project_euler

euler1.rb

#Find the sum of all the multiples of 3 or 5 below 1000.

def multiples
  total = 0
  (1..1000).each { |num| total = total + num if (num % 3 == 0) || (num % 5 == 0) }
  puts total
end

multiples 

euler2.rb

#By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

def even_fib_sum (limit)
  first = 0
  second = 1
  num = 0
  sum_of_evens = 0
  while num < limit
  	num = first + second
  	sum_of_evens += num if num % 2 == 0
  	first = second
  	second = num
  end
  puts sum_of_evens
end

even_fib_sum(4000000)

euler3.rb

#What is the largest prime factor of the number 600851475143 ?

def prime_factors(number)
  return [] if number == 1
  factor = (2..number).find {|x| number % x == 0}
  [factor] + prime_factors(number / factor)
end
puts prime_factors(600851475143).max

euler4.rb

#Find the largest palindrome made from the product of two 3-digit numbers.

array = []
100.upto(999) do |a|
  100.upto(999) do |b|
    product = a * b
    array << product if product.to_s == product.to_s.reverse
  end
end
puts array.max

euler5.rb

#What is the smallest positive number that is evenly divisible by all of the numbers from 1 to 20?

def evenly_divisible(number)
  array = [11, 12, 13, 14, 15, 16, 17, 18, 19]
  array.each do |n|
    if (number%n != 0)
      return false
    end
  end
  return true
end

number = 20
start = 1
until evenly_divisible(number)
  number = start * 20
  start += 1
end

puts number