CODEWITHSUNDEEP
matlab
samirbajaj
samirbajaj

Reputation: 159

Matlab - Flip 1's to 0's in a matrix

I have a very large sparse matrix (each row is several thousand elements--most of the elements are 0's). In addition, I have a vector of row indexes, and I need to perform the following operation on each row:

Flip half the non-zero elements (randomly chosen from all non-zero elements in the row) to zero, and save the column indexes of the flipped elements.

Thanks for any pointers.

Upvotes: 3

Views: 1223

Answers (1)

mythealias
mythealias

Reputation: 350

You can use randperm() to generate a random order of columns that you want to zero out in a row.

% A: sparse matrix (assume 2d)
% ri = vector or row indices

for i = 1:numel(ri)               % Edit one row of A at a time
    row = A( ri(i), : );
    c = find( row );              % Find column index of all non-zero elements a row
    cdel = randperm(length(c));   % Random rearrangement of column index
    cdel(1:end/2) = [];           % Only want to zero out half the columns, so ignore the other half
    % c(cdel) will give the column index of elements to be zeroed.
    row( 1, c(cdel) ) = 0;        % Zero out selected columns
    A( ri(i), : ) = row;          % Update A
end

There maybe some bug in the code as I haven't tested it out. Also some steps are redundant and can be combined.

c(cdel) will give you the required index of the columns that were flipped. You can save it in a cell vector as it size may change for each row. You can do this by,

fcol{i} = c(cdel);

Upvotes: 2

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