Reset DataFrame.RowCount to zero, when DataFrame is empty (dotnet#6698)

* Reset RowCount to zero, when DataFrame is empty

* Fix typo.

---------

Co-authored-by: Michael Sharp <51342856+michaelgsharp@users.noreply.github.com>

Author: asmirnov82

src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs

@@ -58,21 +58,25 @@ public void Insert<T>(int columnIndex, IEnumerable<T> column, string columnName)
         protected override void InsertItem(int columnIndex, DataFrameColumn column)
         {
             column = column ?? throw new ArgumentNullException(nameof(column));
-            if (RowCount > 0 && column.Length != RowCount)
+
+            if (Count == 0)
             {
-                throw new ArgumentException(Strings.MismatchedColumnLengths, nameof(column));
+                //change RowCount on inserting first row to dataframe
+                RowCount = column.Length;
             }
-
-            if (Count >= 1 && RowCount == 0 && column.Length != RowCount)
+            else if (column.Length != RowCount)
             {
+                //check all columns in the dataframe have the same length (amount of rows)
                 throw new ArgumentException(Strings.MismatchedColumnLengths, nameof(column));
             }
 
             if (_columnNameToIndexDictionary.ContainsKey(column.Name))
             {
                 throw new ArgumentException(string.Format(Strings.DuplicateColumnName, column.Name), nameof(column));
             }
+
             RowCount = column.Length;
+
             _columnNameToIndexDictionary[column.Name] = columnIndex;
             for (int i = columnIndex + 1; i < Count; i++)
             {
@@ -108,6 +112,11 @@ protected override void RemoveItem(int columnIndex)
                 _columnNameToIndexDictionary[this[i].Name]--;
             }
             base.RemoveItem(columnIndex);
+
+            //Reset RowCount if the last column was removed and dataframe is empty
+            if (Count == 0)
+                RowCount = 0;
+
             ColumnsChanged?.Invoke();
         }
 
@@ -138,6 +147,9 @@ protected override void ClearItems()
             base.ClearItems();
             ColumnsChanged?.Invoke();
             _columnNameToIndexDictionary.Clear();
+
+            //reset RowCount as DataFrame is now empty
+            RowCount = 0;
         }
 
         /// <summary>

test/Microsoft.Data.Analysis.Tests/DataFrameTests.cs

@@ -271,29 +271,44 @@ public void TestIndexer()
         [Fact]
         public void ColumnAndTableCreationTest()
         {
-            DataFrameColumn intColumn = new Int32DataFrameColumn("IntColumn", Enumerable.Range(0, 10).Select(x => x));
-            DataFrameColumn floatColumn = new SingleDataFrameColumn("FloatColumn", Enumerable.Range(0, 10).Select(x => (float)x));
+            const int rowCount = 10;
+            DataFrameColumn intColumn = new Int32DataFrameColumn("IntColumn", Enumerable.Range(0, rowCount).Select(x => x));
+            DataFrameColumn floatColumn = new SingleDataFrameColumn("FloatColumn", Enumerable.Range(0, rowCount).Select(x => (float)x));
             DataFrame dataFrame = new DataFrame();
             dataFrame.Columns.Insert(0, intColumn);
             dataFrame.Columns.Insert(1, floatColumn);
-            Assert.Equal(10, dataFrame.Rows.Count);
+            Assert.Equal(rowCount, dataFrame.Rows.Count);
             Assert.Equal(2, dataFrame.Columns.Count);
-            Assert.Equal(10, dataFrame.Columns[0].Length);
+            Assert.Equal(2, dataFrame.Columns.LongCount());
+            Assert.Equal(rowCount, dataFrame.Columns[0].Length);
             Assert.Equal("IntColumn", dataFrame.Columns[0].Name);
-            Assert.Equal(10, dataFrame.Columns[1].Length);
+            Assert.Equal(rowCount, dataFrame.Columns[1].Length);
             Assert.Equal("FloatColumn", dataFrame.Columns[1].Name);
 
-            DataFrameColumn bigColumn = new SingleDataFrameColumn("BigColumn", Enumerable.Range(0, 11).Select(x => (float)x));
-            DataFrameColumn repeatedName = new SingleDataFrameColumn("FloatColumn", Enumerable.Range(0, 10).Select(x => (float)x));
+            //add column with bigger length than other columns in the dataframe
+            DataFrameColumn bigColumn = new SingleDataFrameColumn("BigColumn", Enumerable.Range(0, rowCount + 1).Select(x => (float)x));
             Assert.Throws<ArgumentException>(() => dataFrame.Columns.Insert(2, bigColumn));
+            Assert.Throws<ArgumentException>(() => dataFrame.Columns.Add(bigColumn));
+
+            //add column smaller than other columns in the dataframe
+            DataFrameColumn smallColumn = new SingleDataFrameColumn("SmallColumn", Enumerable.Range(0, rowCount - 1).Select(x => (float)x));
+            Assert.Throws<ArgumentException>(() => dataFrame.Columns.Insert(2, smallColumn));
+            Assert.Throws<ArgumentException>(() => dataFrame.Columns.Add(smallColumn));
+
+            //add column with duplicate name
+            DataFrameColumn repeatedName = new SingleDataFrameColumn("FloatColumn", Enumerable.Range(0, rowCount).Select(x => (float)x));
             Assert.Throws<ArgumentException>(() => dataFrame.Columns.Insert(2, repeatedName));
-            Assert.Throws<ArgumentOutOfRangeException>(() => dataFrame.Columns.Insert(10, repeatedName));
+
+            //Insert column at index out of range
+            DataFrameColumn extraColumn = new SingleDataFrameColumn("OtherFloatColumn", Enumerable.Range(0, rowCount).Select(x => (float)x));
+            var columnCount = dataFrame.Columns.Count;
+            Assert.Throws<ArgumentOutOfRangeException>(() => dataFrame.Columns.Insert(columnCount + 1, repeatedName));
 
