Task

Write a boolean function that tells whether a given integer is prime.



Remember that   1   and all non-positive numbers are not prime.

Use trial division.

Even numbers greater than   2   may be eliminated right away.

A loop from   3   to   √ n    will suffice,   but other loops are allowed.



Related tasks

   count in factors
   prime decomposition
   AKS test for primes
   factors of an integer
   Sieve of Eratosthenes
   factors of a Mersenne number
   trial factoring of a Mersenne number
   partition an integer X into N primes
   sequence of primes by Trial Division


def prime(a)
  if a == 2
    true
  elsif a <= 1 || a % 2 == 0
    false
  else
    divisors = (3..Math.sqrt(a)).step(2)
    divisors.none? { |d| a % d == 0 }
  end
end
p (1..50).select{|i| prime(i)}