Extract non zero elements from 2D array

Hi I have an array like this:

I want to extract non-zero elements for each row, e.g for second row, it should be 3,4. (and other array elements are zero) for third row 1,3,4 I think 3d array will be used and i tried this code, but incorrect results.

for m=1:r
  for k=1:row
    for l=1:col
      if(a(k,l)~=0)
          b(k,l,m)=a(k,l);
      end
    end
  end
end

(a for array given above and b is new array with results) Any help will be highly appreciated.

Answer 1

[Stephen23]

No ugly loops are required:

 >> [R,C] = find(X)
 >> out = accumarray(R,X(X~=0),[],@(n){n});
 >> out{2} % second row
 ans =
     4
     3
 >> out{3} % third row
 ans =
     3
     1
     4

[Tha saliem]

Thanks alot. Can I do this without using cell array?

[Stephen23]

@Tha saliem: how do you want unequally sized vectors joined together? In one long column is easy:

>> Y = X.';
>> Y(Y~=0)
ans =
   3
   4
   1
   3
   4
   2
   1
   2
   3
   4
   5

However if you want them arranged in some other way then you have to describe exactly how, because there are a million ways that fit your requirement of "without using cell array", but you did not give any description of what you actually want. It is hard to develop code when you do not give a specification of what you actually want.

[Tha saliem]

Can I do it using 3D array instead of cell array?

[Stephen23]

@Tha saliem: sure, anything is possible, but you need to tell us what you want to do with the unequally-sized vectors. Do you want to pad them to the same size, or what?

For example, exactly how do you want [3;4] and [1;3;4] arranged in this 3D array, considering that they have different lengths?

Answer 2

[Nicolaie Popescu-Bodorin]

% Let A be a 2D Array and NZI the cell containing indices of non-zero elements on each column.

% preallocation:
sa = size(A,2); NZI{sa}=[];
for k=1:sa,
    NZI{k} = find(A(:,k)~=0);
end

Kind regards, Nicolaie Popescu-Bodorin, www.lmrec.org/bodorin/

[Tha saliem]

Thanks alot.