How to order a HashList?

Question

Hi I got next function:
The input ArrayList results is ordered correctly.
The return value returnList is completly unordered.
I think to problem is the HashTable isn't ordered correctly..
Is there a way I can order the Hashtable or should I sort the returnList on someway?
I want to order or sort on a field.

Private Function FilterDepartementenSurveys(ByVal results As ArrayList) As ArrayList
    Dim hashTable As New Hashtable(results.Count)

    For Each resultaat As DTO.Results.Reporting.FilledInSurvey In results
        If Not hashTable.ContainsKey(resultaat.DepartmentCode) Then
            hashTable.Add(resultaat.DepartmentCode, New ArrayList)
        End If
        Dim arraylist As ArrayList = CType(hashTable(resultaat.DepartmentCode), Collections.ArrayList)
        arraylist.Add(resultaat)
    Next

    Dim returnList As New ArrayList
    For Each list As ArrayList In hashTable.Values
        returnList.Add(list)
    Next

    Return returnList
End Function

There's no such thing as a HashList. A Hashtable or Dictionary is fundamentally un-ordered. That's why they are fast. — Hans Passant, Mar 25 '12 at 20:07
In returnList you have a an array of arrays. What does "ordered" mean in this case for you? (there is no generic comparison for arrays) — Ando, Mar 26 '12 at 10:54

score 2 · Answer 1 · answered Mar 25 '12 at 20:05

From MSDN - HashTable:

Represents a collection of key/value pairs that are organized based on the hash code of the key.

This explains why you don't get the items in the order you expect - they get ordered by the hash of the key.

If you need to preserve ordering, use SortedList or OrderedDictionary.

score 1 · Answer 2 · answered Mar 25 '12 at 20:31

The best way to sort is to use a generic class for sorting which can be used across the application for sorting.

Use the below class which uses Linq and Lamda expression.

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Linq.Expressions

Public Class GenericSorter(Of T)

Public Function Sort(ByVal source As IEnumerable(Of T), _
                     ByVal sortBy As String, _
                     ByVal sortDirection As String) As IEnumerable(Of T)

    Dim param = Expression.Parameter(GetType(T), "item")

    Dim sortExpression = Expression.Lambda(Of Func(Of T, Object))_
    (Expression.Convert(Expression.[Property](param, sortBy), _
    GetType(Object)), param)

    Select Case sortDirection.ToLower
        Case "asc"
            Return source.AsQueryable().OrderBy(sortExpression)
        Case Else
            Return source.AsQueryable().OrderByDescending(sortExpression)
    End Select

End Function

End Class

For calling this class use the implementation below

Dim gs As New GenericSorter(Of FileDepartmentSurveyData)
SurveyFormatItems = gs.Sort(SurveyFormatItems.AsQueryable, _
                             sortExpression, sortDirection).ToArray()

http://www.codeproject.com/Articles/37541/Generic-Sorting-with-LINQ-and-Lambda-Expressions

score 0 · Answer 3 · edited May 23 '17 at 10:08

0

The order in a HashList is undetermined. Use a TreeMap instead. See Sort a Map<Key, Value> by values (Java)

edited May 23 '17 at 10:08

Community

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answered Mar 25 '12 at 20:04

Diego

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The question is about VB.NET, not Java. – Oded Mar 25 '12 at 20:05
The same data structure principles still apply though. – seand Mar 25 '12 at 20:09

score 0 · Answer 4 · answered Mar 25 '12 at 20:06

0

Regular hash tables don't define the order of elements. You probably need a tree structure instead. Retrieval from a tree is O(log N) vs. a hash's O(1). That may or may not be an issue.

answered Mar 25 '12 at 20:06

seand

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How to order a HashList?

4 Answers4