sorting - How to check if a list is sorted in Racket? -
i trying define function takes list argument , returns boolean (#t or #f) indicating if list sorted in ascending order. output of function should this:
(sorted? '(1 2 3 4 5)) ; => #t (sorted? '(1 2 5 6 4)) ; => #f (sorted? '("frank" "adam" "eve")) ; => #f (sorted? '("adam" "eve" "frank")) ; => #t here's attempt @ solution:
(define (sorted? lst) (cond [(< (length lst) 2] [(<= (car lst) (cadr lst)) (sorted? (cdr lst))] [(string? lst) (string<=? (car lst) (cadr lst)) (sorted? (cdr lst))] [else #f])) i keep getting errors string part, , can't seem figure out i'm doing wrong. getting errors because have 2 different types in same definiton? i'm assuming since it's conditional statement if arguments in list don't fit condition, should ignored. i'm new @ , need help. please share knowledge if know i'm doing wrong , need fix this. thanks.
usually don't want mix numerical , lexical sort, lets imagine second can compare value:
(define (any<? b) (cond ((and (number? a) (number? b)) (< b)) ((and (string? a) (string? b)) (string<? b)) ;; can add types here. default cast ;; string , compare string ;; nb! format racket specific (else (string<? (format "~a" a) (format "~a" b))))) so lets sort something:
(define unsorted '(#\a 5 "hello" 9 (a c b) 10 (a b c) "50")) ;; nb: sort racket specific, r6rs has list-sort ;; arguments in reverse order. (define sorted (sort test any<?)) test2 ; ==> ((a b c) (a c b) 5 "50" 9 10 #\a "hello") ;; using named let current element ;; makes check null? once each pair (define (sorted? lst <) (or (null? lst) (let loop ((e (car lst)) (lst (cdr lst))) (or (null? lst) (and (not (< (car lst) e)) (loop (car lst) (cdr lst))))))) (sorted? unsorted any<?) ; ==> #f (sorted? sorted any<?) ; ==> #t you wouldn't use any<? specific comparison procedure data if know type of elements in list. e.g.
(sorted? '("a" "b" "cd") string<?) ; ==> #t (sorted? '(4 7 3 5 9 3 4 6) <) ; ==> #f
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