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CS3L: Introduction to Symbolic Programming

CS3L: Introduction to Symbolic Programming. Summer 2008 Colleen Lewis colleenL@berkeley.edu. Lecture 9: Recursion Rocks!. Today. Copies (we need volunteers) Count-evens Find-evens Recursion In Two Directions. (copies 3 ‘ha). (copies 4 ‘ha). (copies 2 ‘ha). (se ‘ha. (se ‘ha.

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CS3L: Introduction to Symbolic Programming

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  1. CS3L: Introduction to Symbolic Programming Summer 2008 Colleen Lewis colleenL@berkeley.edu Lecture 9: Recursion Rocks!

  2. Today • Copies (we need volunteers) • Count-evens • Find-evens • Recursion In Two Directions

  3. (copies 3 ‘ha) (copies 4 ‘ha) (copies 2 ‘ha) (se ‘ha (se ‘ha (copies 1 ‘ha) (se ‘ha (copies 0 ‘ha) Copies (define (copies n wd) (if (< n 1) ‘() (sentence wd (copies (- n 1) wd)))) (se ‘ha ‘()

  4. Count the number of even-numbers (define (count-evens sent) (cond ((empty? sent) ;empty? 0 ) ;base case: return 0 ((even? (first sent)) (+ 1 (count-evens (bf sent))));recurse on the ; rest of sent ((odd? (first sent) (+ 0 (count-evens (bf sent))) ;recurse on the ; rest of sent ))

  5. The Leap of Faith… Dude this seems like a hard problem! I’ll do this small piece and hope that someone can do the rest.

  6. (se 2 sent = ( 3 4 5 6 ) sent = ( 4 5 6 ) (se 4 sent = ( 5 6 ) sent = ( 6 ) (se 6 sent = ( ) > (find-evens '(2 3 4 5 6)) sent = ( 2 3 4 5 6 ) ‘() • (se 2 (se 4 (se 6 ‘()))) • (2 4 6)

  7. Problem: find all the even numbers in a sentence of numbers (define (find-evens sent) (cond ((empty? sent) ;base case '() ) ((odd? (first sent)) ;rec case 1 (find-evens (bf sent)) ) (else ;rec case 2: even (se (first sent) (find-evens (bf sent))) ) ))

  8. The Leap of Faith… Dude this seems like a hard problem! I’ll do this small piece and hope that someone can do the rest. • Sum-in-interval • What was the “small piece”? • What was the “rest”?

  9. 5 10 4 9 Sum-In-Interval 4 10 4 + + 10

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