How can I test if a value is a prime number in Ruby? Both the easy and the hard way?

Question

I am trying to create a program that will test whether a value is prime, but I don't know how. This is my code:

class DetermineIfPrime
def initialize (nth_value)
@nth_value = nth_value
primetest
end

def primetest
  if Prime.prime?(@nth_value)
   puts ("#{@nth_value} is prime")
  else
   puts ("This is not a prime number.")
  end
rescue Exception
puts ("#{$!.class}")
puts ("#{$!}")
 end
end

And every time I run that it returns this.

NameError
uninitialized constant DetermineIfPrime::Prime

I tried other ways to do the job, but I think this is the closest I can get.

I also tried this:

class DetermineIfPrime
def initialize (nth_value)
@nth_value = nth_value
primetest
end

 def primetest
 for test_value in [2, 3, 5, 7, 9, 11, 13] do
  if (@nth_value % test_value) == 0
   puts ("#{@nth_value} is not divisible by #{test_value}")
  else
   puts ("This is not a prime number since this is divisible by #{test_value}")
  break
  end
 end
 end
end

Or am I just doing something wrong?

Answer 1

Ruby has built in method to check if number is prime or not.

require 'prime'

Prime.prime?(2)  #=> true
Prime.prime?(4)  #=> false

Answer 2

def is_prime?(num)
  return false if num <= 1
  Math.sqrt(num).to_i.downto(2).each {|i| return false if num % i == 0}
  true
end

First, we check for 0 and 1, as they're not prime. Then we basically just check every number less than num to see if it divides. However, as explained here, for every factor greater than the square root of num, there's one that's less, so we only look between 2 and the square root.

Update

def is_prime?(num) return if num <= 1 (2..Math.sqrt(num)).none? { |i| (num % i).zero? } end

Answer 3

The error you are getting is because you haven't required Primein your code, You need to do require Prime in your file.

One cool way I found here, to check whether a number is prime or not is following:

class Fixnum
  def prime?
    ('1' * self) !~ /^1?$|^(11+?)\1+$/
  end
end

10.prime?