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using System;
using Microsoft.ML;
using Microsoft.ML.Data;
namespace Samples.Dynamic
{
// This example demonstrates hashing of categorical string and integer data types.
public static class Hash
{
public static void Example()
{
// Create a new ML context, for ML.NET operations. It can be used for
// exception tracking and logging, as well as the source of randomness.
var mlContext = new MLContext(seed: 1);
// Get a small dataset as an IEnumerable.
var rawData = new[] {
new DataPoint() { Category = "MLB" , Age = 18 },
new DataPoint() { Category = "NFL" , Age = 14 },
new DataPoint() { Category = "NFL" , Age = 15 },
new DataPoint() { Category = "MLB" , Age = 18 },
new DataPoint() { Category = "MLS" , Age = 14 },
};
var data = mlContext.Data.LoadFromEnumerable(rawData);
// Construct the pipeline that would hash the two columns and store the
// results in new columns. The first transform hashes the string column
// and the second transform hashes the integer column.
//
// Hashing is not a reversible operation, so there is no way to retrieve
// the original value from the hashed value. Sometimes, for debugging,
// or model explainability, users will need to know what values in the
// original columns generated the values in the hashed columns, since
// the algorithms will mostly use the hashed values for further
// computations. The Hash method will preserve the mapping from the
// original values to the hashed values in the Annotations of the newly
// created column (column populated with the hashed values).
//
// Setting the maximumNumberOfInverts parameters to -1 will preserve the
// full map. If that parameter is left to the default 0 value, the
// mapping is not preserved.
var pipeline = mlContext.Transforms.Conversion.Hash("CategoryHashed",
"Category", numberOfBits: 16, maximumNumberOfInverts: -1)
.Append(mlContext.Transforms.Conversion.Hash("AgeHashed", "Age",
numberOfBits: 8));
// Let's fit our pipeline, and then apply it to the same data.
var transformer = pipeline.Fit(data);
var transformedData = transformer.Transform(data);
// Convert the post transformation from the IDataView format to an
// IEnumerable <TransformedData> for easy consumption.
var convertedData = mlContext.Data.CreateEnumerable<
TransformedDataPoint>(transformedData, true);
Console.WriteLine("Category CategoryHashed\t Age\t AgeHashed");
foreach (var item in convertedData)
Console.WriteLine($"{item.Category}\t {item.CategoryHashed}\t\t " +
$"{item.Age}\t {item.AgeHashed}");
// Expected data after the transformation.
//
// Category CategoryHashed Age AgeHashed
// MLB 36206 18 127
// NFL 19015 14 62
// NFL 19015 15 43
// MLB 36206 18 127
// MLS 6013 14 62
// For the Category column, where we set the maximumNumberOfInverts
// parameter, the names of the original categories, and their
// correspondence with the generated hash values is preserved in the
// Annotations in the format of indices and values.the indices array
// will have the hashed values, and the corresponding element,
// position -wise, in the values array will contain the original value.
//
// See below for an example on how to retrieve the mapping.
var slotNames = new VBuffer<ReadOnlyMemory<char>>();
transformedData.Schema["CategoryHashed"].Annotations.GetValue(
"KeyValues", ref slotNames);
var indices = slotNames.GetIndices();
var categoryNames = slotNames.GetValues();
for (int i = 0; i < indices.Length; i++)
Console.WriteLine($"The original value of the {indices[i]} " +
$"category is {categoryNames[i]}");
// Output Data
//
// The original value of the 6012 category is MLS
// The original value of the 19014 category is NFL
// The original value of the 36205 category is MLB
}
public class DataPoint
{
public string Category { get; set; }
public uint Age { get; set; }
}
public class TransformedDataPoint : DataPoint
{
public uint CategoryHashed { get; set; }
public uint AgeHashed { get; set; }
}
}
}
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