Euler Project in Ruby - Problem 7

euler7.rb

def prime? x
  (2..x-1).each { |y| return false if x % y == 0 }
  true
end
 
x       = 10_001 # We look for 10 001st prime number
n       = 3 # Start at 3 so we can skip even numbers
counter = 1 # Counter at 1 because 2 is prime number
 
while true
  counter += 1 if prime? n
  break if counter == x
  n += 2 # We can skip the odd numbers
end
 
puts "The answer is #{n}." 
# This code takes more than 15s to run, the solution is 104_743

Why don't you use the library ruby provides you with?

require 'Prime'

puts Prime.instance.first(10_001).last

Just remember to write 'Prime' with lowercase => require 'prime'