PROGRAM arraysum
   IMPLICIT NONE
   INTEGER :: i, N
   DOUBLE PRECISION, ALLOCATABLE :: X(:)
   DOUBLE PRECISION :: mysum

   PRINT *, 'Enter the number of elements in the vector (N >= 1)'
   READ *, N
   IF (N .LT. 1) THEN
      PRINT *, 'FATAL ERROR: N must be strictly positive!'
      STOP
   END IF

   ALLOCATE(X(N))               ! get the space I need

   CALL RANDOM_NUMBER(X)        ! initialize X
!
!  Compute the sum using a loop
!
   mysum = 0.0
   DO i = 1, N
      mysum = mysum + X(i)
   END DO

   PRINT *, 'SUM = ', mysum, SUM(X)    ! print both answers for comparison
   PRINT *, 'ERROR = ', mysum-SUM(X) 

   DEALLOCATE(X)

END PROGRAM arraysum