schemerackethtdp

Why return empty and not the list itself?


Chapter 4, HtDP.

Note: I've seen this in other questions as well.

Is it for a clarity reason or an algorithmic reason, that I am unaware of, that the base case returns empty instead of the list itself which is empty.

Example:

; List-of-numbers -> List-of-numbers
; compute the weekly wages for all given weekly hours
(define (wage* alon)
  (cond
    [(empty? alon) empty] ;<---- see here
    [else (cons (wage (first alon)) (wage* (rest alon)))]))

; Number -> Number
; compute the wage for h hours of work
(define (wage h)
  (* 12 h))

I would think this is just as correct.

; List-of-numbers -> List-of-numbers
; compute the weekly wages for all given weekly hours
(define (wage* alon)
  (cond
    [(empty? alon) alon] ;<---- see here
    [else (cons (wage (first alon)) (wage* (rest alon)))]))

; Number -> Number
; compute the wage for h hours of work
(define (wage h)
  (* 12 h))

Solution

  • Both forms are correct and are exactly equivalent, it's just a matter of style. Although it can be argued that this is a bit more clear, because it's more explicit what's being returned:

    (if (empty? lst)
      empty
      ...)
    

    In the end, it's a matter of personal taste vs. coding conventions. If you were a member of a team and everybody was using the first form, then you should use it. On the other hand if you're a lone programmer, then use the form that better suits your tastes.