C function call Numerical Recipes Chapter 10.3 - c

Question

I have this function. I'm kinda green in C and I don't know how to call this function to get a result:
float dbrent(float ax, float bx, float cx, float (*f)(float),float (*df)(float), float tol, float* xmin)
{
int iter, ok1, ok2;
float a, b, d, d1, d2, du, dv, dw, dx, e = 0.0;
float fu, fv, fw, fx, olde, tol1, tol2, u, u1, u2, v, w, x, xm;
a = (ax < cx ? ax : cx);
b = (ax > cx ? ax : cx);
x = w = v = bx;
fw = fv = fx = (*f)(x);
dw = dv = dx = (*df)(x);
for (iter = 1; iter <= ITMAX; iter++)
{
xm = 0.5 * (a + b);
tol1 = tol * fabs(x) + ZEPS;
tol2 = 2.0 * tol1;
if (fabs(x - xm) <= (tol2 - 0.5 * (b - a))) {
*xmin = x;
return fx;
}
if (fabs(e) > tol1) {
d1 = 2.0 * (b - a);
d2 = d1;
if (dw != dx) d1 = (w - x) * dx / (dx - dw);
if (dv != dx) d2 = (v - x) * dx / (dx - dv);
u1 = x + d1;
u2 = x + d2;
ok1 = (a - u1) * (u1 - b) > 0.0 && dx * d1 <= 0.0;
ok2 = (a - u2) * (u2 - b) > 0.0 && dx * d2 <= 0.0;
olde = e;
e = d;
if (ok1 || ok2) {
if (ok1 && ok2)
d = (fabs(d1) < fabs(d2) ? d1 : d2);
else if (ok1)
d = d1;
else
d = d2;
if (fabs(d) <= fabs(0.5 * olde)) {
u = x + d;
if (u - a < tol2 || b - u < tol2)
d = SIGN(tol1, xm - x);
}
else {
d = 0.5 * (e = (dx >= 0.0 ? a - x : b - x));
}
}
else {
d = 0.5 * (e = (dx >= 0.0 ? a - x : b - x));
}
}
else {
d = 0.5 * (e = (dx >= 0.0 ? a - x : b - x));
}
if (fabs(d) >= tol1) {
u = x + d;
fu = (*f)(u);
}
else {
u = x + SIGN(tol1, d);
fu = (*f)(u);
if (fu > fx) {
*xmin = x;
return fx;
}
}
du = (*df)(u);
if (fu <= fx) {
if (u >= x) a = x; else b = x;
MOV3(v, fv, dv, w, fw, dw)
MOV3(w, fw, dw, x, fx, dx)
MOV3(x, fx, dx, u, fu, du)
}
else {
if (u < x) a = u; else b = u;
if (fu <= fw || w == x) {
MOV3(v, fv, dv, w, fw, dw)
MOV3(w, fw, dw, u, fu, du)
}
else if (fu < fv || v == x || v == w) {
MOV3(v, fv, dv, u, fu, du)
}
}
}
cout<<"Too many iterations in routine dbrent";
return 0.0;
}
This code is from Numerical Recipes Chapter 10.3. I need to call this function and get the result but I don't know how to call it, especially when there is a syntax like float (*)(float).
Thanks in advance.

Answer 1

Well, let's suppose you have the following functions:
float df_(float yourFloat) {
...
return yourFloat;
}
and
float f_(float yourFloat) {
...
return yourFloat;
}
Then you could call your function as follows:
float t = dbrent(ax, bx, cx, &f_, &df_, tol, &xmin);
Where &xmin hands over the address of some float variable xmin.
So, in essence, you supply the address of your functions f_ and df_ and your float variable xmin.

Answer 2

We can call dbrent in several ways, the below shown is one of the ways.
Your function prototype
float dbrent ( float ax, float bx, float cx,
float (*f)(float),
float (*df)(float),
float tol, float* xmin
)
what you need is a func_f (you can use any name for function) that takes a float value as an argument and returns a float value as shown below.
float func_f(float num)
{
/* code */
}
you need one more function with same type as mentioned above
float func_df(float num)
{
/* code */
}
Following is the setup you need to make before calling dbrent
some_funtion()
{
// setup to call dbrent
// guess you know you need some values in it.
float ax, bx, cx, tol, xmin;
float (*f)(float);
float (*df)(float);
f = func_f;
df = func_df;
//one way of calling dbrent
dbrent(ax, bx, cx, f, df,tol, xmin);
//you can even directly call using the function names instead of using pointers
dbrent(ax, bx, cx, func_f, func_df, tol, xmin);
}
NOTE: variables have not been initialized for simplicity, make sure you pass valid variables when calling the function.