Extracting only real numbers from a vector containing both real and complex values

Hi, I have a matrix that contains both real and complex elements in it. How can I transfer all the real elements (NOT the real parts of all elements) to another matrix?

For example:

My orginal matrix is A:

A = [  -0.4406 - 1.5696i , -0.4406 + 1.5696i, 1.8812 + 0.0000i]

Here we see that A has 3 elements, the first two of which are complex while the third element is real. How do I write code that extracts only the real element ie, the third element (1.8812)?

I do NOT want to extract the real parts of all three variables.

Answer 1

[Daniele Mascali]

Try this:

real(A(imag(A) == 0))

[Rik]

If you run into any issues with float rounding errors:

A = [  -0.4406 - 1.5696i , -0.4406 + 1.5696i, 1.8812 + 0.0000i];
A(4)= 1 + 1i*(sqrt(2)^2-2); % imaginary part should be 0 but isn't quite
B1=real(A(imag(A) == 0));
B2=real(A(abs(imag(A))<(10*eps)));
disp(A),disp(B1),disp(B2)

  -0.4406 - 1.5696i  -0.4406 + 1.5696i   1.8812 + 0.0000i   1.0000 + 0.0000i

    1.8812

    1.8812    1.0000

[Mathieu NOE]

that's why I suggested to make a test with a given tolerance and not exact zero match with risks of wrong answer due to rounding errors

Answer 2

[Mathieu NOE]

hello

see code below :

A = [  -0.4406 - 1.5696i , -0.4406 + 1.5696i, 1.8812 + 0.0000i];
tol = eps;
ind = find(abs(imag(A))<tol);
B = A(ind);

[Stephen23]

+1 A slightly larger tolerance might also be more robust, e.g. 1e-10

find is not required, logical indexing is simpler and more efficient:

tol = 1e-10;
idx = abs(imag(A))<tol;
B = A(idx)

B = 1.8812

Answer 3

[Ahmadreza Torabi]

Hello

You can use something like this:

A = [1 2 7+i 3 8+i 4 9+i 5 6];
m = 1;
n = length(A);
for k = 1:n
    if isreal(A(k))
        B(m) = A(k);
        m = m+1;
    end
end
B

B = 1×6

1 2 3 4 5 6

Then B is just include real numbers of A.

[Rik]

This will dynamically grow the array. It is better to create a logical vector of the same size as A where you mark wether it should be removed. Then you only need to modify the B once: when you create it.