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

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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.

Accepted Answer

Daniele Mascali
Daniele Mascali on 19 Mar 2021
Try this:
real(A(imag(A) == 0))
Mathieu NOE
Mathieu NOE on 19 Mar 2021
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

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More Answers (2)

Mathieu NOE
Mathieu NOE on 19 Mar 2021
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);
  1 Comment
Stephen on 19 Mar 2021
+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

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Ahmadreza Torabi
Ahmadreza Torabi on 15 Dec 2021
Edited: Ahmadreza Torabi on 15 Dec 2021
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;
B = 1×6
1 2 3 4 5 6
Then B is just include real numbers of A.
  1 Comment
Rik on 15 Dec 2021
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.

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