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

Features : Check a number is Prime, get a number factors and get list of prime numbers and also you can easily transform it in any language you want

As Ruby has its own Prime class so you don't need to worry

but if you want to do it your own without using ruby core things

n=100 #=> a

def prime_numbers(n)
  prime_numbers = []
  (1..n).each do |number|
      prime_numbers << number if is_prime(number)
  end
  prime_numbers
end

def is_prime(n)
   if factors(n).count > 2
      return true
   end
   return false
end

# find factors of a number

def factors(n)
     factors = []
     (1..n).each {|d| factors << d if (n%d).zero?}
     factors
end

Note: There are three functions involved and I deliberately do this for beginners, So that they can easily understand about it

Optimization Guide:

1) You can start loop from 2 and end at n-1 if you want to save iterations

2) use Ruby core functions and enjoy things :)

Answer 2

You can list prime numbers like this as well.

Example Array: ar = (2..30).to_a
ar.select{ |n| (2..n).count{ |d| (n % d).zero? } == 1 }

Answer 3

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.