Matrix Indices Problem

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]

Answer 1

[Sean de Wolski]

[junk, idx] = max(logical(A),[],2)

idx will be you index vector.

[Oleg Komarov]

Nice.

[Paulo Silva]

+1

[mutt]

Very neat trick. But how about the last non-zero element in each row? [3;4;5;4]

[Paulo Silva]

A=[0 1 2 0 0; 0 3 4 5 0; 6 7 8 9 10; 0 0 0 11 0]

[junk, idx] = max(logical(fliplr(A)),[],2)

idx=size(A,2)+1-idx

[Fangjun Jiang]

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]

Thanks to everyone for contributing. In practice I wouldn't use the sort approach because my real matrix is very large.

Answer 2

[Andrei Bobrov]

size(A,2)+1-sum(cumsum(A,2)~=0,2)

more

(sum(cumsum(A')==0)+1)'

[mutt]

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]

Hi Mutt! my variant:

sum(cumsum(flipud(A'))>0)'