Doug's MATLAB Video Tutorials

December 9th, 2008

clear
clc
M(:,:,1) = [1,2;3,4];
M(:,:,2) = [21,22;23,24];
J = [1,2; 2,1];

ans =
[1 22
23 4]

D = M(:,:,J) % This doesn't work,

[nR, nC] = size(J);
D = zeros(nR,nC);
for r = 1:nR
     for c = 1:nC
          D(r,c) = M(r,c,J(r,c));
    end
end     

19:07 UTC | Posted in Format: Video, Level: Basic | Permalink | 4 Comments »

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4 Responses to “Video tutorial: 3d indexing”

1. Martin le Roux replied on December 10th, 2008 at 18:56 UTC :
   

   What about the following:

   [nR,nC] = size(J);
   [r,c] = ndgrid(1:nR,1:nC);
   M(sub2ind(size(M),r,c,J))

   Admittedly it’s harder to read, but it might be faster if J is large.

2. dhull replied on December 10th, 2008 at 19:49 UTC :
   

   Martin,

   Yes, in MATLAB there are often many ways of doing the same thing. I do not know which one is faster, I suspect that the difference is negligible.

   As you said, your code is harder to read, so that would make me want to do it another way. Personally, I try to write code that is easy to read and understand. Once it is working, then if there are sections that are slow, I will try more advanced maneuvers like you pointed out to trade readability for speed.

   As always, thanks for reading and adding,
   Doug

3. Hassan replied on December 10th, 2008 at 19:53 UTC :
   

   I am have problem populating 3D matrix using mex file. I have a 3D int array

   double *outArray;
   dims[0]=4;dims[1]=2;dims[2]=3;

   plhs[0]=mxCreateNumericArray(3,dims,mxINT16_CLASS,mxREAL);
   outArray = mxGetPr(plhs[0]);
   outArray[0]=12;

   It gives random numbers in the output. If i do it using 2D matrix I have no problem.. any ideas?

4. Hassan replied on December 11th, 2008 at 00:57 UTC :
   

   Found the error..should be
   plhs[0]=mxCreateNumericArray(3,dims,mxDOUBLE_CLASS,mxREAL);

   instead of

   plhs[0]=mxCreateNumericArray(3,dims,mxINT16_CLASS,mxREAL);

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<pre class="code">
a = magic(3);
sum(a)
</pre>

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