Sieve of Eratosthenes in Ruby

Question

Rather than scraping a Ruby version of this algorithm off the net I wanted to create my own based on its description here. However I cannot figure out two things

<         


        

Answer 1

This is a reference for those who are interested. The code is from this site.

This code uses Sieve of Eratosthenes as well.

n = 1000000
ns = (n**0.5).to_i + 1
is_prime = [false, false] + [true]*(n-1)
2.upto(ns) do |i|
  next if !is_prime[i]
  (i*i).step(n, i) do |j|
    is_prime[j] = false
  end
end

count = 0
list = (0..n).map do |i|
  count += 1 if is_prime[i]
  count
end

while gets
  puts list[$_.to_i]
end

And here is another one.

def eratosthenes(n)
  nums = [nil, nil, *2..n]
  (2..Math.sqrt(n)).each do |i|
    (i**2..n).step(i){|m| nums[m] = nil}  if nums[i]
  end
  nums.compact
end

p eratosthenes(100)