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

def primeSieve(n)   primes = Array.new    for i in 0..n-2    primes[i] = i+2   end    index = 0   while Math.sqrt(primes.last).ceil > primes[index]     (primes[index] ** 2).step(primes.length - 1, primes[index])        {|x| x % primes[index] == 0 ? primes.delete(x) : ''}     index += 1   end    primes end 

1. Why it doesn’t iterate to the end of the array?
2. According to the description in the link above the loop should be broken out of when the squareroot of the last element in the array is greater than the current prime – mine does this one before.

I’m fairly sure it has something to do with the delete operation modifying the length of the array. For example my function currently yields 2,3,5,7,9,10 when I enter n=10 which is obviously not correct. Any suggestions on how I can go about alterating this to make it work like it’s supposed to?

Answer 1

The following seems to work. I took out the floating point arithmetic and squared instead of square rooting. I also replaced the deletion loop with a ‘select’ call.

while primes[index]**2 <= primes.last       prime = primes[index]       primes = primes.select { |x| x == prime || x%prime != 0 }       index += 1 end 

Edit: I think I figured out how you’re trying to do this. The following seems to work, and seems to be more in line with your original approach.

while Math.sqrt(primes.last).ceil >= primes[index]     (primes[index] * 2).step(primes.last, primes[index]) do       |x|       primes.delete(x)     end     index += 1 end