HTDP2e Exercise 309

[Aron Zvi]

Hey Guys,

I do not see how to apply pattern matching to this problem. 

Using for/list as below makes the most sense to me and I do not see how to fit in pattern matching

; [List-of [List-of String]] -> [List-of Natural]

; produces list of number of Strings per item in a llos

(check-expect (words-on-line empty) empty)

(check-expect (words-on-line (list empty)) (list 0))

(check-expect (words-on-line (list (list "22" "rr" "ff") (list "22" "rr"))) (list 3 2))

(define (words-on-line llos)

  (for/list ((los llos))

    (length los)))

Can I get a hint?

[Daniel Prager]

Hint:

(define (words-on-line llos)
  (match llos
      ['() empty]
      [(list a b ...) <recursive expression goes here>))

Dan

[Aron Zvi]

Thanks. I initially tried as below with first pattern [empty empty] and empty apparently is a variable that will match a non-empty list. I need to use  '() 

(define (words-on-line llos)
  (match llos

      [empty empty]

      [(list a b ...) <recursive expression goes here>))