I was working on a exercism problem named Raindrops.
Problem description:
Convert a number to a string, the contents of which depend on the number's
factors.
If the number has 3 as a factor, output 'Pling'.
If the number has 5 as a factor, output 'Plang'.
If the number has 7 as a factor, output 'Plong'.
If the number does not have 3, 5, or 7 as a factor, just pass the number's
digits straight through.
I came out with two version.
; version 1
(define (convert n)
(define pling (divides? 3 n))
(define plang (divides? 5 n))
(define plong (divides? 7 n))
(if (or pling plang plong)
(string-append (if pling "Pling" "")
(if plang "Plang" "")
(if plong "Plong" ""))
(number->string n)))
; version 2
(define (convert n)
(define table '((3 . "Pling") (5 . "Plang") (7 . "Plong")))
(define result (for/list ([(k v) (in-dict table)] #:when (divides? k n))
v))
(if (empty? result) (number->string n)
(string-append* result)))
(require math/number-theory)
I thought version 1 would be faster, but it turned out to be wrong. Running
with raco test got following timing information.
version 1
cpu time: 9 real time: 9 gc time: 9
version 2
cpu time: 0 real time: 0 gc time: 0
Then I ran both version in DrRacket, both output following result.
cpu time: 0 real time: 0 gc time: 0
It's strange, isn't it?
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