CODEWITHSUNDEEP
racket
Sillage
Sillage

Reputation: 81

Expects 2 arguments but found only 1

I am doing a practice question where it asks to define a function that is supposed to produce all elements in a list, loi, that can be divided by 3 or a natural number, mult, but not both.

Below is my code:

(define (keep-multiples-of-three-or loi mult)
  (cond
    [(empty? loi) empty]
    [else (cond [(empty? loi) empty]
                [(and (not (equal? 3 mult)) 
                      (or (equal? (remainder (first loi) 3) 0)
                          (equal? (remainder (first loi) mult) 0)))   
                 (first loi)]           
                [else (keep-multiples-of-three-or (rest-loi) 
                                                  mult)]))]))

for (keep-multiples-of-three-or (cons 9 (cons 3 empty)) 3) the error message says: cons expects 2 arguments but found only 1.

I don’t know what is wrong here. Can somebody help me?

Upvotes: 1

Views: 1865

Answers (1)

Óscar López
Óscar López

Reputation: 236004

You have a couple of errors:

  • There's no need to nest the cond expression
  • The [(empty? loi) empty] case is repeated two times
  • It's not (rest-loi), it's (rest loi)
  • In the second case, you need to cons the result and call the recursion again
  • The (not (equal? 3 mult)) condition is making your example return an empty list, think about it: you're saying that mult is 3, but then you check it mult is not 3

This should fix the issues:

(define (keep-multiples-of-three-or loi mult)
  (cond [(empty? loi) empty]
        [(or (equal? (remainder (first loi) 3) 0)
             (equal? (remainder (first loi) mult) 0))
         (cons (first loi)
               (keep-multiples-of-three-or (rest loi) mult))]
        [else (keep-multiples-of-three-or (rest loi) mult)]))

For example:

(keep-multiples-of-three-or (list 9 3) 3)
=> '(9 3)
(keep-multiples-of-three-or (list 1 3 5 9) 5)
=> '(3 5 9)

Upvotes: 1

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