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Why does my objective function return complex?

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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
Torsten on 28 Feb 2022
log(x), x^a and sqrt(x) are the most common sources to get complex numbers.

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Accepted Answer

Matt J
Matt J on 28 Feb 2022
Edited: Matt J on 28 Feb 2022
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)
if ~isreal(fval)
Snoopy on 2 Mar 2022
Edited: Snoopy on 2 Mar 2022
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
Walter Roberson on 2 Mar 2022
suggest you add a conditional breakpoint to trigger if any(imag(VALUE))

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