Section 66.48: Select with Func<TSource, int, TResult> selector - Use to get ranking of elements

On of the overloads of the SELECT extension methods also passes the index of the current item in the collection being SELECTed. These are a few uses of it.

Get the "row number" of the items

var rowNumbers = collection.OrderBy(item => item.Property1)

.ThenBy(item => item.Property2)

.ThenByDescending(item => item.Property3)

.Select((item, index) => new { Item = item, RowNumber = index })

.ToList();

Get the rank of an item within its group

var rankInGroup = collection.GroupBy(item => item.Property1)

.OrderBy(group => group.Key)

.SelectMany(group => group.OrderBy(item => item.Property2)

.ThenByDescending(item => item.Property3)

.Select((item, index) => new

{

Item = item, RankInGroup = index

})).ToList();

Get the ranking of groups (also known in Oracle as dense_rank)

var rankOfBelongingGroup = collection.GroupBy(item => item.Property1)

.OrderBy(group => group.Key)

.Select((group, index) => new

{

Items = group, Rank = index

})

.SelectMany(v => v.Items, (s, i) => new

{

Item = i, DenseRank = s.Rank

}).ToList();

For testing this you can use:

public class SomeObject

{

public int Property1 { get; set; } public int Property2 { get; set; } public int Property3 { get; set; }

public override string ToString()

{

return string.Join(", ", Property1, Property2, Property3);

}

}

And data:

List<SomeObject> collection = new List<SomeObject>

{

new SomeObject { Property1 = 1, Property2 = 1, Property3 = 1}, new SomeObject { Property1 = 1, Property2 = 2, Property3 = 1}, new SomeObject { Property1 = 1, Property2 = 2, Property3 = 2}, new SomeObject { Property1 = 2, Property2 = 1, Property3 = 1}, new SomeObject { Property1 = 2, Property2 = 2, Property3 = 1}, new SomeObject { Property1 = 2, Property2 = 2, Property3 = 1}, new SomeObject { Property1 = 2, Property2 = 3, Property3 = 1}

};