How do I make different groups within a matrix?

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Jaime Castiblanques 2021-3-12

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评论： Jaime Castiblanques 2021-3-15

I have a 2904x3 matrix where each column represents the x, y and z coordinates of some vectors. Some of these vectors have the same z-coordinate and I need to group those together. Any ideas?

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Jaime Castiblanques 2021-3-12

Hi Mathieu,

This allows me to have all my z organised, so thank you for that. However, I need to have each of those z values associated to their correpondent x and y values. Is there any way of organizing the z values within the 2904x3 matrix I mentioned, while keeping the x and y coordinates that go with each z value?

Thanks,

Jaime

Adam Danz 2021-3-12

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Extending Mathieu NOE's suggestion, the 3rd output to unique is a grouping variable but you should use the stable flag to ensure that the grouping values correspond to each element of the vector.

% xyz is nx3 matrix of [x,y,z] values
[~,~,zgroup] = unique(xyz(:,3)); 

Alternatively, if you just want to sort the matrix according to the z column,

xyzSort = sortrows(xyz,3);

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Answer 1

Veronica Taurino 2021-3-12

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编辑：Veronica Taurino 2021-3-12

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How do you need to get arrays associated?

Try this:

clear all
close all
x= (50)*rand(50,1);
y= (50)*rand(50,1);
z= (50)*rand(50,1);
% I create 4 equal z coordinates:
z(12)=z(23);
z(34)=z(23);
z(1)=z(23);
z(45)=z(23);
z(23)
% Put all together
AA= [x,y,z];
% Look for index
[~, ind] = unique(AA(:, 3), 'rows');
% DUPLICATE INDEX
duplicate_index = setdiff(1:size(AA, 1), ind) % This finds the indexes of duplicates
% duplicate values
duplicate_value = AA(duplicate_index, :) % This extracts the arrays with duplicates

results:

z(23) =
   12.6859 % the Z value I imposed
duplicate_index = % indexes of duplicates the script finds
    12    23    34    45
duplicate_value =  % array with same Z the script finds
   19.6717   40.5660   12.6859
   47.5017    3.6415   12.6859
   11.9746   35.4336   12.6859
   38.4967   19.0424   12.6859

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Veronica Taurino 2021-3-12

编辑：Veronica Taurino 2021-3-12

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It is just a way to set equal z's coordinates over the arrays I randomly created.

If you need to group points with equal Z's, you can change the previous code as follows:

x= (50)*rand(50,1);
y= (50)*rand(50,1);
z= (50)*rand(50,1);
% I create 2 GROUPS of  equal z coordinates:
% First group
z(12)=z(23);
z(34)=z(23);
z(1)=z(23);
z(45)=z(23);
z(23);
% Second group
z(1)=z(50);
z(28)=z(50);
z(11)=z(50);
z(50);
% Put all together
AA = [x,y,z]; %%%%% YOU WOULD START FROM HERE WITH YOUR DATA %%%%%%%%%
% Sort 
AA_sorted=sortrows(AA,3);
% Look for index
[C,ia,ic] = unique(AA_sorted(:, 3), 'rows');
% DUPLICATE INDEX
duplicate_index2 = setdiff(1:size(AA_sorted, 1), ia);
% duplicate values
duplicate_value2 = AA_sorted(duplicate_index2, :);
% Look for index GROUP
[C3,ia3,ic3] = unique(duplicate_value2(:, 3), 'rows');
% Create group
for jj=1:length(ia3)
    if jj<length(ia3)
        Group{jj}=duplicate_value2(ia3(jj):ia3(jj+1)-1,:);
    else
        Group{jj}=duplicate_value2(ia3(jj):end,:);
    end
end