Program prime_function
!------------------------------------------------------------------------------
!Program: Prime Test Function
!Author: Chris Harper
!Date: 2/23/2008
!------------------------------------------------------------------------------

IMPLICIT NONE

!declare functions and variables
logical :: isPrime
integer :: n

!test prime function with user input
do
	print *, "Enter an integer (0 to exit):"
	read *, n
	if (n == 0) exit
	if (isPrime(n)) then
		print *, n, " is prime"
	else
		print *, n, " is not prime"
	end if
end do

end

!returns true if n is a prime number, false otherwise
logical function isPrime(n)
	IMPLICIT NONE
	integer , intent (IN) :: n
	integer :: i

	select case (n)
		case (:1) ! <=1 is not prime
			isPrime = .false.
		case (2:3) ! 2 and 3 are prime
			isPrime = .true.
		case (4:) ! >=4 is prime if it cannot be evenly divided from 2 to its sqrt
			do i = 2, sqrt(real(n))
				if (mod(n, i) == 0) then
					isPrime = .false.
					return
				end if
			end do
			isPrime = .true.
	end select
end function