# How can I get coordinates from a grouping inside of a matrix.

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Jonathan Salazar on 29 Jul 2017
Answered: Image Analyst on 29 Jul 2017
I have a matrix with 5 columns: atom number, mol_tag, then the x y and z coordinates. The mol_tag seperate the atoms into globules by sharing a common value, I need a matrix that will give three coordinates from each grouping of mol_tag so that I can calculate the normal vector to each of the mol_tag groupings. thank you!

Walter Roberson on 29 Jul 2017
mol_tags = YourMatrix(:,2)
unique_mol_tags = unique( mol_tags );
num_tags = length(unique_mol_tags);
mol_normals = zeros(num_tags, 3);
for K = 1 : num_tags
three_idx = find( mol_tags == unique_mol_tags(K), 3, 'first');
coords = YourMatrix(three_idx, 3:5);
this_normal = ..... something with the coords
mol_normals(K, :) = this_normal;
end
It is possible to code the coordinate finding part without any explicit loop, but I do not think the result would be any clearer, as it involves some advanced accumarray hacks.

Image Analyst on 29 Jul 2017
If by the "three coordinates from each grouping of mol_tag" you mean the mean x, y, and z over all rows with that mol_tag value, you can use grpstats() if you have the Statistics and Machine Learning Toolbox