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

 From: richard gere (diggerrrrr at gmail.com) Date: Thu Apr 3 11:08:57 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]

```Thanks for your kind reply.
I did not got the expected result for :

(check-expect (arrangements '(a t))
'((a t) (t a)))

so i have now re-defined/changed  "for-all-pos" and
"insert-everywhere/in-all-words"  functions .
Here are the new definitions :

(define (for-all-pos n a-word a-sym)
(cond
[(= n (length1 a-word)) (cons (insert-at n a-sym a-word) empty) ]
;;[(zero? n) (cons (insert-at n a-sym a-word) empty) ]

[else (cons (insert-at n a-sym a-word) (for-all-pos (add1 n)
a-word a-sym))]))

(define (insert-everywhere/in-all-words a-sym alow)
(cond
[(empty? alow) empty ]
[else (append (for-all-pos 0 (first alow) a-sym)
(insert-everywhere/in-all-words a-sym (rest alow))) ]))

Tested them with two different data set and  both were successful :

(check-expect (arrangements (cons 'a (cons 't empty))) (cons (cons 'a
(cons 't empty))
(cons
(cons 't (cons 'a empty)) empty)))

(check-expect (arrangements (cons 'a (cons 't (cons 'z empty))))
(cons (cons 'a (cons 't (cons 'z empty)))
(cons (cons 't (cons 'a (cons 'z empty)))
(cons (cons 't (cons 'z (cons 'a empty)))

(cons (cons 'a (cons 'z (cons 't empty)))
(cons (cons 'z (cons 'a (cons 't empty)))
(cons (cons 'z (cons 't (cons 'a empty))) empty)))))))

(generate-report)

Thank you
Veer

>  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.)

On Thu, Apr 3, 2008 at 7:20 PM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
>  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
> > _________________________________________________