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Which category OneHotEncoder drops, and what it does with an unseen value.

public sealed record OneHotEncoderOptions

Properties — Drop chooses which category loses its column, and Unknown what happens to a value the fit never saw — sklearn.preprocessing.OneHotEncoder's drop and handle_unknown, same defaults.

Example — IfBinary drops the first category only where the feature has exactly two.

using Lodestar.Preprocessing;

var options = new OneHotEncoderOptions { Drop = CategoryDrop.IfBinary };

OneHotEncoder<string> binary = Encoders.OneHot(["y", "n", "y"], 1, options);
OneHotEncoder<string> three = Encoders.OneHot(["a", "b", "c"], 1, options);

int binaryColumns = binary.EncodedFeatureCount;  // => 1
int threeColumns = three.EncodedFeatureCount;    // => 3

Remarks — the two settings interact: an ignored unknown encodes to all zeros, and so does a dropped first category, so a row of zeros means either. Transform says so with the example.

Applies to — net10.0, netstandard2.0.

See also — CategoryDrop, UnknownCategory.