Program f08kffe

!     F08KFF Example Program Text

!     Mark 30.2 Release. NAG Copyright 2024.

!     .. Use Statements ..
      Use nag_library, Only: dbdsqr, dgebrd, dorgbr, f06qff, nag_wp, x04caf
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Integer                          :: i, ic, ifail, info, lda, ldc, ldu,   &
                                          ldvt, lwork, m, n
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: a(:,:), c(:,:), d(:), e(:), taup(:), &
                                          tauq(:), u(:,:), vt(:,:), work(:)
!     .. Executable Statements ..
      Write (nout,*) 'F08KFF Example Program Results'
!     Skip heading in data file
      Read (nin,*)
      Do ic = 1, 2
        Read (nin,*) m, n
        lda = m
        ldc = n
        ldu = m
        ldvt = n
        lwork = 64*(m+n)
        Allocate (a(lda,n),c(ldc,n),d(n),e(n-1),taup(n),tauq(n),u(ldu,n),      &
          vt(ldvt,n),work(lwork))

!       Read A from data file

        Read (nin,*)(a(i,1:n),i=1,m)

!       Reduce A to bidiagonal form
!       The NAG name equivalent of dgebrd is f08kef
        Call dgebrd(m,n,a,lda,d,e,tauq,taup,work,lwork,info)

        If (m>=n) Then

!         Copy A to VT and U

          Call f06qff('Upper',n,n,a,lda,vt,ldvt)
          Call f06qff('Lower',m,n,a,lda,u,ldu)

!         Form P**T explicitly, storing the result in VT
!         The NAG name equivalent of dorgbr is f08kff
          Call dorgbr('P',n,n,m,vt,ldvt,taup,work,lwork,info)

!         Form Q explicitly, storing the result in U
!         The NAG name equivalent of dorgbr is f08kff
          Call dorgbr('Q',m,n,n,u,ldu,tauq,work,lwork,info)

!         Compute the SVD of A
!         The NAG name equivalent of dbdsqr is f08mef
          Call dbdsqr('Upper',n,n,m,0,d,e,vt,ldvt,u,ldu,c,ldc,work,info)

!         Print singular values, left & right singular vectors

          Write (nout,*)
          Write (nout,*) 'Example 1: singular values'
          Write (nout,99999) d(1:n)
          Write (nout,*)
          Flush (nout)

!         ifail: behaviour on error exit
!                =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
          ifail = 0
          Call x04caf('General',' ',n,n,vt,ldvt,                               &
            'Example 1: right singular vectors, by row',ifail)

          Write (nout,*)
          Flush (nout)

          Call x04caf('General',' ',m,n,u,ldu,                                 &
            'Example 1: left singular vectors, by column',ifail)

        Else

!         Copy A to VT and U

          Call f06qff('Upper',m,n,a,lda,vt,ldvt)
          Call f06qff('Lower',m,m,a,lda,u,ldu)

!         Form P**T explicitly, storing the result in VT
!         The NAG name equivalent of dorgbr is f08kff
          Call dorgbr('P',m,n,m,vt,ldvt,taup,work,lwork,info)

!         Form Q explicitly, storing the result in U
!         The NAG name equivalent of dorgbr is f08kff
          Call dorgbr('Q',m,m,n,u,ldu,tauq,work,lwork,info)

!         Compute the SVD of A
!         The NAG name equivalent of dbdsqr is f08mef
          Call dbdsqr('Lower',m,n,m,0,d,e,vt,ldvt,u,ldu,c,ldc,work,info)

!         Print singular values, left & right singular vectors

          Write (nout,*)
          Write (nout,*) 'Example 2: singular values'
          Write (nout,99999) d(1:m)
          Write (nout,*)
          Flush (nout)

          ifail = 0
          Call x04caf('General',' ',m,n,vt,ldvt,                               &
            'Example 2: right singular vectors, by row',ifail)

          Write (nout,*)
          Flush (nout)

          Call x04caf('General',' ',m,m,u,ldu,                                 &
            'Example 2: left singular vectors, by column',ifail)

        End If
        Deallocate (a,c,d,e,taup,tauq,u,vt,work)
      End Do

99999 Format (3X,(8F8.4))
    End Program f08kffe