Program f07cefe

!     F07CEF Example Program Text

!     Mark 30.3 Release. nAG Copyright 2024.

!     .. Use Statements ..
      Use nag_library, Only: dgttrf, dgttrs, nag_wp, x04caf
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Integer                          :: i, ifail, info, ldb, n, nrhs
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: b(:,:), d(:), dl(:), du(:), du2(:)
      Integer, Allocatable             :: ipiv(:)
!     .. Executable Statements ..
      Write (nout,*) 'F07CEF Example Program Results'
      Write (nout,*)
      Flush (nout)
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n, nrhs
      ldb = n
      Allocate (b(ldb,nrhs),d(n),dl(n-1),du(n-1),du2(n-2),ipiv(n))

!     Read the tridiagonal matrix A from data file

      Read (nin,*) du(1:n-1)
      Read (nin,*) d(1:n)
      Read (nin,*) dl(1:n-1)

!     Read the right hand matrix B
      Read (nin,*)(b(i,1:nrhs),i=1,n)

!     Factorize the tridiagonal matrix A
!     The NAG name equivalent of dgttrf is f07cdf
      Call dgttrf(n,dl,d,du,du2,ipiv,info)

      If (info==0) Then

!       Solve the equations AX = B
!       The NAG name equivalent of dgttrs is f07cef
        Call dgttrs('No transpose',n,nrhs,dl,d,du,du2,ipiv,b,ldb,info)

!       Print the solution

!       ifail: behaviour on error exit
!              =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
        ifail = 0
        Call x04caf('General',' ',n,nrhs,b,ldb,'Solution(s)',ifail)

      Else
        Write (nout,99999) 'The (', info, ',', info, ')',                      &
          ' element of the factor U is zero'
      End If

99999 Format (1X,A,I3,A,I3,A,A)
    End Program f07cefe