column dimensions of nonzero elements of each row in a matrix

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george pepper 2020-4-29

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评论： george pepper 2020-4-30

采纳的回答： Shunichi Kusano

Hello,

I have a 10000 by 6 matrix that looks like

A=[0 0 0 0 1 1; 0 1 0 0 1 0; 0 0 1 1 0 0]

I would like to get 10000 by 1 vectors called dim1 and dim2 such that dim1 (dim2) contains the column number of the first (second) nonzero element in each row. In the above example,

dim1=[5;2;3];
dim2=[6;5;4];

Is there an easy way to write this without using a loop?

Thanks in advance.

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Walter Roberson 2020-4-29

编辑：Walter Roberson 2020-4-29

dim1 = sum(cumprod(A==0,2),2)+1;

At the moment, a formula for dim2 is not coming to mind without using an assignment.

Watch out for the case where there is no non-zero: the dim1 output would be 1 more than the number of columns.

george pepper 2020-4-29

Thanks! How do you write it with an assignment?

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采纳的回答

Shunichi Kusano 2020-4-30

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编辑：Shunichi Kusano 2020-4-30

Hi george,

How about this way?

[sorted,sorti] = sort(A, 2,'descend');

sorti(:,1) will be dim1 and sorti(:,2) will be dim2. If "A" has values more than 1, it must be

[sorted,sorti] = sort(A>0, 2,'descend');

hope this helps.

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Walter Roberson 2020-4-30

[sorted,sorti] = sort(A~=0, 2,'descend');

rather than A>0, since A values might be negative

george pepper 2020-4-30

great, thanks!

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