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

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Ulrik William Nash
Ulrik William Nash 2019년 10월 19일
댓글: Jos (10584) 2019년 10월 21일
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]

답변 (2개)

Star Strider
Star Strider 2019년 10월 19일
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
  댓글 수: 1
Jos (10584)
Jos (10584) 2019년 10월 21일
nice! One can shorten it to:
[~, v] = max(m')
... but what if there is no 1 in a row

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Jos (10584)
Jos (10584) 2019년 10월 19일
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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