Scilab Function
Last update : 00/00/0000

NDcost - generic external for optim computing gradient using finite differences

### Calling Sequence

  `[f,g,ind]=NDcost(x,ind,fun,varargin)`

### Parameters

- `x`: real vector or matrix
- `ind`: integer parameter (see optim)
- `fun`: Scilab function with calling sequence `F=fun(x,varargin)` varargin may be use to pass parameters `p1,...pn`
- `f`: criterion value at point `x` (see optim)
- `g`: gradient value at point `x` (see optim)

### Description

This function can be used as an external for `optim` to minimize problem where gradient is too complicated to be programmed. only the function `fun` which computes the criterion is required.

This function should be used as follow: `[f,xopt,gopt]=optim(list(NDcost,fun,p1,...pn),x0,...)`

### Examples

    // example #1 (a simple one)
    //function to minimize
    function f=rosenbrock(x,varargin)
       p=varargin(1)
       f=1+sum( p*(x(2:$)-x(1:$-1)^2)^2 + (1-x(2:$))^2)
    endfunction

    x0=[1;2;3;4];
    [f,xopt,gopt]=optim(list(NDcost,rosenbrock,200),x0)

    // example #2: this example (from Rainer von Seggern) shows a quick (*) scilab way
    //             to identify the parameters of a linear differential equation.
    //             The model is a simple damped (linear) oscillator:
    //
    //               x''(t) + c x'(t) + k x(t) = 0
    //
    // and we write it as a system of (2) differential equations of first
    // order with y(1) = x, and y(2) = x':
    //
    //     dy1/dt = y(2)
    //     dy2/dt = -c*y(2) -k*y(1)
    //
    // We suppose to have m measures of x (that is y(1)) at different times
    // t_obs(1), ..., t_obs(m) called x_obs(1), ..., x_obs(m)  (in this example
    // these measures will be simulated) and we want to find the parameters
    // c and k by minimizing the sum of square errors between x_obs and y1(t_obs,p)
    //
    // (*) This method is not the most efficient but is easy to implement.

    function dy = DEQ(t,y,p)
       // the rhs of our first order differential equation system
       c =p(1);k=p(2)
       dy=[y(2);-c*y(2)-k*y(1)]
    endfunction

    function y=uN(p, t, t0, y0)
       // Numerical solution get with ode (in this case an exact analytic
       // solution is found easily but ode could work for any equations...)
       // Note: the ode output must be an approximation of the solution at
       //       times given in the vector t=[t(1),...,t($)]
       y = ode(y0,t0,t,list(DEQ,p))
    endfunction

    function r = cost_func(p, t_obs, x_obs, t0, y0)
       // the function to be minimized : this is the sum of the squared
       // errors between what gives the model and the measuments
       sol = uN(p, t_obs, t0, y0)
       e = sol(1,:) - x_obs
       r = sum(e.*e)
    endfunction

    // Datas
    t0 = 0; y0 = [10;0]; // initial time and initial conditions
    T = 30;  // final time for measurements

    // Here we simulate experimental datas (from which the parameters
    // should be found)
    pe = [0.2;3];  // exact parameters
    m = 60;  t_obs = linspace(t0+2,T,m); // observation times
    sigma = 2;   // noise level
    y_obs = uN(pe, t_obs, t0, y0);
    x_obs = y_obs(1,:) + grand(1,m,"nor",0, sigma);

    // initial guess parameters
    p0 = [0.5 ; 5];

    // for information the cost function before optimization:
    cost0 = cost_func(p0, t_obs, x_obs, t0, y0);
    mprintf("\n\r cost function before optimization = %g",cost0)

    // Solution with optim
    [costopt,popt]=optim(list(NDcost,cost_func, t_obs, x_obs, t0, y0),p0); //,'ar',40,40,1e-3);

    mprintf("\n\r cost function after optimization = %g",costopt)
    mprintf("\n\r values of the parameters found: c = %g, k = %g",popt(1),popt(2))

    // a small plot
    t = linspace(0,T,400);
    y = uN(popt, t, t0, y0);
    clf();
    plot2d(t',y(1,:)',style=2)
    plot2d(t_obs',y_obs(1,:)',style=-2)
    legend(["model","measures"]);
    xtitle("least square fit to identify ode parameters")

### See Also

`optim`, `external`, `derivative`,