find in multidimensional array - MATLAB

Question

I have a 3 by 3 by 2 by 3 array X containing 1s or 0s. Picture this as a row of three 3 by 3 matrices along a row and another such 'layer' behind them (from the '2').

I want to find the positions in X of the 0s in the first layer and second layer separately. I'm not really sure how to do this with find, but heuristically something like:

A = find(X == 0 & 3rd index of X is 1) 

B = find(X == 0 & 3rd index of X is 2)

EDIT

I just realised my attempt to simplify my actual question made it misleading. The array X actually has -1's, 1's and -2's and I want to find the -2's. They're not meant to be logical operators. Also I would prefer any operation proposed to be as fast as possible as this will be part of a recursive backtracking algorithm.

Do you want the returned indices to correspond to your 3x3x2x3 array, or just to the 3x3x3 subarray independently? — Andras Deak -- Слава Україні
– Andras Deak -- Слава Україні, Commented Sep 20, 2015 at 17:48
I want the indices to correspond to the original array please — dcwang
– dcwang, Commented Sep 20, 2015 at 17:59

Daniel · Accepted Answer · 2015-09-20 18:03:57Z

2

solution using logical indexing

I recommend to use logical indexing instead of find. This gives you all indices where X is 1

value_you_want=-2
C=X==value_you_want;

Now you want only parts of these indices in A and B, first initialize A and B with false of the same size as C:

A=false(size(C));
B=A;

And finally copy the slice you want to each of these matrices:

A(:,:,1,:)=C(:,:,1,:);
B(:,:,2,:)=C(:,:,2,:);

If you really want your numeric indices, use find(A) and find(B)

Alternative solution using linear indices and find

%get all indices
C=find(X==value_you_want)
%convert linear indices to subscript indices, only use third dimension
[~,~,S,~]=ind2sub(size(X),find(X==0));
%Use S to split C
A=C(S==1);
B=C(S==2);

edited Sep 20, 2015 at 18:03

answered Sep 20, 2015 at 17:52

Daniel

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4 Comments

Andras Deak -- Слава Україні Over a year ago

Yes, just what I was looking for:)

Andras Deak -- Слава Україні Over a year ago

And of course if you don't need the actual indices, this is much faster than find.

dcwang Over a year ago

sorry again, I realised my question is misleading, actually X contains -1, 1 and -2's. I want to find the -2's. (I changed the question to simplify things but they're not meant to be logical operators).

Daniel Over a year ago

@dcwang: Using logical indexing is not limited to logical matrices. It is a indexing techique which applies to all matrices. I updated my answer to be more clear, insert whatever number you want for value_you_want

aybassiouny · Accepted Answer · 2015-09-20 17:51:25Z

0

Generally use find(condition) to return linear indices in the array satisfying condition.

A = find(A(:,:,1,:)<1)

B = find(A(:,:,2,:)<1)

edited Sep 20, 2015 at 17:51

answered Sep 20, 2015 at 17:50

aybassiouny

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3 Comments

Andras Deak -- Слава Україні Over a year ago

Note that OP is looking for the zeros. And that find automatically looks for nonzeros:)

Daniel Over a year ago

Note that X(B) will not return the zeros. h=X(:,:,2,:);h(B) will return the zeros

Andras Deak -- Слава Україні Over a year ago

@Daniel and the same goes for Adriaan's answer. I'm not sure, however, which one is needed by OP.

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