Class 24: Imperative Programming

cs1120 Fall 2011David Evans19 October 2011

Running time practice throughout!

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Recap: list-append vs. mlist-append!Aliasingmlist-reverse!

I haven’t yet written solutions to the Chapter 9 Exercises, but I will do it soon if at least 3 people provide some evidence that they are trying to solve them! You can do this by posting your solutions or questions as comments on today’s class. I also how to see more comments/suggestions for the ElevenLearning version of the book (post here: http://www.cs.virginia.edu/cs1120/all/2011/on-line-version-of-course-book). If you do at least one of these, you can turn in PS5 at Monday’s class with no penalty.

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Appending vs. Appending!(define (list-append p q) (if (null? p) q (cons (car p) (list-append (cdr p) q))))

(define (mlist-append! p q) (if (null? p) (if (null? (mcdr p)) (mlist-append! (mcdr p) q))))

Running time:

Memory use:

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Appending vs. Appending!(define (list-append p q) (if (null? p) q (cons (car p) (list-append (cdr p) q))))

(define (mlist-append! p q) (if (null? p) (error “Cannot append to empty!”) (if (null? (mcdr p)) (set-mcdr! p q) (mlist-append! (mcdr p) q))))

Running time in (Np), Np is number of elements in p

Number of new cons cells: (Np)

Running time in (Np), number of elements in p

Number of new cons cells: 0

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Does it matter?To test: Define a procedure to make a list of length n.

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> (define p1 (make-list 100000))> (define p2 (make-list 100000))> (time (begin (list-append p1 p2) (void)))cpu time: 78 real time: 77 gc time: 47> (length p1)100000> (define m1 (make-mlist 100000))> (define m2 (make-mlist 100000))> (time (mlist-append! m1 m2))cpu time: 0 real time: 13 gc time: 0> (mlength m1)200000> (time (mlist-append! m1 m2))cpu time: 31 real time: 26 gc time: 0> (time (mlist-append! m1 m2))

(define (make-list n) (if (= n 0) null (cons 0 (make-list (- n 1)))))

(define (make-mlist n) (if (= n 0) null (mcons 0 (make-mlist (- n 1)))))

> (time (begin (list-append p1 p2) (void)))cpu time: 78 real time: 77 gc time: 47> (time (begin (list-append p1 p2) (void)))cpu time: 78 real time: 83 gc time: 31> (time (begin (list-append p1 p2) (void)))cpu time: 78 real time: 73 gc time: 31> (time (begin (list-append p1 p2) (void)))cpu time: 93 real time: 90 gc time: 31

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What about the built-in append?

> (time (begin (append p1 p2) (void)))cpu time: 0 real time: 1 gc time: 0> (time (begin (append p1 p2) (void)))cpu time: 0 real time: 2 gc time: 0> (define p200k (append p1 p2))> (define p400k (append p200k p200k))> (define p800k (append p400k p400k))> (define p1600k (append p800k p800k))> (length p1600k)1600000> (time (begin (append p400k p400k) (void)))cpu time: 0 real time: 6 gc time: 0> (time (begin (append p800k p800k) (void)))cpu time: 109 real time: 110 gc time: 94> (time (begin (append p1600k p1600k) (void)))cpu time: 1045 real time: 1044 gc time: 999> (time (begin (append p1600k p1600k) (void)))cpu time: 171 real time: 160 gc time: 125> (time (begin (append p1600k p1600k) (void)))cpu time: 203 real time: 197 gc time: 171

Possible reason built-in append is so much faster:Internal list representation doesn’t have to be just cons pairs (could jump quickly over many cells) Because lists are immutable, doesn’t have to copy list!

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What about the built-in mappend!

> (require racket/mpair)> (time (begin (mappend! m1 m2) (void)))cpu time: 0 real time: 1 gc time: 0> (time (begin (mappend! m1 m2) (void)))cpu time: 0 real time: 1 gc time: 0> (time (begin (mappend! m1 m2) (void)))

9

Still waiting…Favorite response to Exam Question 14: What topics to you hope to see in the remainder of the

course?:

“I can’t even begin to anticipate what will come next – most of what we’ve done so far has already blown my mind.”

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Why We’re Still Waiting / Blowing our Minds?

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m1:

2 3

(define (mlist-append! p q) (if (null? p) (error “Cannot append to empty!”) (if (null? (mcdr p)) (set-mcdr! p q) (mlist-append! (mcdr p) q))))

(mlist-append! m1 m2)

4m2: 5 6

(mlist-append! m1 m2)(mlist-append! m1 m2)

Aliasing

1

m1:

2 3

4m2: 5 67

> (mlist-append! m1 m2)> m1{1 2 3 4 5 6}> m2{4 5 6}> (set-mcar! m2 7)> m2{7 5 6}> m1{1 2 3 7 5 6}

Aliasing Problems

1

m1:

2 3

4m2: 5 6

> (mlist-append! m1 m2)> (mlist-append! m1 m2)> (mlist-length m1) (define (mlist-length p)

(if (null? p) 0 (+ 1 (mlist-length (mcdr p)))))

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Reversing

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Analyzing list-reverse(define (list-reverse p) (if (null? p) null (list-append (list-reverse (cdr p)) (list (car p)))))

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Reversing(define (list-reverse p) (if (null? p) null (list-append (list-reverse (cdr p)) (list (car p)))))

Running time is in (N2) where N is number of elements in p.

Number of new cons cells: for the list-appends: N-1 + N-2 + … + 1 = N2/ 2 + for the (list (car p)): N memory use is in (N2)

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mlist-reverse!Define a mlist-reverse! that reverses the elements of a mutable list. The output should be a mutable list with the elements in the reverse order. The number of cons cells it creates should not scale with the length of the list. (The input list can be mutated arbitrarily!)

Charge

PS5 Due Friday Except as noted earlier: if you post a comment/suggestion about the ElevenLearning book or a comment that suggests you are trying the Chapter 9 book exercises then it is due Monday