Program f07phfe

!     F07PHF Example Program Text

!     Mark 27.3 Release. NAG Copyright 2021.

!     .. Use Statements ..
      Use nag_library, Only: dsprfs, dsptrf, dsptrs, nag_wp, x04caf
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Integer                          :: i, ifail, info, j, ldb, ldx, len, n, &
                                          nrhs
      Character (1)                    :: uplo
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: afp(:), ap(:), b(:,:), berr(:),      &
                                          ferr(:), work(:), x(:,:)
      Integer, Allocatable             :: ipiv(:), iwork(:)
!     .. Executable Statements ..
      Write (nout,*) 'F07PHF Example Program Results'
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n, nrhs
      ldb = n
      ldx = n
      len = n*(n+1)/2
      Allocate (afp(len),ap(len),b(ldb,nrhs),berr(nrhs),ferr(nrhs),work(3*n),  &
        x(ldx,n),ipiv(n),iwork(n))

!     Read A and B from data file, and copy A to AFP and B to X

      Read (nin,*) uplo
      If (uplo=='U') Then
        Read (nin,*)((ap(i+j*(j-1)/2),j=i,n),i=1,n)
      Else If (uplo=='L') Then
        Read (nin,*)((ap(i+(2*n-j)*(j-1)/2),j=1,i),i=1,n)
      End If
      Read (nin,*)(b(i,1:nrhs),i=1,n)

      afp(1:len) = ap(1:len)
      x(1:n,1:nrhs) = b(1:n,1:nrhs)

!     Factorize A in the array AFP
!     The NAG name equivalent of dsptrf is f07pdf
      Call dsptrf(uplo,n,afp,ipiv,info)

      Write (nout,*)
      Flush (nout)
      If (info==0) Then

!       Compute solution in the array X
!       The NAG name equivalent of dsptrs is f07pef
        Call dsptrs(uplo,n,nrhs,afp,ipiv,x,ldx,info)

!       Improve solution, and compute backward errors and
!       estimated bounds on the forward errors

!       The NAG name equivalent of dsprfs is f07phf
        Call dsprfs(uplo,n,nrhs,ap,afp,ipiv,b,ldb,x,ldx,ferr,berr,work,iwork,  &
          info)

!       Print 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,x,ldx,'Solution(s)',ifail)

        Write (nout,*)
        Write (nout,*) 'Backward errors (machine-dependent)'
        Write (nout,99999) berr(1:nrhs)
        Write (nout,*) 'Estimated forward error bounds (machine-dependent)'
        Write (nout,99999) ferr(1:nrhs)
      Else
        Write (nout,*) 'The factor D is singular'
      End If

99999 Format ((3X,1P,7E11.1))
    End Program f07phfe