Writing fitting functions for lmfit
lmfit expects
fitting functions with prototypes in the form:
double function_name(double, double *);
For example:
double sin_fit(double t, double * p)
{
return p[0] + p[1] * sin ( 2 * M_PI * ( p[2] * t + p[3]));
}
Note that while lm_minimize takes a pointer to an array of parameters, it may not always pass that pointer to the fitting function. Because of this passing fixed parameters to the fitting function using the 3rd argument will yield incorrect results. For example, the following code is wrong:
double wrong_fit(double t, double *p)
{
return p[0]*p[0] + p[1];
}
....
double p[2];
p[0] = 1;
p[1] = magic_number;
...
lm_minimize(..., 1, p, ..., ..., ...., ...);
Here the fitting function wrong_fit relies on a fixed parameter p[1] which it expects to contain the value magic_number. However it will be passed such a p that only p[0] has a valid value and p[1] is undefined. This will lead to incorrect operation.
To work around this, you can use global variables, as follows:
double magic = magic_number;
...
double right_fit(double t, double * p)
{
return p[0]*p[0] + magic;
}
....
double p = 1;
...
lm_minimize(..., 1, &p, ..., ..., ..., ...);
Cheers,
Steve
double function_name(double, double *);
For example:
double sin_fit(double t, double * p)
{
return p[0] + p[1] * sin ( 2 * M_PI * ( p[2] * t + p[3]));
}
Note that while lm_minimize takes a pointer to an array of parameters, it may not always pass that pointer to the fitting function. Because of this passing fixed parameters to the fitting function using the 3rd argument will yield incorrect results. For example, the following code is wrong:
double wrong_fit(double t, double *p)
{
return p[0]*p[0] + p[1];
}
....
double p[2];
p[0] = 1;
p[1] = magic_number;
...
lm_minimize(..., 1, p, ..., ..., ...., ...);
Here the fitting function wrong_fit relies on a fixed parameter p[1] which it expects to contain the value magic_number. However it will be passed such a p that only p[0] has a valid value and p[1] is undefined. This will lead to incorrect operation.
To work around this, you can use global variables, as follows:
double magic = magic_number;
...
double right_fit(double t, double * p)
{
return p[0]*p[0] + magic;
}
....
double p = 1;
...
lm_minimize(..., 1, &p, ..., ..., ..., ...);
Cheers,
Steve
posted by steve @ 11:27 PM
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