From: Lloyd Linklater <lloyd@2live4.com>
Date: 2009-07-25T05:27:07+09:00
Subject: using until

I am writing a little thing to find all the prime numbers to a million.
I got it to work this way:

highestPrime = 1_000_000
primes = Array.new(highestPrime, true)

currentNum = 3
while currentNum < highestPrime do
  (currentNum*currentNum).step(highestPrime, currentNum * 2) { |i|
primes[i] = false }
  currentNum += 2
  while primes[currentNum] == false do
    currentNum += 2
  end
end

I am unhappy with using currentNum += 2 twice.  I very much want to
write something like this:

while currentNum < highestPrime do
  (currentNum*currentNum).step(highestPrime, currentNum * 2) { |i|
primes[i] = false }
  currentNum += 2 until primes[currentNum] == true
end

but that just hangs the program.  What am I missing?
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