Add a sample for copy columns.

docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/CopyColumns.cs

@@ -0,0 +1,86 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.ML.Core.Data;
+using Microsoft.ML.Data;
+using Microsoft.ML.Transforms;
+
+namespace Microsoft.ML.Samples.Dynamic
+{
+    public class CopyColumns
+    {
+        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();
+
+            // Get a small dataset as an IEnumerable and them read it as ML.NET's data type.
+            IEnumerable<SamplesUtils.DatasetUtils.SampleInfertData> data = SamplesUtils.DatasetUtils.GetInfertData();
+            var trainData = mlContext.Data.ReadFromEnumerable(data);
+
+            // Preview of the data.
+            //
+            // Age    Case  Education  induced     parity  pooled.stratum  row_num  ...
+            // 26.0   1.0   0-5yrs      1.0         6.0       3.0      1.0  ...
+            // 42.0   1.0   0-5yrs      1.0         1.0       1.0      2.0  ...
+            // 39.0   1.0   0-5yrs      2.0         6.0       4.0      3.0  ...
+            // 34.0   1.0   0-5yrs      2.0         4.0       2.0      4.0  ...
+            // 35.0   1.0   6-11yrs     1.0         3.0       32.0     5.0  ...
+
+            // CopyColumns is commonly used to rename columns.
+            // For example, if you want to train towards Age, and your learner expects a "Label" column, you can
+            // use CopyColumns to rename Age to Label. Technically, the Age columns still exists, but it won't be
+            // materialized unless you actually need it somewhere (e.g. if you were to save the transformed data
+            // without explicitly dropping the column). This is a general property of IDataView's lazy evaluation.
+            string labelColumnName = "Label";
+            var pipeline = mlContext.Transforms.CopyColumns(labelColumnName, "Age") as IEstimator<ITransformer>;
+
+            // You also may want to copy a column to perform some hand-featurization using built-in transforms or
+            // a CustomMapping transform. For example, we could make an indicator variable if a feature, such as Parity
+            // goes above some threshold. We simply copy the Parity column to a new column, then pass it through a custom function.
+            Action<InputRow, OutputRow> mapping = (input, output) =>output.CustomValue = input.CustomValue > 4 ? 1 : 0;
+            pipeline = pipeline.Append(mlContext.Transforms.CopyColumns("CustomValue", "Parity"))
+                .Append(mlContext.Transforms.CustomMapping(mapping, null));
+
+            // Now we can transform the data and look at the output to confirm the behavior of CopyColumns.
+            // Don't forget that this operation doesn't actually evaluate data until we read the data below.
+            var transformedData = pipeline.Fit(trainData).Transform(trainData);
+
+            // We can extract the newly created column as an IEnumerable of SampleInfertDataTransformed, the class we define below.
+            var rowEnumerable = mlContext.CreateEnumerable<SampleInfertDataTransformed>(transformedData, reuseRowObject: false);
+
+            // And finally, we can write out the rows of the dataset, looking at the columns of interest.
+            Console.WriteLine($"Label, Parity, and CustomValue columns obtained post-transformation.");
+            foreach (var row in rowEnumerable)
+            {
+                Console.WriteLine($"Label: {row.Label} Parity: {row.Parity} CustomValue: {row.CustomValue}");
+            }
+
+            // Expected output:
+            //  Label, Parity, and CustomValue columns obtained post-transformation.
+            //  Label: 26 Parity: 6 CustomValue: 1
+            //  Label: 42 Parity: 1 CustomValue: 0
+            //  Label: 39 Parity: 6 CustomValue: 1
+            //  Label: 34 Parity: 4 CustomValue: 0
+            //  Label: 35 Parity: 3 CustomValue: 0
+        }
+
+        private class SampleInfertDataTransformed
+        {
+            public float Label { get; set; }
+            public float Parity { get; set; }
+            public float CustomValue { get; set; }
+        }
+
+        private class OutputRow
+        {
+            public float CustomValue { get; set; }
+        }
+
+        private class InputRow
+        {
+            public float CustomValue { get; set; }
+        }
+    }
+}

src/Microsoft.ML.Data/Transforms/ExtensionsCatalog.cs

@@ -18,6 +18,13 @@ public static class TransformExtensionsCatalog
         /// <param name="catalog">The transform's catalog.</param>
         /// <param name="outputColumnName">Name of the column resulting from the transformation of <paramref name="inputColumnName"/>.</param>
         /// <param name="inputColumnName">Name of the columns to transform.</param>
+        /// <example>
+        /// <format type="text/markdown">
+        /// <![CDATA[
+        /// [!code-csharp[CopyColumns](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/CopyColumns.cs)]
+        /// ]]>
+        /// </format>
+        /// </example>
         public static ColumnCopyingEstimator CopyColumns(this TransformsCatalog catalog, string outputColumnName, string inputColumnName)
             => new ColumnCopyingEstimator(CatalogUtils.GetEnvironment(catalog), outputColumnName, inputColumnName);
 
@@ -27,6 +34,13 @@ public static ColumnCopyingEstimator CopyColumns(this TransformsCatalog catalog,
         /// </summary>
         /// <param name="catalog">The transform's catalog</param>
         /// <param name="columns">The pairs of input and output columns.</param>
+        /// <example>
+        /// <format type="text/markdown">
+        /// <![CDATA[
+        /// [!code-csharp[CopyColumns](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/CopyColumns.cs)]
+        /// ]]>
+        /// </format>
+        /// </example>
         public static ColumnCopyingEstimator CopyColumns(this TransformsCatalog catalog, params (string outputColumnName, string inputColumnName)[] columns)
             => new ColumnCopyingEstimator(CatalogUtils.GetEnvironment(catalog), columns);
 
@@ -39,7 +53,7 @@ public static ColumnCopyingEstimator CopyColumns(this TransformsCatalog catalog,
         /// <example>
         /// <format type="text/markdown">
         /// <![CDATA[
-        /// [!code-csharp[Concat](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/ConcatTransform.cs)]
+        /// [!code-csharp[Concat](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/ConcatTransform.cs)]
         /// ]]>
         /// </format>
         /// </example>