Count digits in list Racket

问题

I have a doubt, I'm using Racket, and I wanna count the digits of a list, but I can't. I try with length but it doesn't work as I want because

(countDigits '(4 5 6 78)) > 5

the answer has to be 5 but, i don't know how to, i have a code for count digits in a number but I don't knowhow to do it in a list. ¿How could I do it?

回答1:

Here's a possible solution:

(define (countDigits lst)
  (apply +
         (map (compose string-length number->string)
              lst)))

Explanation:

• For each number in the list, we convert it to a string
• Then, we obtain the length of each string - that will tell us the number of digits
• Finally, we add together all the lengths

For example:

(countDigits '(4 5 6 78))
=> 5

回答2:

A more naive example that wouldn't make your professor look twice :)

Regular Recursion:

(define (countDigits list-of-digits)
  (cond [(empty? list-of-digits) 0]
        [else (+ 1 (countDigits (rest list-of-digits)))]))

Tail Recursion:

(define (countDigits list-of-digits sum)
  (cond [(empty? list-of-digits) sum]
        [else (countDigits (rest list-of-digits) (+ 1 sum))]))

来源：https://stackoverflow.com/questions/46019105/count-digits-in-list-racket