Retrieve all non-zeros indices except the first one for each row
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Romeo Tahal
il 27 Feb 2020
Commentato: Romeo Tahal
il 8 Mar 2020
Hi, I have the following question. How can I retrieve all nonzero indices of each row except the first nonzero index in a matrix? Suppose I have the following matrix:
A = [2 0 1 0 0; 4 -9 0 5 0;0 0 2 0 -3;0 -3 2 0 0;1 2 0 0 6];
I would like the output to be: 11, 0, 7, 17, 0, 23, 0, 14, 0, 10, 25, 0 as the linear indices.
If there are no more non-zeros in a row after the last one, then the index is zero.
-Thanks in advance-
Romeo
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Image Analyst
il 27 Feb 2020
Zero cannot be a linear index. This code will work:
A = [2 0 1 0 0; 4 -9 0 5 0;0 0 2 0 -3;0 -3 2 0 0;1 2 0 0 6]
linearIndexes = [];
for row = 1 : size(A, 1)
% Find columns where A is nonzero
indexes = find(A(row,:) ~= 0);
% Tack on the second and other elements more to the right in this row.
for k = 2 : length(indexes)
% Get the linear index for this (row, column) location.
linearIndex = sub2ind(size(A), row, indexes(k));
% Tack it on to our accumulating/growing output variable.
linearIndexes(end+1) = linearIndex;
end
end
linearIndexes % Echo to command window.
It shows
A =
2 0 1 0 0
4 -9 0 5 0
0 0 2 0 -3
0 -3 2 0 0
1 2 0 0 6
linearIndexes =
11 7 17 23 14 10 25
Same as you except for no zeros.
7 Commenti
Image Analyst
il 28 Feb 2020
Seems complicated. MATLAB has capability to deal with sparse matrices. Why not just use it and not have this complicated scheme? Are the matrices so gigantic, even when sparse, that you're running out of memory? Well anyway, good luck.
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