Double precision changes to complex double after calculation
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Commented: Guillaume on 30 Jan 2015
I have a matrix with very simple data in double precision but after doing a calculation (listed below) some of the numbers, not all, change to complex numbers. Even the timestamp which is not used in the calculation changes to complex. The data that changes appears to be the same as the data that doesn't change.
for i = 1:length(ws);
if L(i,1) >= -500 && L(i,1) <= -12
wsstd_uns(i,1) = timestamp(i,1);
wsstd_uns(i,2) = fric(i,1).*(0.35*((-(BLH(i,1)./(vK.*L(i,1)))).^(2/3)) + (2 -(10./BLH(i,1)))).^(1/2);
wsstd_uns(i,3:5) = [ws(i,1),wgst(i,1),(wgst(i,1)-ws(i,1))];
elseif L(i,1) >= 0 && L(i,1) <= 500
wsstd_s(i,1) = timestamp(i,1);
wsstd_s(i,2) = 2.*fric(i,1).*((1 -(10./BLH(i,1))).^(1/2));
wsstd_s(i,3:5) = [ws(i,1),wgst(i,1),(wgst(i,1)-ws(i,1))];
Actually, as per the tip section of the doc you've linked:
- Since i is a function, it can be overridden and used as a variable. However, it is best to avoid using i and j for variable names if you intend to use them in complex arithmetic.
- For speed and improved robustness in complex arithmetic, use 1i and 1j instead of i and j.
In other words, unless you use i or j as the imaginary unit, it doesn't matter. And if you do use i or j as the imaginary unit, you shouldn't and should use 1i or 1j instead (which can't be used as a variable).
Theory one: You use i as a variable. As this is a "reserved word" for complex calculations, there may be an unexpected effect.
Theory two: One of your functions or variables shadows the real MATLAB command and does something unexpected. Like you would assign plot=1 the then try to use the plot command.
More Answers (1)
Well, you take the square root and cubic roots of some numbers so, if these numbers are negative, you'll get some complex numbers.
As for your timestamp, are you sure it's complex, that is imag(x) ~= 0. The real numbers in a matrix containing complex numbers are displayed as complex but with an imaginary part equal to 0.
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