Sort

Elements, one after another, proceed in order. But often collections are not in an order we want. With the VB.NET Sort subroutine we order elements by their values.

Sorting details

We sort collections in-place. Sorting is by default in ascending order: elements go from lowest to highest. We can implement a descending order.

Array

First, we can use Array.Sort on array instances. Here the array is of type String() and it contains 3 strings (vegetables). Array.Sort is called.

Module Module1

    Sub Main()
        ' Create an array of String() with 3 elements.
        Dim vegetables As String() = New String() {"turnip",
            "onion",
            "corn"}

        ' Use the System.Array.Sort shared method.
        System.Array.Sort(vegetables)

        ' Loop through the results and print them with Console.WriteLine.
        For Each value As String In vegetables
            Console.WriteLine(value)
        Next
    End Sub

End Module
corn
onion
turnip

Sort, Reverse

Another way to reorder an array combines the Sort and Reverse methods. Effectively, this will give you a reverse alphabetical sort on strings.

Module Module1
    Sub Main()
        ' Create a String array with 5 elements.
        Dim array As String() = New String() {"X", _
            "B", _
            "A", _
            "Z", _
            "C"}
        ' Use the System.Array.Sort shared method.
        System.Array.Sort(Of String)(array)
        ' Invoke the Reverse method after sorting.
        System.Array.Reverse(array)
        ' Loop through the results.
        For Each value As String In array
            Console.WriteLine(value)
        Next
    End Sub
End Module
Z
X
C
B
A

Sort, descending

Here is a better way to sort descending (from last to first) in VB.NET. No separate reverse call is needed. Instead, we use a special Comparison Function.

Module Module1

    Function DescendingComparison(ByVal valueA As String,
                                  ByVal valueB As String) As Integer
        ' Invert the order of the comparison to sort in descending order.
        Return valueB.CompareTo(valueA)
    End Function

    Sub Main()
        Dim letters As String() = {"X",
            "B",
            "A",
            "Z",
            "C"}
        ' Use custom DescendingComparison to sort in reverse.
        Array.Sort(Of String)(letters, New Comparison(Of String)(AddressOf DescendingComparison))

        ' Display results.
        For Each value As String In letters
            Console.WriteLine("DESCENDING: {0}", value)
        Next
    End Sub

End Module
DESCENDING: Z
DESCENDING: X
DESCENDING: C
DESCENDING: B
DESCENDING: A

List

Sorting a List is also easy. No custom algorithms need to be implemented. Instead, you can invoke the instance Sort method on the List you created.

Module Module1

    Sub Main()
        ' Create a List of Strings and add 4 Strings to it.
        Dim list As New List(Of String)
        list.Add("chair")
        list.Add("table")
        list.Add("desk")
        list.Add("couch")

        ' Use Sort method.
        list.Sort()

        ' Loop through the results and display them.
        For Each value As String In list
            Console.WriteLine(value)
        Next
    End Sub

End Module
chair
couch
desk
table

List, sort with lambda

With a List, we can use a lambda expression to define the Comparison Function. This may make the VB.NET code shorter and easier to read.

Module Module1
    Sub Main()
        Dim numbers = New List(Of Integer)({20, 30, 40})

        ' Use lambda to sort list in descending order.
        numbers.Sort(Function(valueA, valueB) valueB.CompareTo(valueA))

        For Each number As Integer In numbers
            Console.WriteLine("LAMBDA SORTED NUMBER: {0}", number)
        Next
    End Sub
End Module
LAMBDA SORTED NUMBER: 40
LAMBDA SORTED NUMBER: 30
LAMBDA SORTED NUMBER: 20

Copied array, sort

Array.Sort() operates on the original data. So we must make a copy of the original array if we wish to keep it in that order. We apply the Array.Copy Function.

Module Module1
    Sub Main()
        Dim names() As String = {"Zach", "Andrew", "David"}

        ' Create a copy of names array and sort it.
        Dim namesCopy(2) As String
        Array.Copy(names, namesCopy, 3)
        Array.Sort(namesCopy)

        ' Display arrays.
        Console.WriteLine(String.Join(",", names))
        Console.WriteLine(String.Join(",", namesCopy))
    End Sub
End Module
Zach,Andrew,David
Andrew,David,Zach

Sort on 2 items

Support we have a List of Tuples, and we want to sort based on the first value of each tuple, and then the second value. Here we use an ascending and also descending sort.

Module Module1

    Function GetData() As List(Of Tuple(Of Integer, Integer))
        Dim result As New List(Of Tuple(Of Integer, Integer))
        result.Add(New Tuple(Of Integer, Integer)(100, 1))
        result.Add(New Tuple(Of Integer, Integer)(100, 2))
        result.Add(New Tuple(Of Integer, Integer)(10, 2))
        result.Add(New Tuple(Of Integer, Integer)(10, 1))
        Return result
    End Function

    Function ComparisonTwoTuples(ByVal tupleA As Tuple(Of Integer, Integer),
                                 ByVal tupleB As Tuple(Of Integer, Integer)) As Integer
        ' Compare the first Item of each tuple in ascending order.
        Dim part1 As Integer = tupleA.Item1
        Dim part2 As Integer = tupleB.Item1
        Dim compareResult As Integer = part1.CompareTo(part2)
        ' If not equal, return the comparison result.
        If compareResult <> 0 Then
            Return compareResult
        End If
        ' Compare the second item of each tuple in descending order.
        Return tupleB.Item2.CompareTo(tupleA.Item2)
    End Function

    Sub Main()
        Dim data As List(Of Tuple(Of Integer, Integer)) = GetData()
        data.Sort(New Comparison(Of Tuple(Of Integer, Integer))(AddressOf ComparisonTwoTuples))

        For Each value In data
            Console.WriteLine("SORTED ON 2 VALUES: {0}", value)
        Next

    End Sub
End Module
SORTED ON 2 VALUES: (10, 2)
SORTED ON 2 VALUES: (10, 1)
SORTED ON 2 VALUES: (100, 2)
SORTED ON 2 VALUES: (100, 1)

Optimization, SortedSet

Sort() is sometimes a slow part of a program. There is a way to avoid calling Sort: we can keep the data sorted as we go along. Consider SortedSet.

Module Module1

    Sub Main()
        Dim m As Integer = 100000
        Dim s1 As Stopwatch = Stopwatch.StartNew
        ' Version 1: add 3 strings to List, then Sort it.
        For i As Integer = 0 To m - 1
            Dim list = New List(Of String)
            list.Add("zebra")
            list.Add("20")
            list.Add("bird")
            list.Sort()
            If list.Count <> 3 Then
                Return
            End If
        Next
        s1.Stop()

        Dim s2 As Stopwatch = Stopwatch.StartNew
        ' Version 2: add 3 strings to SortedSet.
        For i As Integer = 0 To m - 1
            Dim sorted = New SortedSet(Of String)
            sorted.Add("zebra")
            sorted.Add("20")
            sorted.Add("bird")
            If sorted.Count <> 3 Then
                Return
            End If
        Next
        s2.Stop()

        Dim u As Integer = 1000000
        Console.WriteLine(((s1.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
        Console.WriteLine(((s2.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
    End Sub

End Module
405.52 ns    List, Sort
357.17 ns    SortedSet

Summary

Sorting strings can be done in many ways. Here we examined ways to sort strings in alphabetical order, and also reverse alphabetical order. Built-in functions are well-tested.