Find the indices of the imaginary element of the matrix

Question

Hassan Alkomy 2021-6-8

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https://ww2.mathworks.cn/matlabcentral/answers/851005-find-the-indices-of-the-imaginary-element-of-the-matrix

回答： Hassan Alkomy 2021-6-8

Suppose I have a Matrix like this:

A=[1    2    3;
   4+i  5    6-i;
   7    8+i  9]

Now, I want to know if there is a way to find the indecies of the imaginary elements (by this I mean the complex numbers which have imaginary part). I am looking for an answer like that:

A(2,1)

A(2,3)

A(3,2)

or any other form the conveys the indecies information of the imaginary elements.

Thank you so much.

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Answer 1

David Hill 2021-6-8

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https://ww2.mathworks.cn/matlabcentral/answers/851005-find-the-indices-of-the-imaginary-element-of-the-matrix#answer_719870

在 MATLAB Online 中打开

Lidx=find(imag(A)~=0);

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David Hill 2021-6-8

If your application will allow, I always perfer linear indexing.

Hassan Alkomy 2021-6-8

Thank you so much.

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Answer 2

Joseph Cheng 2021-6-8

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you can do a comparison to the real(A) like

A=[1    2    3;
   4+i  5    6-i;
   7    8+i  9]
A = 
   1.0000 + 0.0000i   2.0000 + 0.0000i   3.0000 + 0.0000i
   4.0000 + 1.0000i   5.0000 + 0.0000i   6.0000 - 1.0000i
   7.0000 + 0.0000i   8.0000 + 1.0000i   9.0000 + 0.0000i
[row col]=find(A~=real(A))
row = 3×1
     2
     3
     2
col = 3×1
     1
     2
     3

then with find you can get the row and column index values of the as real(A)~=A

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Hassan Alkomy 2021-6-8

Thank you so much.

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Answer 3

Scott MacKenzie 2021-6-8

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% test matrix
A=[1    2    3;
   4+i  5    6-i;
   7    8+i  9]
% identify imaginary elements in A (+1 or -1)
B = imag(A)
% generate column vector of indices of imaginary elements in A
find(B~=0)

Output:

A =

1 + 0i 2 + 0i 3 + 0i

4 + 1i 5 + 0i 6 - 1i

7 + 0i 8 + 1i 9 + 0i

B =

0 0 0

1 0 -1

0 1 0

ans =

2

6

8

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Hassan Alkomy 2021-6-8

Thank you so much.

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Answer 4

Hassan Alkomy 2021-6-8

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https://ww2.mathworks.cn/matlabcentral/answers/851005-find-the-indices-of-the-imaginary-element-of-the-matrix#answer_719970

Actually, the three solutions by David, Joseph and Scott give what i want. I chose David's answer because it is a one-line answer, but the rest of the solutions gives the same result.

Thank you all.