Merge two list randomly returns a strange merged list

Question

I've just started to learn Racket and I need to create a procedure that merge two list randomly.

This is my code:

#lang racket
(define merge-randomly
   (lambda (lst1 lst2)
      (cond ((and (null? lst1) (null? lst2)) null)
            ((null? lst1) lst2)
            ((null? lst2) lst1)
            (else
               (cond ((> (random) 0.5) (cons (cons (car lst1) (car lst2)) (merge-randomly (cdr lst1) (cdr lst2))))
                     (else (cons (cons (car lst2) (car lst1)) (merge-randomly (cdr lst1) (cdr lst2))))
               )
            )
      )
   )
)

I need to use with list like these two:

(define l1 '((1 2 3) (7 8 9)))
(define l2 '((4 5 6)))

I need to create a new list like this one:

'((4 5 6) (1 2 3) (7 8 9))

But I get this list:

'(((4 5 6) 1 2 3) (7 8 9))

I'm doing something wrong, probably here, (cons (cons (car lst1) (car lst2)), or here, (cons (cons (car lst2) (car lst1)), at the else instruction in first cond.

How can I get the list I want to get?

I'm wondering, is "randomly" what you really want in the end? Functions that branch randomly like this, while they aren't intrinsically bad, they are harder to write Unit Tests for. If you're learning Racket I would recommend writing functions which you can unit-test easily, or even which you can write examples for before writing the body of the function. — Alex Knauth, Oct 28 '18 at 14:06
@AlexKnauth Thanks for your comment. How can I write unit test for Racket? I'm studying at the university and I'm doing what the professor is asking for. I think that if I append both lists will be good enough. — VansFannel, Oct 28 '18 at 14:50
The normal way is using [`check-equal?`](https://docs.racket-lang.org/rackunit/api.html#%28def._%28%28lib._rackunit%2Fmain..rkt%29._check-equal~3f%29%29) from RackUnit. — Alex Knauth, Oct 28 '18 at 15:05

score 1 · Accepted Answer · answered Oct 28 '18 at 08:28

Your guess is correct. The format for cons is this:

(cons first-element rest-of-the-list)

So, when you use (const '(4 5 6) '(1 2 3)), it returns ((4 5 6) 1 2 3).

The correct format for what you are trying to do would be:

(cons (car lst1) (cons (car ls2) (merge-randomly (cdr lst1) (cdr lst2))))

and similarly for the second case.

Here's what the final function should look like:

(define merge-randomly
   (lambda (lst1 lst2)
      (cond ((and (null? lst1) (null? lst2)) null)
            ((null? lst1) lst2)
            ((null? lst2) lst1)
            (else
               (cond ((> (random) 0.5) (cons (car lst1) (cons (car lst2) (merge-randomly (cdr lst1) (cdr lst2)))))
                     (else (cons (car lst2) (cons (car lst1) (merge-randomly (cdr lst1) (cdr lst2)))))
               )
            )
      )
   )
)

score 0 · Answer 2 · answered Oct 28 '18 at 08:27

0

The problem is with (cons (cons (car lst1) (car lst2)) ...). Change it to (cons (car lst1) (cons (car lst2) ...).

PS Your closing parentheses are not idiomatically placed.

answered Oct 28 '18 at 08:27

hkBst

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Merge two list randomly returns a strange merged list

2 Answers2