Effacer les filtres
Effacer les filtres

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

18 vues (au cours des 30 derniers jours)
Ulrik William Nash
Ulrik William Nash le 19 Oct 2019
Commenté : Jos (10584) le 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]

Réponses (2)

Star Strider
Star Strider le 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
  1 commentaire
Jos (10584)
Jos (10584) le 21 Oct 2019
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) le 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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