CODEWITHSUNDEEP
vb.nethashtable
dg90
dg90

Reputation: 1263

How to order a HashList?

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

Upvotes: 1

Views: 1190

Answers (4)

San
San

Reputation: 427

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

Upvotes: 1

seand
seand

Reputation: 5296

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.

Upvotes: 0

Oded
Oded

Reputation: 499062

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.

Upvotes: 2

Diego
Diego

Reputation: 18359

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

Upvotes: 0

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