MATLAB: Predict function in KNN

Question

Hi, I'm trying to implement a Leave One Out using knn. This is the code. I'm reporting the error. Thanks

for i=1:loo_cycles
        full_index = 1:rows_X;
        index_train = full_index(full_index~=i);
        x_set = X(i, :);
        
        x_train = X(index_train, :);
        y_train = y(index_train, 1);
        
        fprintf("Running loo step %d of %d\n",i, loo_cycles);
        cvp = cvpartition(y_train, 'KFold');
        KNN = fitcknn(x_train, y_train,'CVPartition', cvp);
        y_pred_test(i, 1) = predict(KNN, x_set);
end

Error:

Check for missing argument or incorrect argument data type in call to function 'predict'.
Error in test_knn (line 40)
        y_pred_test(i, 1) = predict(KNN, x_set);

Answer 1

Your KNN is not a single model to make predictions but rather a RegressionPartitionedModel. Which means it has all the statistics of the cross validation and all individual models for each fold. If you want to make predictions you will have to select one of the trained models, as example:

y_pred_test(i, 1) = predict(KNN.Trained{1}, x_set);

I don't really understand why you would need this internal cross-validation since you're already looping to each data point. For me it would make more sense to simply train the model in all training data for each iteration:

KNN = fitcknn(x_train, y_train);
y_pred_test(i, 1) = predict(KNN, x_set);

Then you have only one model and can use the predict function.