MATLAB Answers


extend a Matrix in every iteration

Asked by Fernando Arias on 18 May 2018
Latest activity Edited by jonas
on 18 May 2018

Hi, I'm having some problems with my code. I have a 3 dimension matrix in which index represent a coordinate, for example, a 10x10x10 array where the first index are 1, 1, 1 represent the point x=1, y=1 and z=1. And the value in the array represent the number of points in theese coordinates. Now I want to use ptCloud function, so I have to extract all the points from my array. What I'm trying is this (If I were in the case 10*10*10 matrix A):

for i=1:1000
[Yj Xj Zj]=ind2sub(size(A), i); 
value=A(Yj Xj Zj);
P=[Yj Xj Zj];

With this code I already have a matrix MAT whith every points. But the case is that my array is 200x200x160, and I noticed that in every iteration the time that the code requires increases a lot, so I think there would be something wrong in the for loop.

Thanks so much.


The time for every iteration increases due to the size of MAT increasing with every iteration (can be fixed by preallocation). I don't understand what the desired output is, but I'm guessing there is a more effective way than a loop. What is MAT describing?

MAT is the matrix wich contains all points, for example, if there is a 3 in A(1,1,1) means that I have three points in (1,1,1) so I add 3 rows of [1, 1, 1] to the matrix MAT. Finally I would like to have a matrix with all points where first colum is x coordinate, seconx y and third z.

I think there would be another way quiker, becouse with this code I can't have the matrix in more than a day.

EDIT: nvm, Mr. Cobeldick solved that beautifully

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1 Answer

Answer by Stephen Cobeldick on 18 May 2018
Edited by Stephen Cobeldick on 18 May 2018
 Accepted Answer

A = randi(9,10,10,10);
[X,Y,Z] = ind2sub(size(A),1:numel(A));
mat = repelem([X(:),Y(:),Z(:)],A(:),1,1);

but I don't see much point in generating subscript indices: it would be easier to define and use linear indices:

idx = repelem(1:numel(A),A(:).')


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