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A Common Pitfall set! of a function parameter doesn’t work as you might think!

A Common Pitfall set! of a function parameter doesn’t work as you might think! (define mylist '(1 2 3)) ==> mylist (define (alter1 <function>) (method ((s <list>)) (set! s '(4 5 6)))) ==> alter1 (alter1 mylist) ==> (4 5 6) mylist ==> ???. A Common Pitfall

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A Common Pitfall set! of a function parameter doesn’t work as you might think!

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  1. A Common Pitfall • set! of a function parameter • doesn’t work as you might think! • (define mylist '(1 2 3)) • ==> mylist • (define (alter1 <function>) • (method ((s <list>)) (set! s '(4 5 6)))) • ==> alter1 • (alter1 mylist) • ==> (4 5 6) • mylist • ==> ???

  2. A Common Pitfall • set! of a function parameter • doesn’t work as you might think! • (define mylist '(1 2 3)) • ==> mylist • (define (alter1 <function>) • (method ((s <list>)) (set! s '(4 5 6)))) • ==> alter1 • (alter1 mylist) • ==> (4 5 6) • mylist • ==> (1 2 3)

  3. However: (define mylist '(1 2 3)) ==> mylist (define (alter2 <function>) (method ((l <list>)) (set! (tail l) '(4 5 6)))) (alter2 mylist) ==> (4 5 6) mylist ==> ???

  4. However: (define mylist '(1 2 3)) ==> mylist (define (alter2 <function>) (method ((l <list>)) (set! (tail l) '(4 5 6)))) (alter2 mylist) ==> (4 5 6) mylist ==> (1 4 5 6)

  5. Operations on a priority queue (make-prioq) return empty structure (insert! e pq) put entry e in pq (extract-min! pq) remove highest priority entry in pq and return it

  6. Vectors (a.k.a. arrays) (n-vector k) space for k things, indices 0 to k-1 (vector x1 ...) evaluates x1 ... and puts them into a vector (index vec i) get i'th element in O(1) time (index-setter! i val vec) put val at element i of vector vec

  7. ? (define v (n-vector 13)) ==> v ? v ==> [#f #f #f #f #f #f #f #f #f #f #f #f #f] ? (define u (vector 1 2 3 4 5)) ==> u ? u ==> [1 2 3 4 5] ? (index-setter! 3 u 100) ==> 100 ? u ==> [1 2 3 100 5] ? (index u 0) ==> 1 ? (index u 5) ==> {Error : "Index : Index 5 out of bounds for vector."}

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