Logical indexing a 2D array into a 2D array

I am having trouble finding a way to remove all rows and columns that are all zeros from a 2D matrix to get a resulting 2D matrix using logical indexing.

Z = [56,0,0,0,0,55;
      0,0,0,0,0,0;
     27,0,0,0,0,0;
      0,0,0,0,0,0;
      0,0,0,0,0,0;
     100,0,0,0,0,25];

zidx = Z ~= 0;

Z2 = Z(zidx);

results in column vector of:

Z2 = [56;27;100;55;25]

However I really need it to be:

Z2 = [56,55; 27,0; 100,25];

Any help is greatly appreciated.

Answer 1

[Jan]

Easier and faster without a loop:

Z = [56,0,0,0,0,55;
      0,0,0,0,0,0;
     27,0,0,0,0,0;
      0,0,0,0,0,0;
      0,0,0,0,0,0;
    100,0,0,0,0,25];

Z = Z(:, any(Z, 1));  % Keep only columns with any non-zero value
Z = Z(any(Z, 2), :);  % Keep only rows with any non-zero value

If you really want Z = [56,27,100; 55,0,25] as output, append:

Z = Z.'

[Eric Smith]

Thanks. This works and is closer to what I was thinking as a solution.

[Guillaume]

is closer to what I was thinking as a solution

This is the best solution!

Answer 2

[Wouter Verstraelen]

there's actually a very short to do this that hasn't been mentioned so far

Z =Z.*(Z~= 0)

Answer 3

[Bob Thompson]

So, to clarify, you want to have your columns turned into rows?

Because you are looking to roughly keep the row and column locations I would suggest using a for loop to keep the non-zero values from each row. Before that though, because you have different numbers of values you're going to need to either initialize the output array as the max size in zeroes, or to pad as you add new rows.

Z2 = [];
for k = 1:size(Z,1); % Easier to do row wise because of needing to pad rows, rather than columns.
 row = Z(k,Z(k,:)~=0);
 if isempty(row);  
 elseif isempty(Z2);
  Z2 = row;
 elseif size(Z2,2)<length(row); % Check if previous row was smaller than new row
  Z2(:,size(Z2,2)+1:length(row)) = 0;
  Z2 = vertcat(Z2,row);
 elseif size(Z2,2)>length(row); % Check if previous row was bigger than new row
  row(length(row)+1:size(Z2,2)) = 0;
  Z2 = vertcat(Z2,row);
 else
  Z2 = vertcat(Z2,row);
 end
end

Z2 = Z2';

[Eric Smith]

This works, and sorry that I mistyped column major for output matrix... output should be: Z = [56,27,100; 55,0,25];