i want to find row indices for each column having non zero values

i have a matrix a=[1 1 0 1;1 1 0 0;0 0 1 1] i want row indices for each column having non zero elements as [1,2] [1,2] [3] [1,3]

Answer 1

[Stephen23]

An easy way using accumarray:

>> a = [1,1,0,1;1,1,0,0;0,0,1,1]
a =
   1   1   0   1
   1   1   0   0
   0   0   1   1
>> [row,col] = find(a);
>> C = accumarray(col,row,[],@(r){r});
>> C{:}
ans =
     1
     2
ans =
     1
     2
ans =
     3
ans =
     1
     3

Answer 2

[Jos (10584)]

A = [1 1 0 1; 1 1 0 0; 0 0 1 1]
C = arrayfun(@(k) find(A(:,k)),1:size(A,2),'un',0) 
% C{x} holds the row numbers for which A(:,x) is non-zero