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= k-NN Regression

The Apache Ignite Machine Learning component provides two versions of the widely used k-NN (k-nearest neighbors) algorithm - one for classification tasks and the other for regression tasks.

This documentation reviews k-NN as a solution for regression tasks.

== Trainer and Model

The k-NN regression algorithm is a non-parametric method whose input consists of the k-closest training examples in the feature space. Each training example has a property value in a numerical form associated with the given training example.

The k-NN regression  algorithm uses all training sets to predict a property value for the given test sample.
This predicted property value is an average of the values of its k nearest neighbors. If `k` is `1`, then the test sample is simply assigned to the property value of a single nearest neighbor.

Presently, Ignite supports a few parameters for k-NN regression algorithm:

* `k` - a number of nearest neighbors
* `distanceMeasure` - one of the distance metrics provided by the ML framework such as Euclidean, Hamming or Manhattan
* `isWeighted` - false by default, if true it enables a weighted KNN algorithm.
* `dataCache` -  holds a training set of objects for which the class is already known.
* `indexType` - distributed spatial index, has three values: ARRAY, KD_TREE, BALL_TREE


[source, java]
----
// Create trainer
KNNRegressionTrainer trainer = new KNNRegressionTrainer()
  .withK(5)
  .withIdxType(SpatialIndexType.BALL_TREE)
  .withDistanceMeasure(new ManhattanDistance())
  .withWeighted(true);

// Train model.
KNNClassificationModel knnMdl = trainer.fit(
  ignite,
  dataCache,
  vectorizer
);

// Make a prediction.
double prediction = knnMdl.predict(observation);
----


== Example


To see how kNN Regression can be used in practice, try this https://github.com/apache/ignite/blob/master/examples/src/main/java/org/apache/ignite/examples/ml/knn/KNNRegressionExample.java[example^] that is available on GitHub and delivered with every Apache Ignite distribution.

The training dataset is the Iris dataset which can be loaded from the https://archive.ics.uci.edu/ml/datasets/iris[UCI Machine Learning Repository^].