\(\newcommand{\W}[1]{ \; #1 \; }\) \(\newcommand{\R}[1]{ {\rm #1} }\) \(\newcommand{\B}[1]{ {\bf #1} }\) \(\newcommand{\D}[2]{ \frac{\partial #1}{\partial #2} }\) \(\newcommand{\DD}[3]{ \frac{\partial^2 #1}{\partial #2 \partial #3} }\) \(\newcommand{\Dpow}[2]{ \frac{\partial^{#1}}{\partial {#2}^{#1}} }\) \(\newcommand{\dpow}[2]{ \frac{ {\rm d}^{#1}}{{\rm d}\, {#2}^{#1}} }\)
sub_sparse_hes.cpp¶
View page sourceComputing Sparse Hessian for a Subset of Variables¶
Purpose¶
This example uses multiple levels of AD to compute the Hessian for a subset of the variables without having to compute the sparsity pattern for the entire function.
See Also¶
Function¶
We consider the function \(f : \B{R}^{nu} \times \B{R}^{nv} \rightarrow \B{R}\) defined by
Subset¶
Suppose that we are only interested computing the function
where this Hessian is sparse.
Example¶
The following code shows one way to compute this subset of the Hessian of \(f\).
# include <cppad/cppad.hpp>
namespace {
using CppAD::vector;
template <class Scalar>
Scalar f(const vector<Scalar>& u,const vector<Scalar>& v)
{ size_t i;
Scalar sum_v = Scalar(0);
for(i = 0; i < v.size(); i++)
sum_v += v[i];
Scalar sum_cube_u = Scalar(0);
for(i = 0; i < u.size(); i++)
sum_cube_u += u[i] * u[i] * u[i] / 6.0;
return sum_v * sum_cube_u;
}
}
bool sub_sparse_hes(void)
{ bool ok = true;
using CppAD::AD;
typedef AD<double> adouble;
typedef AD<adouble> a2double;
typedef vector< std::set<size_t> > pattern;
double eps = 10. * std::numeric_limits<double>::epsilon();
size_t i, j;
// start recording with x = (u , v)
size_t nu = 10;
size_t nv = 5;
size_t n = nu + nv;
vector<adouble> ax(n);
for(j = 0; j < n; j++)
ax[j] = adouble(j + 2);
CppAD::Independent(ax);
// extract u as independent variables
vector<a2double> a2u(nu);
for(j = 0; j < nu; j++)
a2u[j] = a2double(j + 2);
CppAD::Independent(a2u);
// extract v as parameters
vector<a2double> a2v(nv);
for(j = 0; j < nv; j++)
a2v[j] = ax[nu+j];
// record g(u)
vector<a2double> a2y(1);
a2y[0] = f(a2u, a2v);
CppAD::ADFun<adouble> g;
g.Dependent(a2u, a2y);
// compue sparsity pattern for Hessian of g(u)
pattern r(nu), s(1);
for(j = 0; j < nu; j++)
r[j].insert(j);
g.ForSparseJac(nu, r);
s[0].insert(0);
pattern p = g.RevSparseHes(nu, s);
// Row and column indices for non-zeros in lower triangle of Hessian
vector<size_t> row, col;
for(i = 0; i < nu; i++)
{ std::set<size_t>::const_iterator itr;
for(itr = p[i].begin(); itr != p[i].end(); itr++)
{ j = *itr;
if( j <= i )
{ row.push_back(i);
col.push_back(j);
}
}
}
size_t K = row.size();
CppAD::sparse_hessian_work work;
vector<adouble> au(nu), ahes(K), aw(1);
aw[0] = 1.0;
for(j = 0; j < nu; j++)
au[j] = ax[j];
size_t n_sweep = g.SparseHessian(au, aw, p, row, col, ahes, work);
// The Hessian w.r.t u is diagonal
ok &= n_sweep == 1;
// record H(u, v) = Hessian of f w.r.t u
CppAD::ADFun<double> H(ax, ahes);
// remove unecessary operations
H.optimize();
// Now evaluate the Hessian at a particular value for u, v
vector<double> u(nu), v(nv), x(n);
for(j = 0; j < n; j++)
x[j] = double(j + 2);
vector<double> hes = H.Forward(0, x);
// Now check the Hessian
double sum_v = 0.0;
for(j = 0; j < nv; j++)
sum_v += x[nu + j];
for(size_t k = 0; k < K; k++)
{ i = row[k];
j = col[k];
ok &= i == j;
double check = sum_v * x[i];
ok &= CppAD::NearEqual(hes[k], check, eps, eps);
}
return ok;
}