# Solution to Project Euler #1 - multiples of 3 and 5

I have been programming for about ~2 years, and mostly wrote OOP and structural code. Recently, I have decided to pick up a functional programming language, and Haskell being too alien for me, looked to Racket (since it is high time I learned a LISP anyways) and am loving it. Since this is a new area of programming for me, I would appreciate any feedback you could give me on this program. It is the solution to the first Project Euler program.

;If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
;Find the sum of all the multiples of 3 or 5 below 1000.

#lang racket

(define max 1000)

(define (multiple_of base test)
(equal? (remainder test base) 0))

(define (primes current total)
(if (< current max)
(primes
(+ current 1)
(+ total (if
(or (multiple_of 5 current) (multiple_of 3 current)) current 0)))
total))

(primes 1 0)

; Output: 233168
; Success


## 2 Answers

1. In Scheme, we use hyphens to separate words, not underscores. Also boolean-returning procedures should end in ?. So it should be multiple-of?
2. Instead of (equal? x 0), use (zero? x).
3. Instead of a recursive loop, you can use for comprehensions:

(for/sum ((i (in-range 1000))
#:when (or (multiple-of? 5 i)
(multiple-of? 3 i)))
i)


Surely, that's much more readable. In my humble opinion. :-)

4. Notwithstanding the last comment, your formatting for your primes procedure is not ideal. Here's a more proper formatting:

(define (primes current total)
(if (< current max)
(primes (add1 current)
(+ total (if (or (multiple-of? 5 current)
(multiple-of? 3 current))
current
0)))
total))

• Why do you start at 1? – itsbruce Jan 23 '15 at 11:13
• thank you! Especially about 1 and 2. I wasn't aware of those tips. About number 3, I haven't learned for loops in racket yet, which is the only reason why i went with recursion, but you are right: That looks more readable. As to number 4, do you have a guide documenting this that you can link to? – DTSCode Jan 23 '15 at 17:03
• @DTSCode You're welcome! for "looks like" a loop but is actually a comprehension (if you've ever played with such things in Python or the like). As for style guide, look at mumble.net/~campbell/scheme/style.txt – Chris Jester-Young Jan 23 '15 at 18:32
• alright! thanks a ton! my python is a bit rusty, but I'm sure if I google list comprehension python Ill get it – DTSCode Jan 23 '15 at 19:13

The Scheme code is quite reasonable. However, this is a very naive solution to the problem. You are wasting processing time by testing each number to see if it is a multiple. Even with this naive solution, why do you start at 1, not 3?.

Think. If you do start at 1 (which you know fails the test), you can create all the multiples there are by working forward.

Don´t forget you need to avoid generating numbers which are multiples of both 5 and 3 more than once.