b.rkt

#lang racket

(define (convert.1 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.2 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)

(collect-garbage)
(collect-garbage)

(time
 (for ([_ 10000])
   (for ([i (in-list '(1 3 5 7 6 8 9 10 14 15 21 25 47 35 49 52 105 3125))])
     (convert.1 i ))))
(collect-garbage)
(collect-garbage)

(time
 (for ([_ 10000])
   (for ([i (in-list '(1 3 5 7 6 8 9 10 14 15 21 25 47 35 49 52 105 3125))])
     (convert.2 i ))))