NReco.PivotData.TopPivotTable Take last N instead of first N

Question

Taking the first N number of rows is pretty straightforward and works well with

var topPvtTbl = new TopPivotTable(runningPvtTbl, 3, 99);
                                topPvtTbl.IncludeOtherGroups = false;

Is there an inverse? My use case is that the data is returning dates in ascending order on the rows and instead of taking the first 3 dates, I want the last 3 dates. I don't see a BottomPivotTable option.

Answer 1

You can achieve what you want in the following way:

• configure base PivotTable class to order axis with dates in reverse order (descending)
• apply TopPivotTable wrapper to select only first 3 dates (= so they will be actually last 3 days)
• reverse axis keys with Array.Reverse. For example, if dates are columns: Array.Reverse(topPvtTbl.ColumnKeys);

-- update --

This approach will not work if underlying table-related calculation relies on the concrete axis order (like RunningValuePivotTable). Alternatively, you can implement your own IPivotTable wrapper that takes last N items. For example:

public class LastNColumnsPivotTable : IPivotTable {
  IPivotTable basePvtTbl;

  public LastNColumnsPivotTable(IPivotTable pvtTbl, int lastN) {
    basePvtTbl = pvtTbl;
    if (basePvtTbl.ColumnKeys.Length>lastN) {
      lastColumnKeys = basePvtTbl.ColumnKeys.Skip(basePvtTbl.ColumnKeys.Length - lastN).ToArray();
    } else {
      lastColumnKeys = basePvtTbl.ColumnKeys;
    }
  }

  public string[] Columns { get { return basePvtTbl.Columns; } }
  public string[] Rows { get { return basePvtTbl.Rows; } }

  ValueKey[] lastColumnKeys;
  public ValueKey[] ColumnKeys { get { return lastColumnKeys; } }

  public ValueKey[] RowKeys { get { return basePvtTbl.RowKeys; } }
  public IPivotData PivotData { get { return basePvtTbl.PivotData; } }

  public IAggregator GetValue(ValueKey rowKey, ValueKey colKey) {
    return basePvtTbl.GetValue(rowKey, colKey);
  }
}