Asked by Fernando Arias
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):

MAT=[] for i=1:1000 [Yj Xj Zj]=ind2sub(size(A), i); value=A(Yj Xj Zj); P=[Yj Xj Zj]; P2=repmat(P,value,1) MAT=[MAT;P2] end

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.

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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## 3 Comments

## jonas (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/401397-extend-a-matrix-in-every-iteration#comment_569313

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?

## Fernando Arias (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/401397-extend-a-matrix-in-every-iteration#comment_569317

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.

## jonas (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/401397-extend-a-matrix-in-every-iteration#comment_569339

EDIT: nvm, Mr. Cobeldick solved that beautifully

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