KNNClassifier
KNNClassifier
A model type for constructing a K-nearest neighbor classifier, based on NearestNeighborModels.jl, and implementing the MLJ model interface.
From MLJ, the type can be imported using
KNNClassifier = @load KNNClassifier pkg=NearestNeighborModels
Do model = KNNClassifier()
to construct an instance with default hyper-parameters. Provide keyword arguments to override hyper-parameter defaults, as in KNNClassifier(K=...)
.
KNNClassifier implements K-Nearest Neighbors classifier which is non-parametric algorithm that predicts a discrete class distribution associated with a new point by taking a vote over the classes of the k-nearest points. Each neighbor vote is assigned a weight based on proximity of the neighbor point to the test point according to a specified distance metric.
For more information about the weighting kernels, see the paper by Geler et.al Comparison of different weighting schemes for the kNN classifier on time-series data.
Training data
In MLJ or MLJBase, bind an instance model
to data with
mach = machine(model, X, y)
OR
mach = machine(model, X, y, w)
Here:
X
is any table of input features (eg, aDataFrame
) whose columns are of scitypeContinuous
; check column scitypes withschema(X)
.y
is the target, which can be anyAbstractVector
whose element scitype is<:Finite
(<:Multiclass
or<:OrderedFactor
will do); check the scitype withscitype(y)
w
is the observation weights which can either benothing
(default) or anAbstractVector
whose element scitype isCount
orContinuous
. This is different fromweights
kernel which is a model hyperparameter, see below.
Train the machine using fit!(mach, rows=...)
.
Hyper-parameters
K::Int=5
: number of neighborsalgorithm::Symbol = :kdtree
: one of(:kdtree, :brutetree, :balltree)
metric::Metric = Euclidean()
: anyMetric
from Distances.jl for the distance between points. Foralgorithm = :kdtree
only metrics which are instances ofUnion{Distances.Chebyshev, Distances.Cityblock, Distances.Euclidean, Distances.Minkowski, Distances.WeightedCityblock, Distances.WeightedEuclidean, Distances.WeightedMinkowski}
are supported.leafsize::Int = algorithm == 10
: determines the number of points at which to stop splitting the tree. This option is ignored and always taken as0
foralgorithm = :brutetree
, sincebrutetree
isn't actually a tree.reorder::Bool = true
: iftrue
then points which are close in distance are placed close in memory. In this case, a copy of the original data will be made so that the original data is left unmodified. Setting this totrue
can significantly improve performance of the specifiedalgorithm
(except:brutetree
). This option is ignored and always taken asfalse
foralgorithm = :brutetree
.weights::KNNKernel=Uniform()
: kernel used in assigning weights to the k-nearest neighbors for each observation. An instance of one of the types inlist_kernels()
. User-defined weighting functions can be passed by wrapping the function in aUserDefinedKernel
kernel (do?NearestNeighborModels.UserDefinedKernel
for more info). If observation weightsw
are passed during machine construction then the weight assigned to each neighbor vote is the product of the kernel generated weight for that neighbor and the corresponding observation weight.
Operations
predict(mach, Xnew)
: Return predictions of the target given featuresXnew
, which should have same scitype asX
above. Predictions are probabilistic but uncalibrated.predict_mode(mach, Xnew)
: Return the modes of the probabilistic predictions returned above.
Fitted parameters
The fields of fitted_params(mach)
are:
tree
: An instance of eitherKDTree
,BruteTree
orBallTree
depending on the value of thealgorithm
hyperparameter (See hyper-parameters section above). These are data structures that stores the training data with the view of making quicker nearest neighbor searches on test data points.
Examples
using MLJ
KNNClassifier = @load KNNClassifier pkg=NearestNeighborModels
X, y = @load_crabs; ## a table and a vector from the crabs dataset
## view possible kernels
NearestNeighborModels.list_kernels()
## KNNClassifier instantiation
model = KNNClassifier(weights = NearestNeighborModels.Inverse())
mach = machine(model, X, y) |> fit! ## wrap model and required data in an MLJ machine and fit
y_hat = predict(mach, X)
labels = predict_mode(mach, X)
See also MultitargetKNNClassifier