I'm exploring ruby by writing a code that checks the sum of all prime numbers under two million. As I am still in the process of learning ruby, I'm unfamiliar with all the standards and best practices. What I have so far works, but I am certain it can be improved. Where can I improve my code for both readability and speed?

def is_prime(n)
    range = n
    i = 2

    while i < range do
        return false if (n % i == 0)
        range = n / i
        i += 1

sum = 2
(3..2000000).each do |num|
    if (is_prime(num))
        #puts "#{num}"
        sum += num

puts "#{sum}"
  • 1
    \$\begingroup\$ As for speed, you should try using athosteles sieve. Also, it's unnecessary to go up to n to decide if it's prime or not, going up to the square root is enough \$\endgroup\$
    – juvian
    Oct 30, 2015 at 14:52
  • \$\begingroup\$ Conceptually, primality is an attribute that belongs to an integer. Idiomatic Ruby is pretty hardcore about being object-oriented, so your primality test should be a method of the Integer class unless you have a good reason for it not to be. if num.prime?, not if (is_prime(num)). \$\endgroup\$
    – ezrast
    Oct 31, 2015 at 1:17
  • \$\begingroup\$ And, as always, notice that except for 2 all primes are odd. So you only need to check 50% of the candidates. Here: i += 2 with i starting at 3. \$\endgroup\$ Oct 31, 2015 at 21:58

1 Answer 1


Ruby favours Functional Programming, let me show you an example.

def prime?(n)
  limit = Math.sqrt(n).to_i + 1
  (2..limit).all? { |i| n % i != 0 }

This improves readability as the code makes use of all? to implicitly loop (Instead you looped explicitly) and is faster as the limit is the sqrt of n and not n and all? short-circuits (returns as soon as one condition is false).

Also the second part would benefit from FP:

puts (2..2000000).select{ |n| prime?(n) }.inject(:+)

In general, in Ruby explicit looping is the exception and not the norm, as there are so many built-ins and shortcuts to make use of.

Each function I made use of here is listed at The Ruby Enumerable Documentation and I warmly suggest studying in detail that page, it will take a bit of time, but the time you will save in the future by using it instead of reinventing the wheel each time will be more than worth it.

  • 1
    \$\begingroup\$ I believe that should be limit = Math.sqrt(n).to_i (no +1). Otherwise the sum comes out two off. \$\endgroup\$
    – Shelby115
    Oct 30, 2015 at 19:34
  • \$\begingroup\$ @Shelby115 3 should be 2 in the range \$\endgroup\$
    – Caridorc
    Oct 30, 2015 at 23:51
  • \$\begingroup\$ @FilipBartuzi Cool trick! I knew &: but not :& \$\endgroup\$
    – Caridorc
    Oct 31, 2015 at 15:51
  • \$\begingroup\$ @Caridorc actually I was very wrong. My codesnippet called Fixnum#even? not function defined in main scope \$\endgroup\$ Oct 31, 2015 at 15:54

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