# 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 on 29 Jun 2022
Commented: Walter Roberson on 29 Jun 2022
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

Walter Roberson on 29 Jun 2022
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
Walter Roberson on 29 Jun 2022
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.

Torsten on 29 Jun 2022
[~,idx] = max(M==0,[],2)
or
idx = arrayfun(@(k)find(M(k,:)==0,1),1:size(M,1))
Walter Roberson on 29 Jun 2022
Interesting use of max() !