List of prime numbers using Array methods

Question

I have a code to get list of prime numbers:

def primes_numbers num
    primes = [2]

    3.step(Math.sqrt(num) + 1, 2) do |i|
        is_prime = true     

          


        

Answer 1

Ruby 1.9 has a very nice Prime class:

http://www.ruby-doc.org/core-1.9/classes/Prime.html

But I'm assuming you don't care about any standard classes, but want to see some code, so here we go:

>> n = 100 #=> 100
>> s = (2..n) #=> 2..100
>> s.select { |num| (2..Math.sqrt(num)).none? { |d| (num % d).zero? }} 
#=> [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]

Note: I wrote it this way because you wanted Enumerable methods, for efficiency's sake you probably want to read up on prime finding methods.