Why does my objective function return complex?

I have a likelihood function that I need to minimize. I use fmincon to search for the minimum. Iterations of fmincon sometimes show "Objective function returned complex; trying a new point…" But in my code I do not take square root of any variable. Why am I then getting this error? A second question is whether there is a counterpart of

dbstop if naninf

for "complex" like the

dbstop if complex

command? Or would

dbstop if error

do?

[Torsten]

log(x), x^a and sqrt(x) are the most common sources to get complex numbers.

[David Goodmanson]

also asin and acos

Answer 1

[Matt J]

There is no "dbstop if complex", but you can set a conditional breakpoints in your objective function to detect when theany particular value is complex.Or, insert a keyboard() command.

function fval=myobjective(x)

fval=....
    

    if ~isreal(fval)
        keyboard
    end

end

[Snoopy]

At https://nl.mathworks.com/help/matlab/matlab_prog/set-breakpoints.html#buvvmfu-1, Pause on Errors to pause on all errors looks like an alternative solution.

[Torsten]

Getting complex is not an error.

[Snoopy]

OK. So "Objective function returned complex; trying a new point..." is not an error and MATLAB will not pause when this warning is displayed.

[Walter Roberson]

suggest you add a conditional breakpoint to trigger if any(imag(VALUE))