take from matrix non-zero rows per column

Question

I have a matrix A. Suppose it is:

and I want to get the non-zero rows subscripts per column in a new matrix.
In my example that will be,

B = [ 1 3 4 6 7 0 0 0; 
      4 5 6 7 8 0 0 0; 
      1 2 3 5 6 7 8 0]

In terms of just size, if A=(m,n), B will be B=(n,m) (the transpose). In terms of content, B contains the subscripts of the non-zero rows in A as described above.

you want to transpose the matrix like this: http://en.wikipedia.org/wiki/Transpose ? if so just do B = A' — Wojciech Sobczyk, Dec 05 '13 at 18:48
is your example correct? it doesn't suite the question you wrote — Wojciech Sobczyk, Dec 05 '13 at 18:49
You may need to explain better how you get B from A. There may be some typos, but they don't seem to be related. — N8TRO, Dec 05 '13 at 18:49
the first column of A has in rows 1,3,4,6,7 non-zero element — user3071587, Dec 05 '13 at 18:56
so basically you want to apply `sort` to each column, but move zeros to the end? — A. Donda, Dec 05 '13 at 19:04
i think sort will sort the numbers in matrix, i do not care about the value but for the number of the row that has the non-zero value — user3071587, Dec 05 '13 at 19:09
@user3071587 Did you get an answer that you needed? With your test data, my answer below does what you need and is fast and concise. — chappjc, Dec 09 '13 at 19:33

chappjc · Answer 1 · 2013-12-05T19:43:56.717

2

Here is one way:

mask = ~sort(~A);     %// destination of row indexes in output
[ii,~]=find(A);       %// get the row indexes
B = zeros(size(A));
B(mask) = ii; B=B.'   %'//write indexes to output and transpose

edited Dec 05 '13 at 19:43

answered Dec 05 '13 at 19:05

chappjc

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score 1 · Answer 2 · edited May 23 '17 at 10:32

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I think this does what you want (get the non-zero row indices per column). It's very similar to this other question:

[r c] = size(A);
M = bsxfun(@times, A~=0, 1:size(A,2)).'; %'// substitute values by indices
[~, rows] = sort(M~=0,'descend'); %//'' push zeros to the end
cols = repmat(1:r,c,1);
ind = sub2ind([c r],rows(:),cols(:));
B = repmat(NaN,c,r);
B(:) = M(ind).';
B = B.';

Result:

>> B

B =

     1     3     4     6     7     0     0     0
     4     5     6     7     8     0     0     0
     1     2     3     5     6     7     8     0

edited May 23 '17 at 10:32

Community

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answered Dec 05 '13 at 19:33

Luis Mendo

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Wowzers, that was quite a showing in that question you referenced! It was before my time at SO, sadly. Anyway, I'd agree with TryHard that your answer is indeed "inspiring in its elegance". ;) +1 – chappjc Dec 05 '13 at 19:36
@chappjc Very nice words! Thanks! It was one of my first answers; it's taken me quite a while to find it now :-) – Luis Mendo Dec 05 '13 at 19:38
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Hmm, I like the `//` commenting workaround for SO's poor MATLAB formatting, although a `'` still messes with it. – chappjc Dec 05 '13 at 19:45
@chappjc So is it the `'` that spoils it? I noticed that it gets wrong sometimes, but I never knew exactly what it was. – Luis Mendo Dec 05 '13 at 19:47
Yeah, ever notice an extra `'` in my comments after a line of code (actually, see my current answer)? That's to trick the highlighting into stopping the string color. – chappjc Dec 05 '13 at 19:48
Yes, just noticed that up here! Good to know! – Luis Mendo Dec 05 '13 at 19:49
@chappjc Looks like the number of `'` has to balance that to the left of the `%` – Luis Mendo Dec 05 '13 at 19:54

take from matrix non-zero rows per column

2 Answers2