CODEWITHSUNDEEP
matlabmatrix
nam
nam

Reputation: 215

Construct a 3D matrix with linear index

I want to construct a 3D matrix of size = 80 * 80 * 2 based on a set of data:

 1        4532   1257.0
 1        4556   1257.0
 1        4622   257.0
 1        4633   257.0
 2        7723   31.0
 2        8024   31.0
 2        8099   31.0
 2        9800   31.0
 2        8524   34.0
 2        8525   34.0
 2        8700   734.0
 2        8701   734.0

What I'm doing now is: I first obtain two 80 * 80 2D matrices A and B and then concatenate them using cat(3, A, B):

Denote the above data be M. 

for i = 1 : size(M,1)
    if (M(:,1)==1)
        [r c]=ind2sub(M(:,2));
        A = accumarray([r c], M(:,3));
    elseif (M(:,1)==2)
        [r c]=ind2sub(M(:,2));
        B = accumarray([r c], M(:,3));
    end
end

cat(3, A, B)

I am curious if there is any solutions that can build the 80*80*2 matrix merely by the linear index (the second column of my data) or any other simpler solution works for the purpose.

I appreciate for your help.

Upvotes: 1

Views: 302

Answers (1)

Stewie Griffin
Stewie Griffin

Reputation: 14939

So, I'm assuming your example data is incorrect, and that all values in column 2 are less than n*n, where nxn is the size of the matrix (80x80 in your case).

If that's the case, the following two lines should do the trick.

out = zeros(n,n,2);
out((M(:,1)-1).*n^2+M(:,2)) = M(:,3)

If the second column contains values up to 2*n*n, and thus are the linear indices, then:

out = zeros(n,n,2);
out(M(:,2)) = M(:,3)

Upvotes: 1

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