/// <summary>
/// Groups the data by the values in the given column, and computes the given aggregate quantity for each group.
/// </summary>
/// <typeparam name="T">The type of the aggregate output.</typeparam>
/// <param name="groupByColumnName">The name of the column to group by.</param>
/// <param name="aggregateColumnName">The name of the column for the aggregate output.</param>
/// <param name="aggregator">A function that computes the aggregate quantity.</param>
/// <returns>A new data frame containing the requested aggregate values for each group.</returns>
/// <remarks>
/// <para>The function that computes the aggregate receives a <see cref="FrameView"/> containing
/// all the rows in the group. To produce an aggregate result, it can use values in any of
/// the columns.</para>
/// <para>To produce more than one aggregate value, use <see cref="GroupBy(string, Func{FrameView, IReadOnlyDictionary{string, object}})"/>.</para>
/// </remarks>
public FrameTable GroupBy <T>(string groupByColumnName, Func <FrameView, T> aggregator, string aggregateColumnName)
{
if (groupByColumnName == null)
{
throw new ArgumentNullException(nameof(groupByColumnName));
}
if (aggregator == null)
{
throw new ArgumentNullException(nameof(aggregator));
}
if (aggregateColumnName == null)
{
throw new ArgumentNullException(nameof(aggregateColumnName));
}
// Collect the rows into groups.
int groupByColumnIndex = GetColumnIndex(groupByColumnName);
NamedList groupByColumn = columns[groupByColumnIndex];
NullableDictionary <object, List <int> > groups = FindGroups(groupByColumn);
// Form destination columns based on group aggregates.
NamedList groupsColumn = NamedList.Create(groupByColumnName, groupByColumn.StorageType);
NamedList <T> aggregateColumn = new NamedList <T>(aggregateColumnName);
foreach (KeyValuePair <object, List <int> > group in groups)
{
FrameView values = new FrameView(this.columns, group.Value);
T aggregateValue = aggregator(values);
aggregateColumn.AddItem(aggregateValue);
object groupKey = group.Key;
groupsColumn.AddItem(groupKey);
}
FrameTable result = new FrameTable(groupsColumn, aggregateColumn);
return(result);
}
internal FrameColumnCollection(FrameView frame)
{
Debug.Assert(frame != null);
this.frame = frame;
}
internal FrameColumn(FrameView frame, int c)
{
this.column = frame.columns[c];
this.map = frame.map;
}
