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

nice solution but has performance issues,taking so long to give the result to rectify that
change def prime? x as follows

def prime? x
a=Math.sqrt(x)
(2..a).each { |y| return false if x % y == 0 }
true
end

Now observe the time difference between the results by ur algo and my algo

i think there is another performance increment that can be done.
We have all primes number smallest than x, so we can check only if x is divisible by one of this.

This is my code

def isPrime?(value, array)
array.each do | x |
return false if value % x == 0
end
return true
end

puts ''
primes = Array.new
startvalue = 2
while (primes.count < 10001)
primes << startvalue if isPrime?(startvalue, primes)
startvalue = startvalue + 1
end
puts primes[-1]

@ignazioc Using the previous primes increases complexity

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'