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


isPrime?: function [n][
    if n=2 -> return true
    if n=3 -> return true
    if or? n=<1 0=n%2 -> return false
 
    high: to :integer sqrt n
    loop high..2 .step: 3 'i [
    	if 0=n%i -> return false
    ]
 
    return true
]
 
loop 1..20 'i [
    print ["isPrime?" i "=" isPrime? i ]
]