add StringArray supporting

Author: Oceania2018

src/NumSharp.Core/Backends/NDArray.String.cs

@@ -32,6 +32,33 @@ public static string AsString(NDArray arr)
             }
         }
 
+        /// <summary>
+        /// Convert to String[] from NDArray
+        /// </summary>
+        /// <param name="arr"></param>
+        /// <returns></returns>
+        public static string[] AsStringArray(NDArray arr)
+        {
+            var chars = arr.ToString();
+            int pos = chars.IndexOf(' ');
+            int numOfRows = int.Parse(chars.Substring(0, pos));
+            int posOfContent = chars.IndexOf(':');
+            string contents = chars.Substring(posOfContent + 1);
+            int[] strLengths = chars.Substring(pos + 1, posOfContent - pos - 1)
+                .Split(' ')
+                .Select(x => int.Parse(x))
+                .ToArray();
+            string[] strArray = new string[numOfRows];
+            int currentPos = 0;
+            for (int i = 0; i < numOfRows; i++)
+            {
+                strArray[i] = contents.Substring(currentPos, strLengths[i]);
+                currentPos += strLengths[i];
+            }
+
+            return strArray;
+        }
+
         /// <summary>
         ///     Get a string out of a vector of chars.
         /// </summary>

src/NumSharp.Core/Creation/np.array.string.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using System.Text;
+using NumSharp.Backends;
+using NumSharp.Backends.Unmanaged;
+
+namespace NumSharp
+{
+    public static partial class np
+    {
+        /// <summary>
+        ///     Create a vector ndarray of type <see cref="string"/>.
+        /// Encode string array.
+        /// format: [numOfRow lenOfRow1 lenOfRow2 contents]
+        /// sample: [2 2 4 aacccc]
+        /// </summary>
+        /// <param name="strArray"></param>
+        /// <returns></returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static NDArray array(string[] strArray)
+        {
+            if (strArray == null)
+                throw new ArgumentNullException(nameof(strArray));
+            if (strArray.Length == 0)
+                return new NDArray(NPTypeCode.String, 0);
+
+            // convert to bytes
+            string meta = $"{strArray.Length}";
+            foreach (var str in strArray)
+                meta += $" {str.Length}";
+            meta += $":{string.Join("", strArray)}";
+            return new NDArray(meta.ToCharArray());
+        }
+    }
+}

test/NumSharp.UnitTest/Backends/Unmanaged/StringArrayApiTests.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Linq;
+using FluentAssertions;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using NumSharp.UnitTest.Utilities;
+
+namespace NumSharp.UnitTest.Backends.Unmanaged
+{
+    [TestClass]
+    public class StringArrayApiTests
+    {
+        private static string[] strArray = new string[] { "Hello,", " SciSharp Team!"};
+
+        [TestMethod]
+        public void StringArrayConverting()
+        {
+            var nd = np.array(strArray);
+            nd.Should().BeOfType<char>()
+                .And.BeShaped(28)
+                .And.BeOfValues(50, 32, 54, 32, 49, 53, 58, 72, 101, 108, 108, 111, 44, 32, 83, 99, 105, 83, 104, 97, 114, 112, 32, 84, 101, 97, 109, 33);
+
+            NDArray.AsStringArray(nd).SequenceEqual(strArray);
+        }
+    }
+}