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TPrimes.f90
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TPrimes.f90
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program primes
implicit none
! integer, parameter :: N = 1000
integer :: p, m, i
logical, allocatable :: is_p(:)
integer, allocatable :: p_list(:)
print*,"enter maximum integer"
read (*,*)p
allocate(is_p(p))
call sieve(is_p, p)
m = 0
do i=1,p
if (is_p(i)) then
m = m + 1
end if
end do
print*, "Found ", m, " primes < ", p
allocate(p_list(m))
call logical_to_integer(p_list, is_p, m, p)
write (*,'(100I5)') (p_list(i), i=1,m)
end program
subroutine sieve(is_prime, n_max)
! =====================================================
! Uses the sieve of Eratosthenes to compute a logical
! array of size n_max, where .true. in element i
! indicates that i is a prime.
! =====================================================
integer, intent(in) :: n_max
logical, intent(out) :: is_prime(n_max)
integer :: i
is_prime = .true.
is_prime(1) = .false.
do i = 2, int(sqrt(real(n_max)))
if (is_prime (i)) is_prime (i * i : n_max : i) = .false.
end do
return
end subroutine
subroutine logical_to_integer(prime_numbers, is_prime, num_primes, n)
! =====================================================
! Translates the logical array from sieve to an array
! of size num_primes of prime numbers.
! =====================================================
integer :: i, j=0
integer, intent(in) :: n
integer, intent(in) :: num_primes
logical, intent(in) :: is_prime(n)
integer, intent(out) :: prime_numbers(num_primes)
! integer, intent(out), dimension(0:num_primes-1) :: prime_numbers
do i = 1, size(is_prime)
if (is_prime(i)) then
j = j + 1
prime_numbers(j) = i
end if
end do
end subroutine