assigning points to clusters using index array

Question

Tibo Huyers 2022-3-7

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https://ww2.mathworks.cn/matlabcentral/answers/1665869-assigning-points-to-clusters-using-index-array

评论： Max Alger-Meyer 2022-3-7

I'm trying to cluster a point cloud (nx2 array with x and y coordinate) based on the distance of the point to a line. For example:

points = [1 5; 2 6; 4 8]
points = 3×2
     1     5
     2     6
     4     8

Using a for loop i have already detemined what line each point is closest to. The for loop puts an index in an nx1 array that represents the cluster/line that point belongs to. The amount of lines is variable. With two possible lines this array for the above points might look like this:

clusterIndex = [1; 2; 1]
clusterIndex = 3×1
     1
     2
     1

Using these two arrays I want to cluster the points with x and y coordinates seperatly. Where the first column represents the first cluster and so on. Resulting in this:

x = [1 2; 4 0]
x = 2×2
     1     2
     4     0
y = [5 6; 8 0]
y = 2×2
     5     6
     8     0

Using the list of indices I can't get the points sorted in clusters. If there is a better way instead of using the indices that would also be apreciated.

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Answer 1

Max Alger-Meyer 2022-3-7

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https://ww2.mathworks.cn/matlabcentral/answers/1665869-assigning-points-to-clusters-using-index-array#answer_912299

在 MATLAB Online 中打开

So here is what you asked for using the nnz function:

%given quantities 
points = [1 5; 2 6; 4 8];
clusterIndex = [1; 2; 1];
x = zeros(1,max(clusterIndex));
y = zeros(1,max(clusterIndex));
%Outputs:
for i = 1:size(points,1)
x(nnz(x(clusterIndex(i)))+1,clusterIndex(i)) = points(i,1);
y(nnz(y(clusterIndex(i)))+1,clusterIndex(i)) = points(i,2);
end
%Here are the results:
x
x = 2×2
     1     2
     4     0
y
y = 2×2
     5     6
     8     0

This method uses the nnz() function which identifies how many nonzero elements there are in an array and adds 1 to that number to determine which row a given x or y coordinate should be placed in.

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Tibo Huyers 2022-3-7

编辑：Tibo Huyers 2022-3-7

在 MATLAB Online 中打开

I think this fixes the problem:

x(nnz(x(:,clusterIndex(i)))+1,clusterIndex(i)) = points(i,1);
y(nnz(y(:,clusterIndex(i)))+1,clusterIndex(i)) = points(i,2);

The

:,

is needed to give the nnz function the correct column and not just an element.

Max Alger-Meyer 2022-3-7

Yep, sorry about that. Forgot to include the colon but yes you are correct because we only want to identify the number of nonzero elements in the corresponding column.

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Answer 2

Davide Masiello 2022-3-7

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https://ww2.mathworks.cn/matlabcentral/answers/1665869-assigning-points-to-clusters-using-index-array#answer_912279

在 MATLAB Online 中打开

points = [1 5; 2 6; 4 8];
N = size(points,1);
n = N+mod(N,2);
x = zeros(1,n);
y = x;
x(1:N) = points(:,1);
y(1:N) = points(:,2);
x = reshape(x,2,2);
y = reshape(y,2,2);

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Max Alger-Meyer 2022-3-7

I think you missed what is being asked. Your code seems to just take all of the x or y coordinates and arrange them into either a 2 x 2 array, but the column should correspond to the segment that that coordinate is closest to which is designated by the corresponding row of clusterIndex. Your code also looks like it will break for more than 4 points.

Davide Masiello 2022-3-7