Scheme return pairs in a list

Question

Hi I got the error mcar: contract violationexpected: mpair? given: () while running these code:

(define helpy 
  (lambda (y listz)
    (map (lambda (z) (list y z))
         listz)))

(define print
  (lambda (listy)
    (cond
      ((null? list) (newline))
      (#t (helpy (car listy) (cdr listy))
          (print (cdr listy))))))

My code is trying to return pairs in a list. For example, when I call (print '(a b c)) it should return ((a b) (a c) (b c)).

I just fix and update my code, now it don't return error but I can only get pair ( (a b) (a c), when running these code:

(define helpy

(lambda (y listz)

(map (lambda (z) (list y z))

listz)))

(define print

(lambda (listy)

(cond

((null? listy) (newline))

(#t (helpy (car listy) (cdr listy)))

(print (cdr listy)))))

I think that I got something wrong with the recursion

Óscar López · Accepted Answer · 2012-11-18 22:15:30Z

0

There are a couple of problems with the code. First, by convention the "else" clause of a cond should start with an else, not a #t. Second, the null? test in print should receive listy, not list. And third, you're not doing anything with the result returned by helpy in print, you're just advancing print over the cdr of the current list without doing anything with the value returned by the recursive call. Try this instead:

(define print
  (lambda (listy)
    (cond
      ((null? listy) (newline))
      (else
       (displayln (helpy (car listy) (cdr listy)))
       (print (cdr listy))))))

displayln is just an example, do something else with the returned result if necessary.

edited Nov 18, 2012 at 22:15

answered Nov 18, 2012 at 21:59

Óscar López

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6 Comments

amalloy Over a year ago

"The last clause of a cond should start with an else, not a #t". Really? I'm don't write much scheme, but I thought that was a stylistic thing that doesn't have universal agreement.

Óscar López Over a year ago

@amalloy #t works, but it's not the usual way to write it - that's more of a Common Lisp convention than Scheme's

Daniel Over a year ago

@ÓscarLópez: Thanks you The racket can't define the term displayln in your code, anw I update my code, it now doesn't return error, but I only can get pair (a b)(a c) insted of (a b)(a c)(b c), I think it has something wrong with recursion???

Óscar López Over a year ago

@Daniel try with display then. The code above was tested in Racket using the language @lang racket and it prints the results

Daniel Over a year ago

@ÓscarLópez: Do you have any ideas, that I can expand the function print, for example I run print with (print '((a b c)(b d e) it can return ((a b)(a c)(b c)(b d)(b e)(d e)

|

simmone · Accepted Answer · 2012-11-19 08:01:08Z

0

I try to implement like this:

#lang racket

(define data '(a b c d))

(define (one-list head line-list)
  (if (null? line-list)
      null
      (cons
       (cons head (car line-list))
       (one-list head (rest line-list)))))

(letrec ([deal-data
           (lambda (data)
             (if (null? data)
                 '()
                 (append
                  (one-list (car data) (rest data))
                  (deal-data (rest data)))))])

  (deal-data data))

run result:

'((a . b) (a . c) (a . d) (b . c) (b . d) (c . d))

answered Nov 19, 2012 at 8:01

simmone

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