bees bees - 1 year ago 47
Ruby Question

Project Euler 1:Find the sum of all the multiples of 3 or 5 below 1000

I am trying to solve math problems with Ruby from the Project Euler. Here is the first one I tried:

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.

Please help me to improve my code.

total = 0

(0...1000).each do |i|
total += i if (i%3 == 0 || i%5 == 0)

puts total

Answer Source

Much faster (constant time) answer:

def sum_multiples(multiple, to)
    n = (to-1) / multiple
    n * (n+1) / 2 * multiple

irb(main):001:0> sum_multiples(3, 10) + sum_multiples(5, 10) - sum_multiples(15, 10)
=> 23
irb(main):002:0> sum_multiples(3, 1000) + sum_multiples(5, 1000) - sum_multiples(15, 1000)
=> 233168

Why does this work? sum_multiples works out the sum of multiples of multiple up to but not including to (it relies on integer division). It first works out the number of number of multiples being summed (n), then multiples the standard formula for the sum of 1..n = n(n+1)/2 by multiple. Using this, we can add together the sums for the multiples of 3 and 5. We must then not forget that some numbers are multiples of both 3 and 5, so we subtract multiples of 15 (3*5).

Although your answer is more than fast enough for the constraints in this problem (it should run in about 1 millisecond on modern hardware), a faster solution such as the one I provide will give a result on very large numbers.