             Assert.Equal(2, dataFrame.Columns.Count);
-            DataFrameColumn intColumnCopy = new Int32DataFrameColumn("IntColumn", Enumerable.Range(0, 10).Select(x => x));
+            DataFrameColumn intColumnCopy = new Int32DataFrameColumn("IntColumn", Enumerable.Range(0, rowCount).Select(x => x));
             Assert.Throws<ArgumentException>(() => dataFrame.Columns[1] = intColumnCopy);
 
-            DataFrameColumn differentIntColumn = new Int32DataFrameColumn("IntColumn1", Enumerable.Range(0, 10).Select(x => x));
+            DataFrameColumn differentIntColumn = new Int32DataFrameColumn("IntColumn1", Enumerable.Range(0, rowCount).Select(x => x));
             dataFrame.Columns[1] = differentIntColumn;
             Assert.True(object.ReferenceEquals(differentIntColumn, dataFrame.Columns[1]));
 
@@ -309,18 +324,68 @@ public void ColumnAndTableCreationTest()
         }
 
         [Fact]
-        public void InsertAndRemoveColumnTests()
+        public void InsertAndRemoveColumnToTheEndOfNotEmptyDataFrameTests()
         {
             DataFrame dataFrame = MakeDataFrameWithAllMutableColumnTypes(10);
-            DataFrameColumn intColumn = new Int32DataFrameColumn("IntColumn", Enumerable.Range(0, 10).Select(x => x));
-            DataFrameColumn charColumn = dataFrame.Columns["Char"];
-            int insertedIndex = dataFrame.Columns.Count;
-            dataFrame.Columns.Insert(dataFrame.Columns.Count, intColumn);
+            DataFrameColumn intColumn = new Int32DataFrameColumn("NewIntColumn", Enumerable.Range(0, 10).Select(x => x));
+
+            int columnCount = dataFrame.Columns.Count;
+            DataFrameColumn originalLastColumn = dataFrame.Columns[columnCount - 1];
+
+            //Insert new column at the end
+            dataFrame.Columns.Insert(columnCount, intColumn);
+            Assert.Equal(columnCount + 1, dataFrame.Columns.Count);
+
+            //Remove first
             dataFrame.Columns.RemoveAt(0);
-            DataFrameColumn intColumn_1 = dataFrame.Columns["IntColumn"];
-            DataFrameColumn charColumn_1 = dataFrame.Columns["Char"];
+            Assert.Equal(columnCount, dataFrame.Columns.Count);
+
+            //Check that int column was inserted
+            DataFrameColumn intColumn_1 = dataFrame.Columns["NewIntColumn"];
             Assert.True(ReferenceEquals(intColumn, intColumn_1));
-            Assert.True(ReferenceEquals(charColumn, charColumn_1));
+
+            //Check that last column of the original dataframe was not removed
+            DataFrameColumn lastColumn_1 = dataFrame.Columns[originalLastColumn.Name];
+            Assert.True(ReferenceEquals(originalLastColumn, lastColumn_1));
+
+            //Check that new column is the last one
+            int newIndex = dataFrame.Columns.IndexOf("NewIntColumn");
+            Assert.Equal(columnCount - 1, newIndex);
+
+            //Check that original last column now has correct index
+            int newIndexForOriginalLastColumn = dataFrame.Columns.IndexOf(originalLastColumn.Name);
+            Assert.Equal(columnCount - 2, newIndexForOriginalLastColumn);
+        }
+
+        [Fact]
+        public void AddAndRemoveColumnToTheEmptyDataFrameTests()
+        {
+            DataFrame dataFrame = new DataFrame();
+            DataFrameColumn intColumn = new Int32DataFrameColumn("NewIntColumn", Enumerable.Range(0, 10).Select(x => x));
+
+            dataFrame.Columns.Add(intColumn);
+            Assert.Single(dataFrame.Columns);
+            Assert.Equal(10, dataFrame.Rows.Count);
+
+            dataFrame.Columns.Remove(intColumn);
+            Assert.Empty(dataFrame.Columns);
+            Assert.Equal(0, dataFrame.Rows.Count);
+        }
+
+        [Fact]
+        public void ClearColumnsTests()
+        {
+            //Arrange
+            DataFrame dataFrame = MakeDataFrameWithAllMutableColumnTypes(10);
+
+            //Act
+            dataFrame.Columns.Clear();
+
+            //Assert
+            Assert.Empty(dataFrame.Columns);
+
+            Assert.Equal(0, dataFrame.Rows.Count);
+            Assert.Equal(0, dataFrame.Columns.LongCount());
         }
 
         [Fact]