感謝Andres Hernandez的回答:
目前這是不可能的。不過,擴展QL以允許它很容易,但我認爲它需要在C++上完成。因此,即使您在Python中使用QL,您是否可以修改C++代碼並導出新的綁定?如果是的話,那麼你可以使用下面的代碼,如果沒有,那麼我可以將它檢入到代碼中,但是需要一段時間才能接受pull請求。在nonlinearfittingmethods.cpp
class NelsonSiegelConstrainedFitting
: public FittedBondDiscountCurve::FittingMethod {
public:
NelsonSiegelConstrainedFitting(const Array& lower, const Array& upper,
const Array& weights = Array(),
boost::shared_ptr<OptimizationMethod> optimizationMethod
= boost::shared_ptr<OptimizationMethod>());
std::auto_ptr<FittedBondDiscountCurve::FittingMethod> clone() const;
private:
Size size() const;
DiscountFactor discountFunction(const Array& x, Time t) const;
Array lower_, upper_;
};
:如果你可以觸摸的代碼,你可以添加這樣的事情:
在nonlinearfittingmethods.hpp
NelsonSiegelConstrainedFitting::NelsonSiegelConstrainedFitting(
const Array& lower, const Array& upper, const Array& weights,
boost::shared_ptr<OptimizationMethod> optimizationMethod)
: FittedBondDiscountCurve::FittingMethod(true, weights, optimizationMethod),
lower_(lower), upper_(upper){
QL_REQUIRE(lower_.size() == 4, "Lower constraint must have 4 elements");
QL_REQUIRE(upper_.size() == 4, "Lower constraint must have 4 elements");
}
std::auto_ptr<FittedBondDiscountCurve::FittingMethod>
NelsonSiegelConstrainedFitting::clone() const {
return std::auto_ptr<FittedBondDiscountCurve::FittingMethod>(
new NelsonSiegelFitting(*this));
}
Size NelsonSiegelConstrainedFitting::size() const {
return 4;
}
DiscountFactor NelsonSiegelConstrainedFitting::discountFunction(const Array& x,
Time t) const {
///extreme values of kappa result in colinear behaviour of x[1] and x[2], so it should be constrained not only
///to be positive, but also not very extreme
Real kappa = lower_[3] + upper_[3]/(1.0+exp(-x[3]));
Real x0 = lower_[0] + upper_[0]/(1.0+exp(-x[0])),
x1 = lower_[1] + upper_[1]/(1.0+exp(-x[1])),
x2 = lower_[2] + upper_[2]/(1.0+exp(-x[2])),;
Real zeroRate = x0 + (x1 + x2)*
(1.0 - std::exp(-kappa*t))/
((kappa+QL_EPSILON)*(t+QL_EPSILON)) -
x2*std::exp(-kappa*t);
DiscountFactor d = std::exp(-zeroRate * t) ;
return d;
}
然後,您需要將其添加到swig界面,但這樣做應該是微不足道的。