CODEWITHSUNDEEP
schemeracket
ivanka georgieva
ivanka georgieva

Reputation: 21

Filter high-order function in Scheme

Using 'filter' I need to write a code to reate a procedure called (count-zeros lst) that counts how many 0s are in a number. Sample output: (count-zeros '(1 1 0 1 0 0)) should give 3

(define filter (lambda (pred a-list)
    (if (null? a-list)
        '()
        (if (pred (car a-list))
            (cons (car a-list) (filter pred (cdr a-list)))
            (filter pred (cdr a-list)))))) 

(define (count-zeros lst)
  (filter zero? (length lst)))

I know the logic is right but I have troubles with the implementation and making the code work. Please help me to improve the code.

Upvotes: 0

Views: 189

Answers (1)

Óscar López
Óscar López

Reputation: 236034

Read the problem statement carefully, and compare it against what you wrote: it says count the number of zeroes, but you're taking the length of the list (which returns a number!) and then trying to filter the zeroes out of that number, which doesn't make sense. It's the other way around:

(define (count-zeros lst)
  (length (filter zero? lst)))

First we filter just the zeroes, which returns a list, and then we get the length of that list.

Upvotes: 1

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