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## NLPFDD Call

approximates derivatives by finite differences method

CALL NLPFDD( f, g, h, "fun", x0, <,par, "grd">);

See "Nonlinear Optimization and Related Subroutines" for a listing of all NLP subroutines. See Chapter 11, "Nonlinear Optimization Examples," for a description of the inputs to and outputs of all NLP subroutines.

The NLPFDD subroutine can be used for the following tasks:
• If the module "fun" returns a scalar, the NLPFDD subroutine computes the function value f, the gradient vector g, and the Hessian matrix h, all evaluated at the point x0.
• If the module "fun" returns a column vector of m function values, the subroutine assumes that a least-squares function is specified, and it computes the function vector f, the Jacobian matrix J, and the crossproduct of the Jacobian matrix J'J at the point x0. Note that in this case, you must set the first element of the par argument to m.
If any of the results cannot be computed, the subroutine returns a missing value for that result.

You can specify the following input arguments with the NLPFDD subroutine:
• The "fun" argument refers to an IML module that returns either a scalar value or a column vector of length m. This module returns the value of the objective function or, for least-squares problems, the values of the m functions that comprise the objective function.
• The x0 argument is a vector of length n that defines the point at which the functions and derivatives should be computed.
• The par argument is a vector that defines options and control parameters. Note that the par argument in the NLPFDD call is different from the one used in the optimization subroutines.
• The "grd" argument is optional and refers to an IML module that returns a vector defining the gradient of the function at x0. If the "fun" argument returns a vector of values instead of a scalar, the "grd" argument is ignored.
If the "fun" module returns a scalar, the subroutine returns the following values:
• f is the value of the function at the point x0.
• g is a vector containing the value of the gradient at the point x0. If you specify the "grd" argument, the gradient is computed from that module. Otherwise, the approximate gradient is computed by a finite difference approximation using calls of the function module in a neighborhood of x0.
• h is a matrix containing a finite difference approximation of the value of the Hessian at the point x0. If you specify the "grd" argument, the Hessian is computed by calls of that module in a neighborhood of x0. Otherwise, it is computed by calls of the function module in a neighborhood of x0.
If the "fun" module returns a vector, the subroutine returns the following values:
• f is a vector containing the values of the m functions comprising the objective function at the point x0.
• g is the m×n Jacobian matrix J, which contains the first-order derivatives of the functions with respect to the parameters, evaluated at x0. It is computed by finite difference approximations in a neighborhood of x0.
• h is the n×n crossproduct of the Jacobian matrix, JTJ. It is computed by finite difference approximations in a neighborhood of x0.
The par argument is a vector of length 3.
• par[1] corresponds to the opt[1] argument in the optimization subroutines. This argument is relevant only to least-squares optimization methods, in which case it specifies the number of functions returned by the module "fun". If par[1] is missing or is smaller than 1, it is set to 1.
• par[2] corresponds to the opt[8] argument in the optimization subroutines. It determines what type of approximation is to be used and how the finite difference interval, h, is to be computed. See "Finite Difference Approximations of Derivatives" for details.
• par[3] corresponds to the par[8] argument in the optimization subroutines. It specifies the number of accurate digits in evaluating the objective function. The default is , where is the machine precision.
If you specify a missing value in the par argument, the default value is used.

The NLPFDD subroutine is particularly useful for checking your analytical derivative specifications of the "grd", "hes", and "jac" modules. You can compare the results of the modules with the finite difference approximations of the derivatives of f at the point x0 to verify your specifications.

In the unconstrained Rosenbrock problem (see "Unconstrained Rosenbrock Function" ), the objective function is
f(x) = 50(x2-x12)2 + (1/2)(1-x1)2
Then the gradient and the Hessian, evaluated at the point x=(2,7), are
The following statements define the Rosenbrock function and use the NLPFDD call to compute the gradient and the Hessian. They are shown in Figure 17.2.
   proc iml;
start F_ROSEN(x);
y1 = 10. * (x[2] - x[1] * x[1]);
y2 = 1. - x[1];
f  = .5 * (y1 * y1 + y2 * y2);
return(f);
finish F_ROSEN;
x = {2 7};
CALL NLPFDD(crit,grad,hess,"F_ROSEN",x);
print grad;
print hess;


                                                  GRAD

-1199 300.00001

HESS

1000.9998 -400.0018
-400.0018 99.999993


Figure 17.2: Finite Difference Approximations for Gradient and Hessian

If the Rosenbrock problem is considered from a least-squares perspective, the two functions are
Then the Jacobian and the crossproduct of the Jacobian, evaluated at the point x=(2,7), are
The following statements define the Rosenbrock problem in a least-squares framework and use the NLPFDD call to compute the Jacobian and the crossproduct matrix. Since the value of the PARMS variable, which is used for the par argument, is 2, the NLPFDD subroutine allocates memory for a least-squares problem with two functions, f1(x) and f2(x). The output is shown in Figure 17.3.
   proc iml;
start F_ROSEN(x);
y = j(2,1,0.);
y[1] = 10. * (x[2] - x[1] * x[1]);
y[2] = 1. - x[1];
return(y);
finish F_ROSEN;
x     = {2 7};
parms = 2;
CALL NLPFDD(fun,jac,crpj,"F_ROSEN",x,parms);
print jac;
print crpj;


                                                  JAC

-40        10
-1         0

CRPJ

1601      -400
-400       100


Figure 17.3: Finite Difference Approximations for Jacobian

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