How do I find the index of the first element equal to 0 for each row of a matrix in a fast way

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Simone Fiumi 2022-6-29

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评论： Walter Roberson 2022-6-29

Hi,

I have a NxL matrix, how can I obtain a vector containing the index of the first element equal to 0 of each row of the matrix in in a fast way? Each row always has an element equal to 0 so don't worry about possible errors of that kind. I would like to use the command 'find' in such a way that with one single call it gives me back the entire vector idx which, at the moment, I obtain in the following way:

(M is the given matrix)

idx=zeros(N,1)
for k=1:N
    idx(k)=find(M(k,:)==0,1);
end

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Answer 1

Walter Roberson 2022-6-29

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You cannot do it with a single find()

A vectorized way to do it is

M = ones(5,10); M(randperm(50,10)) = 0;
M
M = 5×10
     1     1     1     1     0     1     1     1     0     0
     1     1     0     1     1     1     1     0     1     1
     1     0     1     1     0     0     1     0     1     1
     1     1     1     1     1     1     1     1     1     1
     0     1     1     1     1     1     1     1     1     1
idx = sum(cumprod(logical(M),2),2)+1;
idx
idx = 5×1
     5
     3
     2
    11
     1

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Walter Roberson 2022-6-29

Sometimes when people have these kinds of questions, their setup does not have negative values or complex values. In that case, you can get the results in a single call using min() with two outputs.

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