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Lists of Lists of Lists of ...

CS 5010 Program Design Paradigms “Bootcamp”

Week 03, Lesson 4

Lists of Lists of ... strings

"alice""bob""carole"("alice" "bob")(("alice" "bob") "carole")("alice" (("alice" "bob") "dave") "eve")

Data Definition

An S-expression of Strings (SoS) is either-- a String-- a List of SoS's

A List of SoS's (LoSS) is either-- empty-- (cons SoS LoSS)

This is mutual recursion

SoS LoSS

defined in terms of

defined in terms of

Mutually Recursive Data Definitions

• One task, one function• So: one function per data definition• Here, functions come in pairs• Write the contracts and purpose statements

together, or• Write one, and the other one will appear as a

wishlist function

Template: functions come in pairs

;; sos-fn : SoS -> ??(define (sos-fn s) (cond [(string? s) ...] [else (loss-fn s)]))

;; loss-fn : LoSS -> ??(define (loss-fn los) (cond [(empty? los) ...] [else (... (sos-fn (first los)) (loss-fn (rest los)))]))

occurs-in?;; occurs-in? : Sos String -> Boolean;; returns true if the given string occurs somewhere

in the given sos.;; occurs-in-loss? : Loss String -> Boolean;; returns true if the given string occurs somewhere

in the given loss.

Examples/Tests(check-expect (occurs-in? "alice" "alice") true)(check-expect (occurs-in? "bob" "alice") false)

(check-expect (occurs-in? (list "alice" "bob") "cathy") false)

(check-expect (occurs-in? (list (list "alice" "bob") "carole") "bob") true)

(check-expect (occurs-in? (list "alice" (list (list "alice" "bob") "dave") "eve") "bob") true)

See book re list vs. cons

The Code(define (occurs-in? sos str) (cond [(string? sos) (string=? sos str)] [else (occurs-in-loss? sos str)]))

(define (occurs-in-loss? loss str) (cond [(empty? loss) false] [else (or (occurs-in? (first loss) str) (occurs-in-loss? (rest loss) str))]))

Finger Exercises;; number-of-strings : Sos -> Number;; number-of-strings-in-loss : Loss -> Number;; returns the number of strings in the given sos or

loss.

;; characters-in : Sos -> Number;; characters-in-loss : Loss -> Number;; returns the total number of characters in the

strings in the given sos or loss.

Summary

• Trees of structures occur all the time• Mutually recursive data definitions• Mutual recursion in the data definition leads to mutual

recursion in the template• Mutual recursion in the template leads to mutual

recursion in the code• Study the tests: – always ask: how could a program pass the tests and still be

wrong?– always ask: how could a program fail the tests and still be

correct?