!***********************************************************************************************************************************
!
!                                                       M 4 4 D E T _ M A I N
!
!  Program:      M44DET_MAIN
!
!  Programmer:   David G. Simpson
!                NASA Goddard Space Flight Center
!                Greenbelt, Maryland  20771
!
!  Date:         February 6, 2009
!
!  Language:     Fortran-90
!
!  Version:      1.00a
!
!  Description:  This program is a short "driver" to call function M44DET, which computes the determinant a 4x4 matrix.
!
!  Files:        Source files:
!
!                   m44det.f90                   Main program
!
!***********************************************************************************************************************************

      PROGRAM M44DET_MAIN

      IMPLICIT NONE

      INTEGER :: I, J
      DOUBLE PRECISION, DIMENSION(4,4) :: MAT
      DOUBLE PRECISION :: DET

      DOUBLE PRECISION :: M44DET

!-----------------------------------------------------------------------------------------------------------------------------------

!
!     Get user input.
!

      WRITE (UNIT=*, FMT='(/A/)') ' Enter matrix:'

      DO I = 1, 4
         DO J = 1, 4
            WRITE (UNIT=*, FMT='(A,I1,1H,,I1,A)', ADVANCE='NO') ' A(', I, J, ') = '
            READ (UNIT=*, FMT=*) MAT(I,J)
         END DO
      END DO

!
!     Compute the determinant of the input matrix.
!

      DET = M44DET (MAT)

!
!     Print the result.
!

      WRITE (UNIT=*, FMT='(/A,ES25.15)') ' Det = ', DET

      STOP

      END PROGRAM M44DET_MAIN



!***********************************************************************************************************************************
!  M44DET  -  Compute the determinant of a 4x4 matrix.
!***********************************************************************************************************************************

      FUNCTION M44DET (A) RESULT (DET)

      IMPLICIT NONE

      DOUBLE PRECISION, DIMENSION(4,4), INTENT(IN)  :: A

      DOUBLE PRECISION :: DET


      DET =  A(1,1)*(A(2,2)*(A(3,3)*A(4,4)-A(3,4)*A(4,3))+A(2,3)*(A(3,4)*A(4,2)-A(3,2)*A(4,4))+A(2,4)*(A(3,2)*A(4,3)- &
             A(3,3)*A(4,2)))-A(1,2)*(A(2,1)*(A(3,3)*A(4,4)-A(3,4)*A(4,3))+A(2,3)*(A(3,4)*A(4,1)-A(3,1)*A(4,4))+ &
             A(2,4)*(A(3,1)*A(4,3)-A(3,3)*A(4,1)))+A(1,3)*(A(2,1)*(A(3,2)*A(4,4)-A(3,4)*A(4,2))+A(2,2)*(A(3,4)*A(4,1)- &
             A(3,1)*A(4,4))+A(2,4)*(A(3,1)*A(4,2)-A(3,2)*A(4,1)))-A(1,4)*(A(2,1)*(A(3,2)*A(4,3)-A(3,3)*A(4,2))+ &
             A(2,2)*(A(3,3)*A(4,1)-A(3,1)*A(4,3))+A(2,3)*(A(3,1)*A(4,2)-A(3,2)*A(4,1)))


      RETURN

      END FUNCTION M44DET