FORTRAN logic notproblem -

if use function once, works properly. if make loop down below, 4 lines of commented code, code malfunctions. can't figure out why return t or f every other number after initial value.

asterisks in parenthesis of write , read doesn't show here reason.

program prime  integer :: n = 0, = 1,x = 0  logical :: ip  write (*,*) "enter number:"  read (*,*) n  !do while ( n < 1000)  ip = isprime(n)  write (*,*) ip, n  !n = n + 1  !end  read(*,*) x  contains  function isprime(n)  logical :: isprime  integer, intent(in) :: n  isprime = .true.  if (n == 2)  write (*,*) n  else   while (i <= (n/2))     = + 2     if (mod(n,i) == 0)      isprime = .false.     end if  end  end if   return  end function isprime  end program prime 

you're forgetting reset i 1 during each call isprime.

the first time isprime called, i=1 top of program main. however, i incremented during first isprime call other 1 second call starts i/=0.

note because isprime contained within program main, isprime inherits i program main.

i'm bound remind use implicit none everywhere avoid other errors, although it's not problem in case.