How do I generate the first n prime numbers?

Question

I am learning Ruby and doing some math stuff. One of the things I want to do is generate prime numbers.

I want to generate the first ten prime numbers and the first ten

Answer 1

require 'prime'

Prime.first(10) # => [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

Answer 2

Implemented the Sieve of Eratosthene (more or less)

def primes(size)
    arr=(0..size).to_a
    arr[0]=nil
    arr[1]=nil
    max=size
    (size/2+1).times do |n|
        if(arr[n]!=nil) then
            cnt=2*n
            while cnt <= max do
                arr[cnt]=nil
                cnt+=n
            end
        end
    end
    arr.compact!
end

Moreover here is a one-liner I like a lot

def primes_c a
    p=[];(2..a).each{|n| p.any?{|l|n%l==0}?nil:p.push(n)};p
end

Of course those will find the primes in the first n numbers, not the first n primes, but I think an adaptation won't require much effort.

Answer 3

Not related at all with the question itself, but FYI:

• if someone doesn't want to keep generating prime numbers again and again (a.k.a. greedy resource saver)
• or maybe you already know that you must to work with subsequent prime numbers in some way
• other unknown and wonderful cases

Try with this snippet:

  require 'prime'

  for p in Prime::Generator23.new
    # `p` brings subsequent prime numbers until the end of the days (or until your computer explodes)
    # so here put your fabulous code
    break if #.. I don't know, I suppose in some moment it should stop the loop
  end
  fp

If you need it, you also could use another more complex generators as Prime::TrialDivisionGenerator or Prime::EratosthenesGenerator. More info

Answer 4

People already mentioned the Prime class, which definitely would be the way to go. Someone also showed you how to use an Enumerator and I wanted to contribute a version using a Fiber (it uses your Integer#is_prime? method):

primes = Fiber.new do
  Fiber.yield 2
  value = 3
  loop do
    Fiber.yield value if value.is_prime?
    value += 2
  end
end

10.times { p primes.resume }

Answer 5

# First 10 Prime Numbers

number = 2
count = 1
while count < 10
  j = 2
  while j <= number
    break if number%j == 0
    j += 1
  end
  if j == number
    puts number 
    count += 1
  end
  number += 1
end

Answer 6

In Ruby 1.9 there is a Prime class you can use to generate prime numbers, or to test if a number is prime:

require 'prime'

Prime.take(10) #=> [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
Prime.take_while {|p| p < 10 } #=> [2, 3, 5, 7]
Prime.prime?(19) #=> true

Prime implements the each method and includes the Enumerable module, so you can do all sorts of fun stuff like filtering, mapping, and so on.