! --------------------------------------------------------------- ! This program finds all prime numbers in the range of 2 and an ! input integer. ! --------------------------------------------------------------- PROGRAM Primes IMPLICIT NONE INTEGER :: Range, Number, Divisor, Count WRITE(*,*) 'What is the range ? ' DO ! keep trying to read a good input READ(*,*) Range ! ask for an input integer IF (Range >= 2) EXIT ! if it is GOOD, exit WRITE(*,*) 'The range value must be >= 2. Your input = ', Range WRITE(*,*) 'Please try again:' ! otherwise, bug the user END DO Count = 1 ! input is correct. start counting WRITE(*,*) ! since 2 is a prime WRITE(*,*) 'Prime number #', Count, ': ', 2 DO Number = 3, Range, 2 ! try all odd numbers 3, 5, 7, ... Divisor = 3 ! divisor starts with 3 DO IF (Divisor*Divisor > Number .OR. MOD(Number,Divisor) == 0) EXIT Divisor = Divisor + 2 ! if does not evenly divide, next odd END DO IF (Divisor*Divisor > Number) THEN ! are all divisor exhausted? Count = Count + 1 ! yes, this Number is a prime WRITE(*,*) 'Prime number #', Count, ': ', Number END IF END DO WRITE(*,*) WRITE(*,*) 'There are ', Count, ' primes in the range of 2 and ', Range END PROGRAM Primes