Multiplying complex vector with its complex conjugate results in complex vector

Hi,

Within a scrip I am multiplying a complex vector (added in dataset.mat) with its complex conjugate.

I.idmPSD_LT=idmLSD_LT.*conj(idmLSD_LT);

Now I expect my result to be real, as it is in for example

A=(1+2i)*(1-2i);

gives real 5. However the resulting vector is partly purely real(idx65:299968) and partly real and complex (and partly stuffed with NaN). For the complex part, Matlab displays x+0i. I can offcourse solve this by taking the real part of the vector, but I am wondering if anyone knows what causes this resulting vector to be complex. Does someone know this?

[Mathieu NOE]

hello

this is simply because your data vector contains NaN complex values - that will not give a "real" NaN output

remove first the NaN, do the product and then you get a real output

all the best

Answer 1

[Abhishek]

Hello @DdeR,

The output is complex because the input vector “idmLSD_LT” contains NaN values. Any calculation involving a complex NaN (of the form “NaN + NaNi”) will produce a complex NaN as a result. Such entries should be removed before performing the calculation.

In MATLAB, you can remove the NaNs with the help of “isnan()” function and “~” operator. Create a new vector that would only contain non-NaN elements.

idmLSD_LT_clean = idmLSD_LT(~isnan(idmLSD_LT)); 

Then perform the operation on the new vector:

I.idmPSD_LT=idmLSD_LT_clean.*conj(idmLSD_LT_clean); 

I tried this approach on the data posted in the question on MATLAB R2025a and got the expected results.

You can refer to the official documentation by MATLAB on “isnan()” function: https://mathworks.com/help/releases/R2025a/matlab/ref/double.isnan.html

I hope this helps.