MATLAB: Why I get “complex number” for the common calculation

prestd;complex number

RSTR=sum(wt.*log(rt1./rft1));
'wt', 'rt1' and 'rft1' are all real numbers and why 'RSTR' is complex numbers?
Z=log( bsxfun(@rdivide,(1+rt21),(1+rft21)) );
'rt21', and 'rft21' are both real numbers and why 'Z' is complex numbers?

Best Answer

There is only one reason why you are getting a complex result there if everything is truly a real number. At least one of the elements of the set of ratios (rt21./rft21) is a negative number. It will take only one such pair that yields something negative.
log(-eps)
ans =
          -36.0436533891172 +      3.14159265358979i
As you can see, the log of a negative number, even a very tiny one will have an imaginary part that is i*pi, where i is sqrt(-1).
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