Question

I am trying to fit collected data to a polynomial equation and I found the lfit function from Numerical Recipes. I only have access to the second edition, so am using that.

I have read about the lfit function and its parameters, one of which is a function pointer, given in the documentation as

void (*funcs)(float, float [], int))

with the help

The user supplies a routine funcs(x,afunc,ma) that returns the ma basis functions evaluated at x = x in the array afunc[1..ma].

I am struggling to understand how this lfit function works. An example function I found is given below:

void fpoly(float x, float p[], int np)
/*Fitting routine for a polynomial of degree np-1, with coefficients in the array p[1..np].*/
{
    int j;
    p[1]=1.0;
    for (j=2;j<=np;j++)
        p[j]=p[j-1]*x;
}

When I run through the source code for the lfit function in gdb I can see no reference to the funcs pointer. When I try and fit a simple data set with the function, I get the following error message.

Numerical Recipes run-time error...
gaussj: Singular Matrix
...now exiting to system...

Clearly somehow a matrix is getting defined with all zeroes.

I am going to involve this function fitting in a large loop so using another language is not really an option. Hence why I am planning on using C/C++.

For reference, the test program is given here:

int main()
{

    float x[5] = {0., 0., 1., 2., 3.};
    float y[5] = {0., 0., 1.2, 3.9, 7.5};
    float sig[5] = {1., 1., 1., 1., 1.};

    int ndat = 4;
    int ma = 4; /* parameters in equation */
    float a[5] = {1, 1, 1, 0.1, 1.5};
    int ia[5] = {1, 1, 1, 1, 1};
    float **covar = matrix(1, ma, 1, ma);

    float chisq = 0;

    lfit(x,y,sig,ndat,a,ia,ma,covar,&chisq,fpoly);

    printf("%f\n", chisq);  
    free_matrix(covar, 1, ma, 1, ma);


    return 0;
}

Also confusing the issue, all the Numerical Recipes functions are 1 array-indexed so if anyone has corrections to my array declarations let me know also!

Cheers

edit: Ok just discovered the problem to this.

When copying the numerical recipes code, I accidentally commented out some real code thinking it was just a comment. I no longer get the singular value error

Answer 1

I have since switched to the 3rd edition of NR which has a more understandable routine.