NAG Library Manual, Mark 28.5
```    Program f11jdfe

!     F11JDF Example Program Text

!     Mark 28.5 Release. NAG Copyright 2022.

!     .. Use Statements ..
Use nag_library, Only: f11gdf, f11gef, f11gff, f11jdf, f11xef, nag_wp
!     .. Implicit None Statement ..
Implicit None
!     .. Parameters ..
Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
Real (Kind=nag_wp)               :: anorm, omega, sigerr, sigmax,        &
sigtol, stplhs, stprhs, tol
Integer                          :: i, ifail, ifail1, irevcm, iterm,     &
itn, its, liwork, lwneed, lwork,     &
maxitn, maxits, monit, n, nnz
Character (1)                    :: ckjdf, ckxef, norm, precon, sigcmp,  &
weight
Character (6)                    :: method
!     .. Local Arrays ..
Real (Kind=nag_wp), Allocatable  :: a(:), b(:), rdiag(:), wgt(:),        &
work(:), x(:)
Integer, Allocatable             :: icol(:), irow(:), iwork(:)
!     .. Executable Statements ..
Write (nout,*) 'F11JDF Example Program Results'
!     Skip heading in data file

liwork = n + 1
lwork = 6*n + 120

Allocate (a(nnz),b(n),rdiag(n),wgt(n),work(lwork),x(n),icol(nnz),        &
irow(nnz),iwork(liwork))
Read (nin,*) precon, sigcmp, norm, iterm

Do i = 1, nnz
End Do

!     Read right-hand side vector b and initial approximate solution x

!     Call F11GDF to initialize solver

weight = 'N'
monit = 0

!     ifail: behaviour on error exit
!             =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
ifail = 0
Call f11gdf(method,precon,sigcmp,norm,weight,iterm,n,tol,maxitn,anorm,   &
sigmax,sigtol,maxits,monit,lwneed,work,lwork,ifail)

!     Calculate reciprocal diagonal matrix elements.

iwork(1:n) = 0

Do i = 1, nnz
If (irow(i)==icol(i)) Then
iwork(irow(i)) = iwork(irow(i)) + 1
If (a(i)/=0.0E0_nag_wp) Then
rdiag(irow(i)) = 1.0E0_nag_wp/a(i)
Else
Write (nout,*) 'Matrix has a zero diagonal element'
Go To 100
End If
End If
End Do

Do i = 1, n
If (iwork(i)==0) Then
Write (nout,*) 'Matrix has a missing diagonal element'
Go To 100
End If
If (iwork(i)>=2) Then
Write (nout,*) 'Matrix has a multiple diagonal element'
Go To 100
End If
End Do

!     Call F11GEF to solve the linear system

irevcm = 0
ckxef = 'C'
ckjdf = 'C'

ifail = 1
loop: Do
Call f11gef(irevcm,x,b,wgt,work,lwork,ifail)

If (irevcm/=4) Then
ifail1 = -1
Select Case (irevcm)
Case (1)
!           Compute matrix vector product

Call f11xef(n,nnz,a,irow,icol,ckxef,x,b,ifail1)

ckxef = 'N'
Case (2)
!           SSOR preconditioning

Call f11jdf(n,nnz,a,irow,icol,rdiag,omega,ckjdf,x,b,iwork,ifail1)

ckjdf = 'N'
End Select
If (ifail1/=0) Then
irevcm = 6
End If
Else If (ifail/=0) Then
Write (nout,99996) ifail
Go To 100
Else
Exit loop
End If
End Do loop

!     Termination

Call f11gff(itn,stplhs,stprhs,anorm,sigmax,its,sigerr,work,lwork,ifail)

Write (nout,99999) 'Converged in', itn, ' iterations'
Write (nout,99998) 'Final residual norm =', stplhs

!     Output x

Write (nout,99997) x(1:n)

100   Continue

99999 Format (1X,A,I10,A)
99998 Format (1X,A,1P,E16.3)
99997 Format (1X,1P,E16.4)
99996 Format (1X,/,1X,' ** F11GEF returned with IFAIL = ',I5)
End Program f11jdfe
```