CODEWITHSUNDEEP
lispcommon-lisp
Simm
Simm

Reputation: 89

Lisp sum of multiples of 3

so I'm new to LISP and I was wondering how I could find the sum of numbers that are divisible by 3. Here's what I have done so far: 

(defun sumel (li)

 (cond 
  ((null li) 0)

 ( (zerop (rem (car li) 3))  (+ (car li) (sumel (cdr li))))

 )

)

A little hint to what I'm doing wrong would be great.

Upvotes: 0

Views: 237

Answers (2)

ptb
ptb

Reputation: 2148

You can also take a different approach which utilizes the large standard library of Common Lisp (remove-if-not and reduce in this case)

(defun sumel (list)    
    (reduce '+ (remove-if-not (lambda (x) (zerop (rem x 3)))
                              list)))

Upvotes: 0

Sylwester
Sylwester

Reputation: 48745

You hanve the case where the list is empty and when you actually find a multiple of 3, but what happens if it is a nother number you should skip? You have do default term so Common Lisp will return nil. Eg your code works like this:

(defun sumel (li)
 (cond 
  ((null li) 0)
  ((zerop (rem (car li) 3))  
   (+ (car li) (sumel (cdr li))))
  (t nil))) ; when not a multiple of 3 we are finished, return nil

Try evaluating (cond) and you'll see it is the same as (cond (t nil)).

You fix this by adding just (sumel (cdr l)) as the alternative. It should evaluate to the sum of the multiples of the rest of the list, which is what you want.

Upvotes: 3

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