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How do I combine two index vectors to form a 2D index matrix?

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Dean Ranmar
Dean Ranmar on 20 Dec 2016
Edited: Jan on 26 Dec 2016
I have two methods for identifying which elements in a matrix have certain properties (e.g.; exceed a threshold).
ndx = Amat > THR; % matrix of threshold crossing locations
and:
[rdx, cdx] = find(Amat>THR); % vectors of threshold crossing locations
that I use for two different purposes [and actually apply to different matrices.] Results from the two methods are compared eventually. I'd like to take the two vectors (rdx, cdx) and convert them to a matrix so I can do an operation such as:
Adx = Amat(ndx); % matrix of threshold crossing amplitudes
which is done for the former case. I'm sure there's a simple way to take the two vectors and combine them to produce "pointer" matrix "ndx," but I haven't found it.
Help?

  1 Comment

Dean Ranmar
Dean Ranmar on 20 Dec 2016
Of course "ndx" is a true/false matrix of same dimension as matrix Amat.

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Accepted Answer

Jan
Jan on 20 Dec 2016
Edited: Jan on 26 Dec 2016
Does not look elegant:
[rdx, cdx] = find(Amat > THR);
Index = sub2ind(size(Amat), rdx, cdx); % [EDITED, was ind2sub...]
ndx = false(size(Amat));
ndx(Index) = true;

  2 Comments

Dean Ranmar
Dean Ranmar on 20 Dec 2016
Actually, it should be sub2ind, not ind2sub. Thanks again. BTW, I should have neat rather than elegant and your solution is neat.

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More Answers (1)

Mischa Kim
Mischa Kim on 20 Dec 2016
Would this do the trick?
Adx = Amat(Amat(:)>THR(:))

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Dean Ranmar
Dean Ranmar on 20 Dec 2016
sorry! I switched the index names on you: ndr == rdx and ndc == rdc.

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