Passing Data to User-Supplied Functions

In some cases it may be advantageous to pass problem-specific data to a user-supplied function through the IMSL C Math Library interface. This ability can be useful if a user-supplied function requires data that is local to the user's calling function, and the user wants to avoid using global data to allow the user-supplied function to access the data. Functions in IMSL C Math Library that accept user-supplied functions have an optional argument(s) that will accept an alternative user-supplied function, along with a pointer to the data, that allows user-specified data to be passed to the function. The example below demonstrates this feature using the IMSL C Math Library function imsl_f_min_uncon and optional argument IMSL_FCN_W_DATA.

Example

 

 

        #include <imsl.h>

        #include <math.h>

        #include <stdio.h>

 

        float fcn_w_data(float x, void *data);

 

        int main()

        {

            float a = -100.0;

            float b = 100.0;

            float fx, x;

            float usr_data[] = {5.0, 10.0};

            x = imsl_f_min_uncon (NULL, a, b,

                IMSL_FCN_W_DATA, fcn_w_data, usr_data,

                0);

            fx = fcn_w_data(x, (void*)usr_data);

 

            printf ("The solution is: %8.4f\n", x);

            printf ("The function evaluated at the solution is: %8.4f\n",

                fx);

        }

 

        /*

        * User function that accepts additional data in a (void*) pointer.

        * This (void*) pointer can be cast to any type and dereferenced to

        * get at any sort of data-type or structure that is needed.

        * For example, to get at the data in this example

        *  *((float*)data)   and usr_data[0] contains the value  5.0

        *  *((float*)data+1) and usr_data[1] contains the value 10.0

        */

        float fcn_w_data(float x, void *data)

        {

            float *usr_data = (float*)data;

 

            return exp(x) - usr_data[0]*x + usr_data[1];

        }