# how to find index from matrix in another matrix?

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Lilya on 26 Jul 2022
Commented: Fangjun Jiang on 27 Jul 2022
Hi all,
I have two different 3d matrices (A=72*46*2192) and (B=72*46*2192), in which I want to find the indices equal to 4 and 5 from A in B.
The result should be a 3d matrix as well, not linear indices.
Any help would be appreciated.
##### 2 CommentsShowHide 1 older comment
Lilya on 27 Jul 2022
A = rand(2,3,4);
B=rand(2,3,4);
Matrix A should have elements = 4 and 5.
I want to use the index of those 4 and 5 in A to find the corresponding elements in B.

Fangjun Jiang on 26 Jul 2022
Edited: Fangjun Jiang on 27 Jul 2022
%C is the logical index matrix.
A=zeros(2,3,4);
A(:,:,4)=4;
A(:,3,4)=5;
C=or(A==4,A==5)
C = 2×3×4 logical array
C(:,:,1) = 0 0 0 0 0 0 C(:,:,2) = 0 0 0 0 0 0 C(:,:,3) = 0 0 0 0 0 0 C(:,:,4) = 1 1 1 1 1 1
%To use it to select corresponding elements in B
B=rand(2,3,4);
B_select=B(C)
B_select = 6×1
0.2953 0.0888 0.5716 0.4577 0.5288 0.3675
LinearIndex=find(C);
[SubX,SubY,SubZ]=ind2sub(size(C),LinearIndex)
SubX = 6×1
1 2 1 2 1 2
SubY = 6×1
1 1 2 2 3 3
SubZ = 6×1
4 4 4 4 4 4
##### 2 CommentsShowHide 1 older comment
Fangjun Jiang on 27 Jul 2022
See the updated answer to understand
1. Logical index
2. Linear index
3. Subscript index