Re: Sort a list of dotted pairs

BastiL2001@[EMAIL PROTECTED]
 writes:

> Dear group,
>
> I have a list of dotted pairs that may look something like this:
> ((4 . something) (2 . maybe ) (3 . this ) (7 . last ) (5 . some))
>
> I want it to be sorted after the car of each dotted pair starting with
> the lowest number...
>
> ((2 . maybe) (3 . this) (4 . something) (5 . some) (7 . last))
>
> Only thing I have done so far is the sort function which does not seem
> to help in that case... How could this be done? Thanks.

It depends on how you made this sort function.
If you made it like Common Lisp sort, then you can just write:

     (sort a-list < 'key car)

But if you made it like the sort we usually find in scheme libraries,
then  you will have to write:

(define (identity x)
   x)

(define (get plist key . default)
   (cond
      ((or (null? plist) (null? (cdr plist)))
         (if (pair? default)
            (car default)
            #f))
      ((eqv? key (car plist))
         (cadr plist))
      (else
          (get (cddr plist) key (if (pair? default)
                                    (car default)
                                    #f)))))
       

(define (cl-sort list less? . options)
   (if (null? options)
      (scheme-sort list less?)
      (let ((key (get options 'key identity)))
         (scheme-sort list (lambda (a b) (less? (key a) (key b)))))))


(cl-sort '((4 . something) (2 . maybe ) (3 . this ) (7 . last ) (5 .
some))
         <
         'key car)

--> ((2 . maybe) (3 . this) (4 . something) (5 . some) (7 . last))



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__Pascal Bourguignon__                     http://www.informatimago.com/

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