Today’s start-of-class procedure
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Approximate optimistic overview
Scholarly
Artistic
Multicultural
Peer
Musical, theatric, sporting, and academic events involving this section’s students are welcome.
Wellness
Misc
These do not earn tokens, but are worth your consideration.
I saw many of you doing something like this.
(define ccc
(lambda (c1 c2 c3)
(overlay (solid-circle 20 c1)
(solid-circle 40 c2)
(solid-circle 60 c3))))
; Instructions: Make three concentric circles with red, a mix of red and
; blue, and then blue.
(define red-blue-mix
(overlay (solid-circle 20 (rgb 255 0 0))
(solid-circle 40 (rgb 255 0 255))
(solid-circle 60 (rgb 0 0 255))))
Why might I object to that definition of red-blue-mix? (TPS)
Given that we alreaday wrote ccc, we SHOULD write
(define red-blue-mix (ccc (rgb 255 0 0) (rgb 255 0 255) (rgb 0 0 255)))
How does the describe-image procedure work?
When you make an image (using almost any image-making procedure), you can specify a description of the image.
describe-imagethen grabs that description. If you don’t provide a description, it does the best job it can. (We’ll do an example in class.)
> (define tri (solid-polygon (list (pt 0 0) (pt 25 75) (pt 0 100))))
. . solid-polygon: arity mismatch;
the expected number of arguments does not match the given number
given: 1
> (define tri (solid-polygon (list (pt 0 0) (pt 25 75) (pt 0 100)) "red"))
> tri
.
> (describe-image tri)
"a solid red polygon built from the points (list (pt 0 0) (pt 25 75) (pt 0 100))"
> (define tri (solid-polygon (list (pt 0 0) (pt 25 75) (pt 0 100)) "red"
"the best red triangle ever"))
> tri
.
> (describe-image tri)
"the best red triangle ever"
> (define something (above (solid-isosceles-triangle 50 40 "red")
(solid-square 40 "brown")))
> (describe-image something)
"a center-aligned stack of images (a solid red 50-by-40 isosceles triangle above a solid brown square with side length 40)"
> something
.
> (define something (above (solid-isosceles-triangle 50 40 "red")
(solid-square 40 "brown")
"a simple picture of a house"))
> something
.
> (describe-image something)
"a simple picture of a house"
Can you provide more clarification on this part of the assignment: “You should use outlined-polygon for this part of the assignment. Note that outlined-polygon only works correctly if you create the points in clockwise order. Otherwise, the outline goes on the “inside” rather than the “outside”.”
I’ll try drawing pictures in class
> (define tri2 (outlined-polygon (list (pt 0 0) (pt 0 100) (pt 25 75))
"red"
20
"the best triangle ever, now with a red outline"))
> (describe-image tri2)
"the best triangle ever, now with a red outline"
> tri2
.
> (define tri2 (outlined-polygon (list (pt 0 0) (pt 25 75) (pt 0 100))
"red"
20
"the best triangle ever, now with a red outline"))
> (describe-image tri2)
"the best triangle ever, now with a red outline"
> tri2
.
What is meant by “Code has been reformatted with Ctrl-I before submitting.”
If you hit Ctrl-I, the code is re-indented for readability. You’re now at the stage where you should be striving for moderately well organized code..
What does the “arity mismatch” error message mean and how can we fix the error?
It usually means that you’ve provided the wrong number of arguments in calling a procedure. You fix it by providing the correct number of arguments. You may have to look at the documentation to figure out what arguments you are missing.
Will you please describe DrRacket’s understanding of the cordinate system? A visual representation would be useful.
I’ll draw one in class.
I still don’t quite understand what the program does when it shifts the image. “As the documentation for solid-polygon suggests, no matter what coordinates you use for the vertices, the polygon gets shifted so that the leftmost point is at the left edge of the image and the topmost point is at the top edge of the image. You can likely use this feature to your advantage. For example, if you’re making a somewhat symmetrical image, you can use (0,0) as the center.”
I’ll try to go over that in class.
> (above (solid-polygon (list (pt 0 0) (pt 25 75) (pt 0 100)) "red")
(solid-polygon (list (pt 50 0) (pt 75 75) (pt 50 100)) "blue"))
.
> (above (solid-polygon (list (pt 0 0) (pt 25 75) (pt 0 100)) "red")
(hshift (solid-polygon (list (pt 50 0) (pt 75 75) (pt 50 100)) "blue")
50))
.
> (above/align "left"
(solid-polygon (list (pt 0 0) (pt 25 75) (pt 0 100)) "red")
(hshift (solid-polygon (list (pt 50 0) (pt 75 75) (pt 50 100)) "blue")
50))
.
Can we make a pentagon “by hand” so that I understand all of the steps?
Certainly.
> (define theta (* 2/5 pi))
> (define radius (/ 50 (sin (* 1/5 pi))))
> (make-polar radius theta)
26.286555605956682+80.90169943749473i
> (define pt1x (real-part (make-polar radius theta)))
> (define pt1y (imag-part (make-polar radius theta)))
> pt1y
80.90169943749473
> (define pt2x (real-part (make-polar radius (* 2 theta))))
> (define pt2y (imag-part (make-polar radius (* 2 theta))))
> (define pt3 (pt (real-part (make-polar radius (* 2 theta)))
(imag-part (make-polar radius (* 2 theta)))))
> (define pt3 (pt (real-part (make-polar radius (* 3 theta)))
(imag-part (make-polar radius (* 3 theta)))))
> (define pt4 (pt (real-part (make-polar radius (* 4 theta)))
(imag-part (make-polar radius (* 4 theta)))))
> (define pentagon (solid-polygon (list (pt radius 0)
(pt pt1x pt1y)
(pt pt2x pt2y)
pt3
pt4)
"brown"
"a brown pentagon with side length 100"))
> pentagon
.
> (describe-image pentagon)
"a brown pentagon with side length 100"
> (beside (solid-square 100 "black")
pentagon)
.
For part three, we have to make something creative. “What is particularly creative?”
Something that’s not an immediate consequence of parts one and two.
As the SCOTUS says, “I’ll know it when I see it.”
What does “non-trivial” mean?
“Trivial” means “as simple as possible”. Non-trivial means “not trivial”. In Scheme, if it needs to be further evaluated, it’s non-trivial.
Alternately, for a typical expression, if there are parentheses around any part, that part is non-trivial. If there aren’t parentheses, it’s trivial.
3is trivial."Hello"is trivial.(equal? 3 "hello")is non-trivial.
In the example: (add-3 (* 2 3) (+ 8 3) (/ 1 2)), do we start with solving
(/ 1 2) first? Or (* 2 3) first? Or all together?
We usually go left to right on the subexpressions. (The official Scheme standard says that we can do them in any order.)
Today’s lab is on paper! Grab a sheet of paper and a pen.