why I get "complex number" for the common calculation?

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?

Answer 1

[John D'Errico]

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).

Answer 2

[Naty Shemer]

Most probably (rt1./rft1) is negative and the log output of a negative number is imaginary...