CODEWITHSUNDEEP
arraysmatlabmatrixmultidimensional-array
Tidbits
Tidbits

Reputation: 43

sortrows in the 2nd dimension of 3d matrix

I am trying to sort first row in the 2nd dimension of a 3d matrix in Matlab, but sortrows() doesn't work in this case.

For example, I have a matrix like

a(:,:,1)
1 2 4
3 1 6
2 0 5

a(:,:,2)
3 5 6
1 2 4
0 2 2

and I need to get the result

a(:,:,1)
1 2 4
2 0 5
3 1 6

a(:,:,2)
0 2 2
1 2 4
3 5 6

Is there an efficient way to achieve that? Thanks a lot!

Upvotes: 4

Views: 537

Answers (2)

Luis Mendo
Luis Mendo

Reputation: 112659

If I understand correctly, you want to sort rows within each third-dim slice according to the value of the first column.

[m,n,p] = size(a);
[~, row_ind] = sort(a(:,1,:), 1);
lin_ind = bsxfun(@plus, bsxfun(@plus, row_ind, (0:n-1)*m), reshape((0:p-1)*m*n, 1, 1, p));
result = a(lin_ind);

How this works:

Sort a(:,1:,) along the first dimension (rows), and get the indices of the sorting (using the second output of sort; line 2). From those row indices, generate the linear indices (using bsxfun; line 3) that will give the desired result (line 4).

Example:

For input

a(:,:,1) = [1 2 4
            3 1 6
            2 0 5];
a(:,:,2) = [3 5 6
            1 2 4
            0 2 2];

this produces

result(:,:,1) =
     1     2     4
     2     0     5
     3     1     6
result(:,:,2) =
     0     2     2
     1     2     4
     3     5     6

Upvotes: 4

Jerry
Jerry

Reputation: 4408

Just use sortrows with each 2d matrix:

a(:,:,1)=sortrows(a(:,:,1));
a(:,:,2)=sortrows(a(:,:,2));

Of course you can do it in a loop

Upvotes: 2

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