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soerenbnoergaard's solution

to Raindrops in the Common Lisp Track

Published at Nov 10 2019 · 0 comments
Test suite

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.


  • 28's factors are 1, 2, 4, 7, 14, 28.
    • In raindrop-speak, this would be a simple "Plong".
  • 30's factors are 1, 2, 3, 5, 6, 10, 15, 30.
    • In raindrop-speak, this would be a "PlingPlang".
  • 34 has four factors: 1, 2, 17, and 34.
    • In raindrop-speak, this would be "34".


Check out Installing Common Lisp for instructions to get started or take a look at the guides available in the track's side bar.


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A variation on a famous interview question intended to weed out potential candidates. http://jumpstartlab.com

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


(ql:quickload "lisp-unit")
#-xlisp-test (load "raindrops")

(defpackage #:raindrops-test
  (:use #:common-lisp #:lisp-unit))

(in-package #:raindrops-test)

(define-test test-1
    (assert-equal "1"
                  (raindrops:convert 1)))

(define-test test-3
    (assert-equal "Pling"
                  (raindrops:convert 3)))

(define-test test-5
    (assert-equal "Plang"
                  (raindrops:convert 5)))

(define-test test-7
    (assert-equal "Plong"
                  (raindrops:convert 7)))

(define-test test-6
    (assert-equal "Pling"
                  (raindrops:convert 6)))

(define-test test-9
    (assert-equal "Pling"
                  (raindrops:convert 9)))

(define-test test-10
    (assert-equal "Plang"
                  (raindrops:convert 10)))

(define-test test-15
    (assert-equal "PlingPlang"
                  (raindrops:convert 15)))

(define-test test-21
    (assert-equal "PlingPlong"
                  (raindrops:convert 21)))

(define-test test-25
    (assert-equal "Plang"
                  (raindrops:convert 25)))

(define-test test-35
    (assert-equal "PlangPlong"
                  (raindrops:convert 35)))

(define-test test-49
    (assert-equal "Plong"
                  (raindrops:convert 49)))

(define-test test-52
    (assert-equal "52"
                  (raindrops:convert 52)))

(define-test test-105
    (assert-equal "PlingPlangPlong"
                  (raindrops:convert 105)))

(define-test test-12121
    (assert-equal "12121"
                  (raindrops:convert 12121)))

(let ((*print-errors* t)
      (*print-failures* t))
  (run-tests :all :raindrops-test))
(defpackage #:raindrops
  (:use #:common-lisp)
  (:export #:convert))

(in-package #:raindrops)

(defun empty-string-alternative (s alternative)
  (if (zerop (length s)) alternative s)) 

(defun factor-p (n factor)
  (zerop (mod n factor)))

(defun convert (n)
   (format nil "~A~A~A"
           (if (factor-p n 3) "Pling" "")
           (if (factor-p n 5) "Plang" "")
           (if (factor-p n 7) "Plong" ""))
   (format nil "~D" n)))

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