NAG Library Manual, Mark 27.3
Interfaces:  FL   CL   CPP   AD 

NAG FL Interface Introduction
Example description
    Program f07gvfe

!     F07GVF Example Program Text

!     Mark 27.3 Release. NAG Copyright 2021.

!     .. Use Statements ..
      Use nag_library, Only: nag_wp, x04dbf, zpprfs, zpptrf, zpptrs
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Integer                          :: aplen, i, ifail, info, j, ldb, ldx,  &
                                          n, nrhs
      Character (1)                    :: uplo
!     .. Local Arrays ..
      Complex (Kind=nag_wp), Allocatable :: afp(:), ap(:), b(:,:), work(:),    &
      Real (Kind=nag_wp), Allocatable  :: berr(:), ferr(:), rwork(:)
      Character (1)                    :: clabs(1), rlabs(1)
!     .. Executable Statements ..
      Write (nout,*) 'F07GVF Example Program Results'
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n, nrhs
      ldb = n
      ldx = n
      aplen = n*(n+1)/2
      Allocate (afp(aplen),ap(aplen),b(ldb,nrhs),work(2*n),x(ldx,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:aplen) = ap(1:aplen)
      x(1:n,1:nrhs) = b(1:n,1:nrhs)

!     Factorize A in the array AFP
!     The NAG name equivalent of zpptrf is f07grf
      Call zpptrf(uplo,n,afp,info)

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

!       Compute solution in the array X
!       The NAG name equivalent of zpptrs is f07gsf
        Call zpptrs(uplo,n,nrhs,afp,x,ldx,info)

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

!       The NAG name equivalent of zpprfs is f07gvf
        Call zpprfs(uplo,n,nrhs,ap,afp,b,ldb,x,ldx,ferr,berr,work,rwork,info)

!       Print solution

!       ifail: behaviour on error exit
!              =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
        ifail = 0
        Call x04dbf('General',' ',n,nrhs,x,ldx,'Bracketed','F7.4',             &

        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 ((5X,1P,4(E11.1,7X)))
    End Program f07gvfe