non-linear fitting with conditions

[Carnë Draug]

Hi

the function leasqr (part of the optim package) is used to fit non-linear data. In my case, some values for the parameters (there's 3 of them) make no physical sense so I made a condition inside the function so it returns before evaluating such values.

if (rCon < 0 || sigma < 0 || theta < 0 || theta  > 1)
    return
endif

However, when leasqr attempts to fit such values, it returns an error (at least that's my understanding of the origin of the error, when I looked into the source) which makes sense because he tries to subtract two values, one with zero size. Here's the error

error: operator -: nonconformant arguments (op1 is 1x26, op2 is 0x0)
error: called from:
error:   /usr/share/octave/packages/3.2/optim-1.0.10/leasqr.m at line 320, column 3

If I got it right, op1 = y (argument of leasqrand) and op2 = f (return of feval with function F).

I guess it makes sense for the code to check this but in that case how can I check for this and make sure that leasqr doesn't give me certain values? Or should this be considered a bug?
I though about making a while loop with a try statement inside to catch any errors and change the initial guess of the parameters values until leasqr doesn't try one that makes no sense on its iterations. But I fear that sometimes this will create an infinite loop.

By the way, here's the function that I am using

function y = func_radial_profile(pf_distances, parameters)

rCon 	= parameters(1);
sigma	= parameters(2);

theta	= parameters(3);

if (rCon < 0 || sigma < 0 || theta < 0 || theta  > 1)

    return
endif


for i=rows(pf_distance)
    y(i) = 1-(1-theta)*exp(-(pf_distances(i)-rCon)^2/(2*sigma^2))

endfor

Thanks in advance,
Carnë Draug