CODEWITHSUNDEEP
functional-programmingconditional-statementsschemeracket
Shawn
Shawn

Reputation: 331

How to get a value other than a boolean in a predicate?

I am trying to add an element into a list if it is zero as part of a recursive function that removes all non-zeroes from a list. if it is non-zero then I just want the code to continue on. Here's what I have so far:

(cons 
   (cond [(zero? (first list_of_numbers)) 
          (first list_of_numbers)]) 
   (continue the recursive function))

However, I get the following error:

cond: all question results were false

Upvotes: 1

Views: 86

Answers (1)

Martin Půda
Martin Půda

Reputation: 7568

You're asked to write a recursive function, so you should start with the terminating condition/ base case:

(define (keep-zeros lst)
  (cond ((empty? lst) '())

In the next cond branch, you will use your condition (zero?) and if (first lst) passes, you will cons it to the result of (keep-zeros (rest lst)):

        ((zero? (first lst))
         (cons (first lst)
               (keep-zeros (rest lst))))

Final cond branch- if that number is non-zero, you will just drop it and call your function on the rest of list:

        (else (keep-zeros (rest lst)))))

All together:

(define (keep-zeros lst)
  (cond ((empty? lst) '())
        ((zero? (first lst))
         (cons (first lst)
               (keep-zeros (rest lst))))
        (else (keep-zeros (rest lst)))))

Test:

> (keep-zeros (cons 0 (cons 3 (cons 4 (cons 0 empty)))))
(cons 0 (cons 0 '()))

Upvotes: 2

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