column dimensions of nonzero elements of each row in a matrix

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george pepper
george pepper 2020-4-29
评论： george pepper 2020-4-30
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.
  2 个评论
Walter Roberson
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.

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Shunichi Kusano
Shunichi Kusano 2020-4-30
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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