# [plt-scheme] Permutation correct way to do it ?

 From: Matthias Felleisen (matthias at ccs.neu.edu) Date: Thu Apr 3 09:50:55 EDT 2008 Previous message: [plt-scheme] Permutation correct way to do it ? Next message: [plt-scheme] Permutation correct way to do it ? Messages sorted by: [date] [thread] [subject] [author]

```Tests should be formulated as
expression
expected result

Usually we use the testing.ss teachpack like this:
(check-expect (arrangements '(a t))
'((a t) (t a)))
followed by
(generate-report)
at the very end. (This will go away in 4.0.)

Why don't you do that and see what happens. -- Matthias

On Apr 3, 2008, at 7:56 AM, richard gere wrote:
> After struggling for hours , finally i was able to solve the
> excercise 12.4.2 .
>
> I want to know , if this is the correct approach , and does my
> solution reflects what the author of the exercise had in its mind ?
> Here is my code :
>
>
>
> ;;words
>
> ;;A word is either
> ;;1. empty or
> ;;2. (cons a wd) where a is a symbol in 'a,'b,'c,...'z and wd is a
> word
> ;;eg (cons 'b empty) is a word , and
> ;; (cons 'r (cons 'e (cons 'a (cons 'd empty) ))) is a word
>
> ;;A list-of-words is either
> ;;1. empty or
> ;;2 (cons wd lw) where wd is a word and lw is a list-of-words
> ;;(cons (cons 'r (cons 'e (cons 'a (cons 'd empty))))
> ;;      (cons (cons 'd (cons 'e (cons 'a (cons 'r empty)))) empty))
>
>
> ;;length1 : word -> number
> ;;to compute the length of a-word or for any list
> (define (length1 a-word)
>   (cond
>     [(empty? a-word) 0]
>     [ else (+ 1 (length1 (rest a-word)))]))
>
> ;;tests
> ;;(length1 (cons 'a (cons 'b empty)))
>
>
> ;;insert-at : number symbol word -> word
> ;;to insert a-sym at pos i in a-word
> (define (insert-at i a-sym a-word)
>   (cond
>
>     [(zero? i) (cons a-sym  a-word) ]
>     [(empty? a-word) empty]
>     [else (cons (first a-word) (insert-at (sub1 i) a-sym (rest a-
> word)))]))
>
> ;;tests
> ;;(insert-at 1 'z (cons 'a (cons 'b empty)))
> ;;(insert-at 0 'z (cons 'a (cons 'b empty)))
>
>
>
> ;;for-all-pos: number word symbol : list-of-words
> ;;to create a list of words by inserting a-sym in a-word at all
> positions i.e 0..n
> ;;each forming a word
> (define (for-all-pos n a-word a-sym)
>   (cond
>     [(zero? n) (cons (cons a-sym a-word) empty) ]
>     [else (cons (insert-at n a-sym a-word) (for-all-pos (sub1 n)
> a-word a-sym))]))
>
>
> ;;tests
> ;;(define A-WORD (cons 'a (cons 'b empty)))
> ;;(for-all-pos (length1 A-WORD) A-WORD 'z)
>
>
>
> ;;insert-everywhere/in-all-words : symbol list-of-words -> list-of-
> words
> ;;to create a list-of-words by inserting a-sym everywhere in word
> for all word
> ;;in alow
> (define (insert-everywhere/in-all-words a-sym alow)
>   (cond
>     [(empty? alow) empty ]
>     [else (append (for-all-pos (length1 (first alow)) (first alow)
> a-sym)
>           (insert-everywhere/in-all-words a-sym (rest alow))) ]))
> ;;tests
> ;;(insert-everywhere/in-all-words 'z (cons (cons 'a (cons 'b empty))
> (cons (cons 'b (cons 'a empty)) empty)))
>
>
>
> ;;arrangements : word -> list-of-words
> ;;to create a list of all re-arrangement of letters in a-word
> (define (arrangements a-word)
>   (cond
>     [(empty? a-word) (cons empty empty)]
>     [else (insert-everywhere/in-all-words (first a-word)
>                                          (arrangements (rest a-
> word)))]))
>
>
> ;;tests
> ;;(arrangements (cons 'd (cons 'e (cons 'r empty) )))
>
> ;;(length1 (arrangements (cons 'd (cons 'e (cons 'r empty) ))))
> ;;(length1 (arrangements (cons 'd (cons 'e (cons 'r (cons 'a
> empty) ) ))))
>
> ;;Hurray !!!
>
>
>
> Thanks
>
> Veer
> _________________________________________________