NAG Library Manual, Mark 28.5
Interfaces:  FL   CL   CPP   AD 

NAG FL Interface Introduction
Example description
    Program f07fhfe

!     F07FHF Example Program Text

!     Mark 28.5 Release. NAG Copyright 2022.

!     .. Use Statements ..
      Use nag_library, Only: dporfs, dpotrf, dpotrs, f06qff, nag_wp, x04caf
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Integer                          :: i, ifail, info, lda, ldaf, ldb, ldx, &
                                          n, nrhs
      Character (1)                    :: uplo
!     .. Local Arrays ..
      Real (Kind=nag_wp), Allocatable  :: a(:,:), af(:,:), b(:,:), berr(:),    &
                                          ferr(:), work(:), x(:,:)
      Integer, Allocatable             :: iwork(:)
!     .. Executable Statements ..
      Write (nout,*) 'F07FHF Example Program Results'
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n, nrhs
      lda = n
      ldaf = n
      ldb = n
      ldx = n
      Allocate (a(lda,n),af(ldaf,n),b(ldb,nrhs),berr(nrhs),ferr(nrhs),         &

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

      Read (nin,*) uplo
      If (uplo=='U') Then
        Read (nin,*)(a(i,i:n),i=1,n)
      Else If (uplo=='L') Then
        Read (nin,*)(a(i,1:i),i=1,n)
      End If
      Read (nin,*)(b(i,1:nrhs),i=1,n)

      Call f06qff(uplo,n,n,a,lda,af,ldaf)

      x(1:n,1:nrhs) = b(1:n,1:nrhs)

!     Factorize A in the array AF
!     The NAG name equivalent of dpotrf is f07fdf
      Call dpotrf(uplo,n,af,ldaf,info)

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

!       Compute solution in the array X
!       The NAG name equivalent of dpotrs is f07fef
        Call dpotrs(uplo,n,nrhs,af,ldaf,x,ldx,info)

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

!       The NAG name equivalent of dporfs is f07fhf
        Call dporfs(uplo,n,nrhs,a,lda,af,ldaf,b,ldb,x,ldx,ferr,berr,work,      &

!       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)
        Write (nout,*) 'A is not positive definite'
      End If

99999 Format ((3X,1P,7E11.1))
    End Program f07fhfe