Reversing Arrays by Using Subroutines

PROGRAM Elem_Rev_SUB

! This program includes a SUBROUTINE which reverses the elements

! of a run-time array.

IMPLICIT NONE

INTEGER :: N

REAL, DIMENSION(:), ALLOCATABLE :: A

! Now read the size of array A. Notice that the indices start at 1.

PRINT *, "Enter size of array A:"

READ *, N

! Now allocate memory for array A:

ALLOCATE(A(N))

! Now read elements of A.

PRINT *, "Enter elements of A. Notice that there are ", N, " of them."

READ *, A

CALL Rev_SUB(A)

! Now we print the reversed array:

PRINT *, A

! Now free memory allocated for A.

! You use DEALLOCATE and ALLOCATE only for run-time arrays.

DEALLOCATE(A)

CONTAINS

SUBROUTINE Rev_SUB(A)

! Reverses elements of array A.

! Input : run-time array A.

! Output : run-time array A.

REAL, DIMENSION(:), INTENT(INOUT) :: A

INTEGER :: I, N

REAL :: Tmp

N=SIZE(A)

! Now we start the reversing process.

! =======================

! Reversing process starts here.

DO I=1, N/2

! We swap A(I) with A(N-I+1)

Tmp=A(I)

A(I)=A(N-I+1)

A(N-I+1)=Tmp

END DO

! Reversing process Ends here.

!=====================

END SUBROUTINE Rev_SUB

END PROGRAM Elem_Rev_SUB