MATLAB: Dont want the number if its a part of a complex number

complex numbersimaginary numbersreal part of a complex number

So I know that doing;

Z = [0.5i 1+3i -2.2];

X = real(Z)

would give me

X = 0 1.0000 -2.2000

but what should I do if I only want pure real numbers and not even the real part of the complex number?

for example;

Z = [0.5i 1+3i -2.2];

"…"

would give me

X = 0 0 -2.2000

Thanks any help is appreciated.

Best Answer

Z = [0.5 1+3i -2.2];

Z(~imag(Z))

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MATLAB: Disable automatic conversion to reals

MATLAB's behaviour has always been to drop the imaginary part on indexing operations (where it is all zero!). gpuArray is somewhat incompatible with standard MATLAB behaviour in this regard - this was a deliberate choice to improve performance of gpuArray indexing (since the very beginning, gpuArray has used interleaved-complex format).

Anyway, probably your best bet is to preprocess your MEX arguments using a simple MATLAB wrapper, something like this:

function varargout = matchComplexity(varargin)
% Require precisely as many outputs as inputs.
nargoutchk(nargin, nargin);
varargout = varargin;
% Check which are stored as real
isReal = cellfun(@isreal, varargout);
% If not all arrays stored as real, complexify those that are real
if ~all(isReal)
    varargout(isReal) = cellfun(@complex, varargout(isReal), ...
        'UniformOutput', false);
end
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Clearly, you'll want to apply this only to your data arguments that are expected to match in complexity.

MATLAB: Matlab is giving conflicting results, please help.

"...but Matlab still says they aren't equal?"

Because they aren't equal.

This is very easy to check (you already used one method, here is another):

>> r1Xf - r2Xf
ans =
  -8.8818e-016  4.4409e-016  0
>> x - r1Xf
ans =
  0  -1.1102e-016  0
>> x - r2Xf
ans =
  -8.8818e-016  3.3307e-016  0

"... the cross products should be equal "

Nope, I don't see any reason to expect them to be equal. Operations on floating point numbers collect floating point error. You need to learn about the limits of floating point arithmetic.

"I don't understand why the arrays are all [-2.2, -0.4, 1.0]..."

Because the operations collect different floating point error for different input values. What you see printed in the command window is the closest representation to 5 or 16 significant digits, depending on your current format setting. To see the "real" value download James Tursa's FEX submission:

https://www.mathworks.com/matlabcentral/fileexchange/22239-num2strexact-exact-version-of-num2str

Use James Tursa's num2strexact and you will see that none of those values really have the exact value -2.2 or -0.4. All you are looking at is a representation of those floating point numbers displayed in the command window, to the precision defined by your format setting. Just because you see -2.2 or -0.4 displayed tells you nothing about the "real" floating point number's value.

Note that you can change how the value is displayed by changing the format.

You need to learn about the limits of floating point numbers. Start by reading these:

https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

https://www.mathworks.com/matlabcentral/answers/316889-why-two-equal-numbers-are-not-equal

http://www.mathworks.com/matlabcentral/answers/102419-how-do-i-determine-if-the-error-in-my-answer-is-the-result-of-round-off-error-or-a-bug

http://www.mathworks.com/matlabcentral/answers/140656-about-numerical-difficulty-of-equality-comparison-operation-between-two-double-real-numbers-in-matlab

This is worth reading as well:

https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

http://faculty.tarleton.edu/agapie/documents/cs_343_arch/papers/1991_Goldberg_FloatingPoint.pdf

Best Answer

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