Back to F Example Codes Page
program Sieve_Of_Eratosthenes
! Find prime numbers using array processing.
! Strike the Twos and strike the Threes
! The Sieve of Eratosthenes!
! When the multiplies sublime
! The numbers that are left, are prime.
! From: Drunkard's Walk, by Frederik Pohl
integer, parameter :: last_number = 100
integer, dimension(last_number) :: numbers
integer :: i, number_of_primes, ac
! Initialize numbers array to 0, 2, 3, ..., last_number--
! Zero instead of 1 because 1 is a special case.
numbers = (/ 0, (ac, ac = 2, last_number) /)
do i = 2, last_number
if (numbers(i) /= 0) then ! if this number is prime
numbers(2*i : last_number : i) = 0 ! eliminate all multiples
endif
enddo
! Count the primes.
number_of_primes = count (numbers /= 0)
! Gather them into the front of the array.
numbers(1:number_of_primes) = pack(numbers, numbers /= 0)
! Print them
print *, "There are ", number_of_primes, &
" prime numbers less than", last_number
print "(5i7)", numbers(1:number_of_primes)
! Sample output:
! There are 25 prime numbers less than 100
! 2 3 5 7 11
! 13 17 19 23 29
! 31 37 41 43 47
! 53 59 61 67 71
! 73 79 83 89 97
end program Sieve_Of_Eratosthenes
Back to F Example Codes Page
Back to F Homepage
Questions or Comments?