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We are back to the standard start-of-class procedure.
Approximate optimistic overview
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Brand new! Please keep me posted on confusing or long parts.
Summary: Like mini-project 3, but with conditionals.
Also:
What is (and)? Why?
What is (or)? Why?
is-even?Here’s how many people defined is-even?.
(define is-even?
(lambda (val)
(if (= (remainder val 2) 0)
#t
#f)))
But the #t and #f are icky. Let’s think about a better way.
If val is even, what do we get from (= (remainder val 2) 0)? #t
If val is odd, what do we get from (= (remainder val 2) 0)? #f
(define is-even?
(lambda (val)
(= (remainder val 2) 0)))
zero?(define is-even?
(lambda (val)
(= (remainder val 2) 0)))
We make it a little clearer using the amazing zero? predicate.
(define is-even?
(lambda (val)
(zero? (remainder val 2))))
Can we make that even more concise with cut and composition?
(define is-even?
(o zero? (cut (remainder <> 2))))
is-even-integer?Here’s one approach.
(define is-even-integer?
(lambda (val)
(and (integer? val) (zero? (remainder val 2)))))
Can we do better?
(define is-even-integer?
(lambda (val)
(and (integer? val) (is-even? val))))
Reminder: Once you’ve written a procedure and need something similar in another procedure, call the previous procedure, don’t copy the body.
(define median-of-three
(lambda (x y z)
(cond
[(or (<= x y z) (<= z y x))
y]
[(or (<= y x z) (<= z x y))
x]
[(or (<= x z y) (<= y z x))
z])))
Hmmm … shouldn’t we have a default?
(define median-of-three
(lambda (x y z)
(cond
[(or (<= x y z) (<= z y x))
y]
[(or (<= y x z) (<= z x y))
x]
[(or (<= x z y) (<= y z x))
z]
[else
#f])))
Or, better yet,
(define median-of-three
(lambda (x y z)
(cond
[(or (<= x y z) (<= z y x))
y]
[(or (<= y x z) (<= z x y))
x]
[else
z])))
If we didn’t have if or cond, we could write
(define median-of-three
(lambda (x y z)
(or (and (or (<= x y z) (<= z y x))
y)
(and (or (<= y x z) (<= z x y))
x)
z)))
No, I don’t want you to take this last approach.
There’s also a math-based trick.
(define median-of-three
(lambda (x y z)
(- (+ x y z) (min x y z) (max x y z))))
One solution to 2b.
(define dark-circles
(apply beside
(map thickly-outlined-circle
(map rgb-darker
(map color-name->rgb
(list "red" "orange" "yellow" "green" "blue" "indigo" "violet"))))))
Another solution to 2b.
(define rainbox-colors
(list "red" "orange" "yellow" "green" "blue" "indigo" "violet"))
(apply beside
(map thickly-outlined-circle
(map (lambda (c) (rgb-darker (rgb-darker (color-name->rgb c))))
rainbow-colors)))
How would composition help? (TPS)
(apply beside
(map (o thickly-outlined-circle rgb-darker rgb-darker color-name->rgb)
rainbow-colors))
Do we have a final for this class?
We have an optional SoLA 5, which gives you one last chance to make up any missing LAs (except for the paper-only LAs). It will be online, so you can take it anywhere with Interweb access.
Will I have another chance to make up the tracing LA?
Yes. We will have an in-class tracing quiz each week until one of the following situations happen: (a) everyone has passed or (b) we reach the end of the semester.
Are there any other in-class-only LAs?
Yes. There’s one on diagramming structures in phase 3.
When is the next SoLA?
Week 8, right after spring break.