NAG Library Manual, Mark 27.2
```/* F01EC_T1W_F C++ Header Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
* Mark 27.2, 2021.
*/

#include <dco.hpp>
#include <iostream>
#include <nag.h>
#include <nagx04.h>
#include <stdio.h>
#include <string>
using namespace std;

int main(void)
{
int      exit_status = 0;
Integer  ifail       = 0;
NagError fail;
INIT_FAIL(fail);

cout << "F01EC_T1W_F C++ Header Example Program Results\n\n";
// Skip heading in data file
string mystr;
getline(cin, mystr);

// Read problem size and number of right-hand-sides
Integer n;
cin >> n;

// Allocate arrays containing A and its factorized form, B
// and the solution X.
nagad_t1w_w_rtype *a = 0, *a_in = 0;
double *           ar = 0;

a    = new nagad_t1w_w_rtype[n * n];
a_in = new nagad_t1w_w_rtype[n * n];
ar   = new double[n * n];

// Read the matrix A, register and copy
double dd;
for (int i = 0; i < n; ++i)
{
for (int j = 0; j < n; ++j)
{
cin >> dd;
Integer k = i + j * n;
a_in[k]   = dd;
}
}

// Create AD configuration data object
ifail = 0;

for (int i = 0; i < n; ++i)
{
{
double  inc              = 1.0;
Integer k                = i * n + i;
dco::derivative(a_in[k]) = inc;
}
for (int j = 0; j < n * n; ++j)
{
a[j] = a_in[j];
}
// Find exp(A)
ifail = 0;

if (i == 0)
{
// Print exp(A)
for (int j = 0; j < n * n; j++)
{
ar[j] = dco::value(a[j]);
}

cout << endl;
x04cac(Nag_ColMajor, Nag_GeneralMatrix, Nag_NonUnitDiag, n, n, ar, n,
"  Exp(A)", 0, &fail);
}

for (int j = 0; j < n; j++)
{
Integer l = j + j * n, p = i * n + j;
double  dd = dco::derivative(a[l]);
ar[p]      = dd;
}
{
double  zero             = 0.0;
Integer k                = i * n + i;
dco::derivative(a_in[k]) = zero;
}
}

cout << "\n\n Derivatives calculated: First order tangents\n";
cout << " Computational mode    : algorithmic\n";
cout << "\n Derivatives of diagonal of exp(A) w.r.t diagonal of A:\n";

// Print derivatives
cout << endl;
x04cac(Nag_ColMajor, Nag_GeneralMatrix, Nag_NonUnitDiag, n, n, ar, n,
"     d(expA(i,i)/dA(j,j)", 0, &fail);

// Remove computational data object
ifail = 0;