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Indexing every value from matrix not only unique values

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Bianca Brisc
Bianca Brisc on 20 Jul 2021
Commented: Jan on 21 Jul 2021
Hello
For example if I have a matrix like:
A =[ 2 , 2 , 4 , 1 ,3 ;
3 , 1 , 10 , 100]
Is there a posibility to find the index of each value? I tried with find function but it is not working for same values and based on my research I found only solutions either for the same values from the matrix or unique. Not a combination.
Thank you!

Accepted Answer

Jan
Jan on 20 Jul 2021
Of course you can use find():
A = [2, 2, 4, 1, 3; ...
3, 1, 10, 100, 200]; % 200 added to match number of elements per row...
[i1, i2] = find(A == 2)
i1 = 2×1
1 1
i2 = 2×1
1 2
Now you get the row and column indices of the elements, which are 2.
Do you want to distinguish all elements even if they have the same number? Then this is trivial: Just use the indices.
  8 Comments
Jan
Jan on 21 Jul 2021
Faster alternatives:
A = [2, 2, 4, 1, 3; ...
3, 1, 10, 100, 200];
S = size(A);
[i1, i2] = ind2sub(S, 1:numel(A))
i1 = 1×10
1 2 1 2 1 2 1 2 1 2
i2 = 1×10
1 1 2 2 3 3 4 4 5 5
% Or:
[S1, S2] = size(A);
Index = [repelem(1:S1, S2); repmat(1:S2, 1, S1)]
Index = 2×10
1 1 1 1 1 2 2 2 2 2 1 2 3 4 5 1 2 3 4 5

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