CODEWITHSUNDEEP
arraysmatlab
Ziggurat
Ziggurat

Reputation: 257

Getting value and index of array matlab

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!

Upvotes: 0

Views: 179

Answers (2)

athypes
athypes

Reputation: 185

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.

Upvotes: 1

Daniel
Daniel

Reputation: 36710

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;

Upvotes: 3

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