
;;; Type:
;;;   drawing
;;; Summary:
;;;   A simple scene
(define scene
  (drawing-group d1 d2 d3))

;;; Procedure:
;;;   drawing-munge
;;; Parameters:
;;;   drawing, a drawing
;;; Purpose:
;;;   Create a new drawing, based on the original drawing
;;; Produces:
;;;   new-drawing, a drawing
(define drawing-munge
  (lambda (d)
    (drawing-recolor (drawing-scale d 1.1) "blue")))

• Puzzlement about what what happens with parameters and what happens when we don't name drawings

• Disappearing values

  3 + (4*5)
  

  Just as intermediate values in a calculation disappear, so do intermedia drawings.

• Behind the scenes: Procedures

• When does order matter?

  • Scale vs. shift (traditionally scale first)
  • Group vs. almost anything

• What's the difference between drawing-bottom and drawing-height

• Where should your detail be: In the parameters or the pre/post?

  • I like detail in the pre/post
  • But documentation is like an essay, not code - it need not be perfertct at this stage of your career. As long as it makes sense, I'll probably accept it
  • Is it always possible to express postconditions formally? NO

• Can we derive immense satisfaction from watching SamR screw up while trying to do relatively easy things in DrRacket/Gimp?

  • Yes
  • And you should take that as a moral of "it's okay to screw up"
  • [No, I don't do so intentionally, just to make that point.]

Quiz

• Is the code in part 1 in the definitions pane or the interactions pane?
  • It shouldn't matter - It will give the same result either way

Lab

• Quick.