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

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!

Answer 1

[Walter Roberson]

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.

Answer 2

[Image Analyst]

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