The following is a program which contains

(1) A non-recursive FUNCTION called FUN_SUM.

(2) A recursive FUNCTION called REC_ FUN_SUM.

(3) A non-recursive SUBROUTINE called SUB_SUM.

(4) A recursive SUBROUTINE called REC_SUB_SUM.

All of the above internal FUNCTIONS/SUBROUTINES compute the sum of the positive integers greater than or equal to 1 and less than or equal to N. Thus the input for each one of them is N, where N is a positive integer, and the output is the sum of the integers greater than or equal to 1 and less than or equal to N.

The program asks the reader to enter a positive integer, which will be called N, and it displayes the sum (described above) as it is computed by each FUNCTION/SUBROUTINE.

PROGRAM PROG_SUM

IMPLICIT NONE

INTEGER :: N, Z

PRINT *, “Enter a positive integer N”

READ *, N

PRINT *, “Sum= “, FUN_SUM(N), “ by non-recursive FUNCTION”

PRINT *, “Sum= “, REC_FUN_SUM(N), “ by recursive FUNCTION”

CALL SUB_SUM(N,Z)

PRINT *, “Sum= “, Z, “ by non-recursive SUBROUTINE”

CALL REC_SUB_SUM(N,Z)

PRINT *, “Sum= “, Z, “ by recursive SUBROUTINE”

CONTAINS

FUNCTION FUN_SUM(N)

INTEGER, INTENT(IN) :: N

INTEGER :: FUN_SUM, I

FUN_SUM=0

DO I=1, N

FUN_SUM=FUN_SUM+I

END DO

END FUNCTION FUN_SUM

RECURSIVE FUNCTION REC_FUN_SUM(N) RESULT (MySum)

INTEGER, INTENT(IN) :: N

INTEGER :: MySum

IF (N==1) THEN

MySum=1

ELSE

MySum=N+REC_FUN_SUM(N-1)

END IF

END FUNCTION REC_FUN_SUM

SUBROUTINE SUB_SUM(N, MySum)

INTEGER, INTENT(IN) :: N

INTEGER, INTENT(OUT) :: MySum

INTEGER :: I

MySum=0

DO I=1, N

MySum=MySum+I

END DO

END SUBROUTINE SUB_SUM

RECURSIVE SUBROUTINE REC_SUB_SUM(N, MySum)

INTEGER, INTENT(IN) :: N

INTEGER, INTENT(OUT) :: MySum

IF (N==1) THEN

MySum=1

ELSE

CALL REC_SUB_SUM(N-1, MySum)

MySum=N+MySum

END IF

END SUBROUTINE REC_SUB_SUM

END PROGRAM PROG_SUM