Added samples for loading text (#3793)

* Added samples for loading text

* Simplification

Author: Shahab Moradi

docs/samples/Microsoft.ML.Samples/Dynamic/DataOperations/LoadingText.cs

@@ -0,0 +1,77 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+using Microsoft.ML;
+using Microsoft.ML.Data;
+
+namespace Samples.Dynamic.DataOperations
+{
+    public static class LoadingText
+    {
+        // This examples shows all the ways to load data with TextLoader.
+        public static void Example()
+        {
+            // Create 5 data files to illustrate different loading methods.
+            var dataFiles = new List<string>();
+            var random = new Random();
+            var dataDirectoryName = "DataDir";
+            Directory.CreateDirectory(dataDirectoryName);
+            for (int i = 0; i < 5; i++)
+            {
+                var fileName = Path.Combine(dataDirectoryName, $"Data_{i}.csv");
+                dataFiles.Add(fileName);
+                using (var fs = File.CreateText(fileName))
+                    // Write random lines without header
+                    for (int line = 0; line < 10; line++)
+                        fs.WriteLine(random.NextDouble().ToString());
+            }
+
+            // Create a TextLoader.
+            var mlContext = new MLContext();
+            var loader = mlContext.Data.CreateTextLoader(
+                columns: new[]
+                    {
+                        new TextLoader.Column("RandomFeature", DataKind.Single, 0)
+                    },
+                hasHeader: false
+            );
+
+            // Load a single file from path.
+            var singleFileData = loader.Load(dataFiles[0]);
+            PrintRowCount(singleFileData);
+
+            // Expected Output:
+            //   10
+
+
+            // Load all 5 files from path.
+            var multipleFilesData = loader.Load(dataFiles.ToArray());
+            PrintRowCount(multipleFilesData);
+
+            // Expected Output:
+            //   50
+
+
+            // Load all files using path wildcard.
+            var multipleFilesWildcardData = 
+                loader.Load(Path.Combine(dataDirectoryName, "*"));
+            PrintRowCount(multipleFilesWildcardData);
+
+            // Expected Output:
+            //   50
+        }
+
+        private static void PrintRowCount(IDataView idv)
+        {
+            // IDataView is lazy so we need to iterate through it
+            // to get the number of rows.
+            long rowCount = 0;
+            using (var cursor = idv.GetRowCursor(idv.Schema))
+                while (cursor.MoveNext())
+                    rowCount++;
+
+            Console.WriteLine(rowCount);
+        }
+    }
+}

src/Microsoft.ML.Data/DataLoadSave/DataLoaderExtensions.cs

@@ -17,6 +17,13 @@ public static class DataLoaderExtensions
         /// </summary>
         /// <param name="loader">The loader to use.</param>
         /// <param name="path">One or more paths from which to load data.</param>
+        /// <example>
+        /// <format type="text/markdown">
+        /// <![CDATA[
+        /// [!code-csharp[Load](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/DataOperations/LoadingText.cs)]
+        /// ]]>
+        /// </format>
+        /// </example>
         public static IDataView Load(this IDataLoader<IMultiStreamSource> loader, params string[] path)
             => loader.Load(new MultiFileSource(path));
     }

src/Microsoft.ML.Data/DataLoadSave/Text/TextLoader.cs

@@ -1432,6 +1432,13 @@ void ICanSaveModel.Save(ModelSaveContext ctx)
         /// Loads data from <paramref name="source"/> into an <see cref="IDataView"/>.
         /// </summary>
         /// <param name="source">The source from which to load data.</param>
+        /// <example>
+        /// <format type="text/markdown">
+        /// <![CDATA[
+        /// [!code-csharp[Load](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/DataOperations/LoadingText.cs)]
+        /// ]]>
+        /// </format>
+        /// </example>
         public IDataView Load(IMultiStreamSource source) => new BoundLoader(this, source);
 
         internal static TextLoader CreateTextLoader<TInput>(IHostEnvironment host,