I have an array in Matlab, let say of (256, 256). Now i need to build a new array of dimensions (3, 256*256) containing in each row the value, and the index of the value in the original array. I.e:
test = [1,2,3;4,5,6;7,8,9]
test =
1 2 3
4 5 6
7 8 9
I need as result:
[1, 1, 1; 2, 1, 2; 3, 1, 3; 4, 2, 1; 5, 2, 2; and so on]
Any ideas? Thanks in advance!
What you want is the output of meshgrid
[C,B]=meshgrid(1:size(test,1),1:size(test,2))
M=test;
M(:,:,2)=B;
M(:,:,3)=C;
here's what i came up with
test = [1,2,3;4,5,6;7,8,9]; % orig matrix
[m, n] = size(test); % example 1, breaks with value zero elems
o = find(test);
test1 = [o, reshape(test, m*n, 1), o]
Elapsed time is 0.004104 seconds.
% one liner from above
% (depending on data size might want to avoid dual find calls)
test2=[ find(test) reshape(test, size(test,1)*size(test,2), 1 ) find(test)]
Elapsed time is 0.008121 seconds.
[r, c, v] = find(test); % just another way to write above, still breaks on zeros
test3 = [r, v, c]
Elapsed time is 0.009516 seconds.
[i, j] =ind2sub([m n],[1:m*n]); % use ind2sub to build tables of indicies
% and reshape to build col vector
test4 = [i', reshape(test, m*n, 1), j']
Elapsed time is 0.011579 seconds.
test0 = [1,2,3;0,5,6;0,8,9]; % testing find with zeros.....breaks
% test5=[ find(test0) reshape(test0, size(test0,1)*size(test0,2), 1 ) find(test0)] % error in horzcat
[i, j] =ind2sub([m n],[1:m*n]); % testing ind2sub with zeros.... winner
test6 = [i', reshape(test0, m*n, 1), j']
Elapsed time is 0.014166 seconds.
Using meshgrid from above: Elapsed time is 0.048007 seconds.
tic toc to find out how it scales with your data.