PROGRAM PCA$PRIMES

C	This program generates all the prime numbers in a given integer
C	range.  Prime numbers are placed in PRIMES_TABLE as they are
C	generated.  After being verified, these prime numbers are written
C	out to a text file PRIMES.DAT.
C
    	INTEGER LOW,HIGH,COUNT,ERROR_COUNT,PRIMES_TABLE
    	LOGICAL PRIME
    	DIMENSION PRIMES_TABLE(10000)

C	Read in the desired integer range from a file and range-check it.
C
	CALL READ_RANGE(LOW, HIGH)
	LOW = MAX (1, LOW)
	HIGH = MIN (HIGH, 10000)
	HIGH = MAX (LOW, HIGH)

C	Generate all prime numbers in the given range.
C
	COUNT = 0
	DO 10 I = LOW, HIGH
	IF (PRIME(I)) THEN
	    COUNT = COUNT + 1
	    PRIMES_TABLE (COUNT) = I
	END IF
   10	CONTINUE

C	Verify that the numbers in PRIMES_TABLE really are prime.
C
	ERROR_COUNT = 0
	DO 20 I = 1, COUNT
	IF (.NOT. PRIME(PRIMES_TABLE(I))) THEN
	    ERROR_COUNT = ERROR_COUNT + 1
	END IF
   20	CONTINUE
	IF (ERROR_COUNT .NE. 0) THEN
	    TYPE 30, ERROR_COUNT
   30	    FORMAT (I5, ' wrong prime numbers generated')
	END IF

C	Write the prime numbers out to PRIMES.DAT and type summary data
C	on the terminal.
C
	CALL OUTPUT_TO_DATAFILE(PRIMES_TABLE, COUNT)
	TYPE 40,COUNT,LOW,HIGH
   40	FORMAT(I5, ' prime numbers generated between', I5, ' and', I5)
	STOP
	END

C	Function to identify whether a given number is prime or not.
C	If it is prime, the returned function value is TRUE.
C
	LOGICAL FUNCTION PRIME(NUMBER)
	PRIME = .TRUE.
	DO 10 I = 2, NUMBER/2
	IF ((NUMBER - ((NUMBER / I) * I)) .EQ. 0) THEN
	    PRIME = .FALSE.
	    RETURN
	ENDIF
   10	CONTINUE
	RETURN
	END

C	Subroutine to read in the integer range in which primes are to
C	be found.
C
	SUBROUTINE READ_RANGE(LOW, HIGH)
	INTEGER LOW, HIGH

	OPEN(UNIT=1, FILE='PRIMESIN.DAT', STATUS='OLD')
	READ(1,100,END=110,ERR=120) LOW, HIGH
  100	FORMAT(2I5)
	CLOSE(UNIT=1)
	RETURN
  110	CALL READ_END_OF_FILE
	RETURN
  120	CALL READ_ERROR
	RETURN
	END

C	Subroutine to print an error message for a read end-of-file.
C
	SUBROUTINE READ_END_OF_FILE

	TYPE 150
  150	FORMAT(' Unexpected end-of-file reading input file')
	RETURN
	END

C	Subroutine to print an error message for an input read-error.
C
	SUBROUTINE READ_ERROR

	TYPE 160
  160	FORMAT(' Read error reading input file')
	RETURN
	END

C	Subroutine to output the prime numbers to a data file.
C
	SUBROUTINE OUTPUT_TO_DATAFILE(IPRIMES_TABLE, ICOUNT)
	DIMENSION IPRIMES_TABLE(ICOUNT)

	OPEN(UNIT=2, FILE='PRIMESOUT.DAT', STATUS='NEW')
	WRITE (2, 200) (IPRIMES_TABLE(I), I=1,ICOUNT)
  200	FORMAT(I5)
	CLOSE(UNIT=2)
	RETURN
	END