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The most efficient way to locate the first 1's in each row of a matrix

Asked by Ulrik William Nash on 19 Oct 2019
Latest activity Commented on by Jos (10584)
on 21 Oct 2019
I have a potentially large r by c matrix, m, consisting of 1's and 0's. What is the most efficient way create a r by 1 vector, v, containing the index of the first 1 in each row of m.
For example,
m = [0 1 1;1 0 0; 0 0 1]
should result in
v = [2;1;3]

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2 Answers

Answer by Star Strider
on 19 Oct 2019

The max function will return the indices of the first instances of the maximum value it encounters, in the desired dimension. Since your matrix only has (0,1), it is appropriate here.
This:
m = [0 1 1;1 0 0; 0 0 1];
[~,v] = max(m,[],2)
produces:
v =
2
1
3

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nice! One can shorten it to:
[~, v] = max(m')
... but what if there is no 1 in a row

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Answer by Jos (10584)
on 19 Oct 2019

m = [0 1 1 1 ;1 0 1 0; 0 0 1 1 ; 0 0 0 1]
% a simple solution
[r, c] = find(m') ;
clear v1
v1(r,1) = c
% another solution
[v2, ~] = find(cumsum(cumsum(m,2),2)==1)

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