Matrix Indices Problem

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mutt
mutt 2011-8-15
Given a matrix e.g. A = [0 1 2 0 0; 0 3 4 5 0; 6 7 8 9 10; 0 0 0 11 0]
what MATLAB code will generate a vector of the column numbers of the first non-zero element in each row?
For this example the vector returned should be: [2;2;1;4]

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采纳的回答

Sean de Wolski
Sean de Wolski 2011-8-15
[junk, idx] = max(logical(A),[],2)
idx will be you index vector.
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Fangjun Jiang
Fangjun Jiang 2011-8-15
Use sort().
A = [0 1 2 0 0; 0 3 4 5 0; 6 7 8 9 10; 0 0 0 11 0];
[dummy,Ind]=sort(A~=0,2)
Ind=Ind(:,end)
To find first non-zero,
A = [0 1 2 0 0; 0 3 4 5 0; 6 7 8 9 10; 0 0 0 11 0];
[dummy,Ind]=sort(A~=0,2,'descend')
Ind=Ind(:,1)
mutt
mutt 2011-8-16
Thanks to everyone for contributing. In practice I wouldn't use the sort approach because my real matrix is very large.

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更多回答（1 个）

Andrei Bobrov
Andrei Bobrov 2011-8-15
size(A,2)+1-sum(cumsum(A,2)~=0,2)
more
(sum(cumsum(A')==0)+1)'
  2 个评论
mutt
mutt 2011-8-15
Very good. And re-using that logic I have for the last non-zero elements:
size(A,2)+1-sum(not(logical(cumsum(A,2)-repmat(max(cumsum(A,2),[],2),1,size(A,2))))~=0,2)
Could this be simplified?
Andrei Bobrov
Andrei Bobrov 2011-8-16
Hi Mutt! my variant:
sum(cumsum(flipud(A'))>0)'

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