On 11/06/2012 03:23 PM, Ryan Culpepper wrote:
On 11/06/2012 02:59 PM, Grant Rettke wrote:
Hi,
Usually there are homework problems and stuff for tweaking strings but I
was just playing around for the fun of it. I wondered what is the "best
way" to do a couple of simple tasks but without regex. Here are two of
them. They are not optimized I just wrote them in a way that seemed the
simplest. What is a better way or different way?
#lang racket
(require rackunit)
;; string -> string
;; Removes vowels from the input
(define remove-vowels
(lambda (word)
(let ((test (lambda (c) (not (member c (list #\a #\e #\i #\o
#\u)))))
(ls (string->list word)))
(list->string (filter test ls)))))
Here's one alternative that should allocate half as many intermediate
pairs (by switching from string->list and filter to in-string and
#:unless):
I take that back; for/list accumulates and reverses its result, so it
creates almost as many pairs as your version does.
Whoops,
Ryan
(define (remove-vowels s)
(list->string
(for/list ([c (in-string s)]
#:unless (memv c '(#\a #\e #\i #\o #\u)))
c)))
If I were really worried about performance, I would allocate a new
string the size of the old string and copy characters from old to new,
skipping vowels, and take the appropriate substring at the end. I might
also delay the allocation of the copy until I found the first vowel; if
there are no values, then the function can just return the string itself
(unless the function needs to return a fresh string because of mutation
elsewhere in the program).
(check-equal? (remove-vowels "") "")
(check-equal? (remove-vowels "f") "f")
(check-equal? (remove-vowels "a e i o u") " ")
;; string -> string
;; Replaces spaces in the input with dashes
(define replace-spaces
(lambda (word)
(let ((test (lambda (c) (if (equal? c #\space) #\- c)))
(ls (string->list word)))
(list->string (map test ls)))))
(check-equal? (replace-spaces "") "")
(check-equal? (replace-spaces " ") "-")
(check-equal? (replace-spaces "a b c") "a-b-c")
Similar, except here you wouldn't have to worry about resizing the copy
at the end.
Ryan
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