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

   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
   mysum = vecsum(N, X)                 ! compute sum using loop-based function
   PRINT *, 'SUM = ', mysum, SUM(X)     ! print both answers for comparison
   PRINT *, 'ERROR = ', abs(mysum-SUM(X))

   DEALLOCATE(X)

END PROGRAM arraysum

FUNCTION vecsum(N, V)
   IMPLICIT NONE
   DOUBLE PRECISION :: vecsum
   INTEGER, INTENT(IN) :: N
   DOUBLE PRECISION, INTENT(IN) :: V(*)
   INTEGER :: i

   vecsum = 0.0
   DO i = 1, N
      vecsum = vecsum + V(i)
   END DO
END FUNCTION vecsum