NAG Library Manual, Mark 28.6
```    Program e01ea_a1w_fe

!     E01EA_A1W_F Example Program Text
!     Mark 28.6 Release. NAG Copyright 2022.

!     .. Use Statements ..
Use iso_c_binding, Only: c_ptr
Use nagad_library, Only: e01ea_a1w_f, e01eb_a1w_f,                       &
x10aa_a1w_f, x10ab_a1w_f, Assignment (=)
Use nag_library, Only: nag_wp
!     .. Implicit None Statement ..
Implicit None
!     .. Parameters ..
Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
Real (Kind=nag_wp)               :: fr, xr, yr
Integer                          :: i, ifail, m, n, r
!     .. Local Arrays ..
Type (nagad_a1w_w_rtype), Allocatable :: f(:), pf(:), px(:), py(:),      &
x(:), y(:)
Real (Kind=nag_wp), Allocatable  :: df(:), dx(:), dy(:)
Integer, Allocatable             :: triang(:)
!     .. Executable Statements ..
Write (nout,*) 'E01EA_A1W_F Example Program Results'

!     Skip heading in data file

m = 1
Allocate (f(n),x(n),y(n),dx(n),dy(n),df(n),px(m),py(m),pf(m),            &
triang(7*n))

!     Initialize x, y and f arrays using data
Do r = 1, n
x(r) = xr
y(r) = yr
f(r) = fr
End Do

!     Create AD configuration data object
ifail = 0

!     Register variables to differentiate w.r.t.

ifail = 0
Call e01ea_a1w_f(n,x,y,triang,ifail)
!     Evaluate interpolant and derivatives at a fixed point
px(1) = 0.5_nag_wp*(x(n/2)%value+x(n/2+1)%value)
py(1) = 0.5_nag_wp*(y(n/2)%value+y(n/2+1)%value)
ifail = 0

Write (nout,*)
Write (nout,*) ' Derivatives calculated: First order adjoints'
Write (nout,*) ' Computational mode    : algorithmic'

!     Get derivative of interpolated value w.r.t data points and values

Write (nout,*)
Write (nout,99999) 'Interpolation point: x = ', px(1)%value, ' y = ',    &
py(1)%value
Write (nout,99999) 'Interpolated value = ', pf(1)%value
99999 Format (1X,A,F7.4,A,F7.4)
Write (nout,*)
Write (nout,*) ' Derivatives of interpolated value w.r.t. x,  y and f:'
Write (nout,*)
Write (nout,*) '    i      d/dx         d/dy         d/df'
Do i = 1, n
Write (nout,99998) i, dx(i), dy(i), df(i)
End Do
99998 Format (1X,I5,3(1X,F12.5))

!     Remove computational data object and tape
ifail = 0