Indexing during assignment

Question

Say I have this sample data

A =
1.0000     6.0000   180.0000    12.0000
1.0000     5.9200   190.0000    11.0000
1.0000     5.5800   170.0000    12.0000
1.0000     5.9200   165.0000    10.0000
2.0000     5.0000   100.0000     6.0000
2.0000     5.5000   150.0000     8.0000
2.0000     5.4200   130.0000     7.0000
2.0000     5.7500   150.0000     9.0000

I wish to calculate the variance of each column, grouped by class (the first column).

I have this working with the following code, but it uses hard coded indices, requiring knowledge of the number of samples per class and they must be in specific order.

Is there a better way to do this?

variances = zeros(2,4);
variances = [1.0 var(A(1:4,2)), var(A(1:4,3)), var(A(1:4,4));
             2.0 var(A(5:8,2)), var(A(5:8,3)), var(A(5:8,4))];

disp(variances);

1.0 3.5033e-02   1.2292e+02   9.1667e-01
2.0 9.7225e-02   5.5833e+02   1.6667e+00

Answer 1

Separate the class labels and the data into different variables.

cls = A(:, 1);
data = A(:, 2:end);

Get the list of class labels

labels = unique(cls);

Compute the variances

variances = zeros(length(labels), 3);
for i = 1:length(labels)
  variances(i, :) = var(data(cls == labels(i), :)); % note the use of logical indexing
end