Arraylist sort and binarysearch

[T120]

Guys - I have a windows app and I'm trying to cache images in memory. I have:-

Public TileCache As New ArrayList

    Structure TileCacheDescriptor

        Public TileURL As String
        Public TileLayer As String
        Public TileImage As Image

    End Structure

   Dim TileDetails As New TileCacheDescriptor
   TileDetails.TileImage = ThisMap.TileGrid(C - Col + w, T - CentreTile + w).TileImage
   TileDetails.TileURL = src
   TileDetails.TileLayer = Layer
   TileCache.Add(TileDetails)
   TileCache.Sort()
                   

I think I have to sort the array to be able to use binarysearch.
So I need to sort the arraylist on TileDetails.TileURL(if possible) and then binarysearch on it.

Any hints much appreciated

[Grimfort]

An object needs to implement IComparable for you to tell it how to sort.

Change your TileCacheDescriptor into a class and implement it. I can't tell why you need to sort it however .

[amrita]

Create a class which implements Icomparer
Use its compare function to sort the array items.

After that use ArrayList.Sort(New ComparereClassYouCreated)

[T120]

Thanks Guys but I'm a bit lost now

Should that be ICompare or IComarable?

Imports Microsoft.VisualBasic

Public Class TileCache : Implements IComparable

    Public Structure TileCacheDescriptor

        Public TileURL As String
        Public TileLayer As String
        Public TileImage As Image

    End Structure

End Class

It's now telling me:-

Error	1	Class 'TileCache' must implement 'Function CompareTo(obj As Object) As Integer' for interface 'System.IComparable'.	C:\Dot Net 2008 Code\BSGPSV3530\TileCache.vb	3	37	BSGPS

I read in several places that an arraylist needs to be sorted so that binarysearch can be used. And if it uses binary chop then it must be sorted.

[amrita]

implements Icomparer
and implements its compare method

[amrita]

This may help to start with

http://www.knowdotnet.com/articles/icomparer.html


http://weblogs.asp.net/jan/archive/2003/05/05/6479.aspx

[.paul.]

try this. it uses a List(Of TileCacheDescriptor) instead of an arraylist, + array.find instead of binarysearch:

Public Class Form1
 
    Public TileCache As New List(Of TileCacheDescriptor)
 
    Structure TileCacheDescriptor
        Public TileURL As String
        Public TileLayer As String
        Public TileImage As Image
    End Structure
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
 
        Dim TileDetails As New TileCacheDescriptor
        TileDetails.TileImage = ThisMap.TileGrid(c - Col + w, T - CentreTile + w).TileImage
        TileDetails.TileURL = "testURL1" 'src
        TileDetails.TileLayer = "testLayer1" 'Layer
        TileCache.Add(TileDetails)
        TileDetails = New TileCacheDescriptor
        TileDetails.TileImage = ThisMap.TileGrid(c - Col + w, T - CentreTile + w).TileImage
        TileDetails.TileURL = "testURL2" 'src
        TileDetails.TileLayer = "testLayer2" 'Layer
        TileCache.Add(TileDetails)
        TileDetails = New TileCacheDescriptor
        TileDetails.TileImage = ThisMap.TileGrid(c - Col + w, T - CentreTile + w).TileImage
        TileDetails.TileURL = "testURL3" 'src
        TileDetails.TileLayer = "testLayer3" 'Layer
        TileCache.Add(TileDetails)
        TileDetails = New TileCacheDescriptor
        TileDetails.TileImage = ThisMap.TileGrid(c - Col + w, T - CentreTile + w).TileImage
        TileDetails.TileURL = "testURL4" 'src
        TileDetails.TileLayer = "testLayer4" 'Layer
        TileCache.Add(TileDetails)
 
        Dim c As New comparer
        c.sortBy = comparer.sortOptions.TileURL
 
        TileCache.Sort(c)
 
        PictureBox1.Image = Array.Find(TileCache.ToArray, Function(item As TileCacheDescriptor) item.TileURL = "testURL4").TileImage
 
    End Sub
End Class
 
Public Class comparer
    Implements System.Collections.Generic.IComparer(Of WindowsApplication1.Form1.TileCacheDescriptor)
 
    Public Enum sortOptions
        TileURL
        TileLayer
    End Enum
 
    Private _sortBy As sortOptions
    Public Property sortBy() As sortOptions
        Get
            Return _sortBy
        End Get
        Set(ByVal value As sortOptions)
            _sortBy = value
        End Set
    End Property
 
    Public Function Compare(ByVal x As Form1.TileCacheDescriptor, ByVal y As Form1.TileCacheDescriptor) As Integer Implements System.Collections.Generic.IComparer(Of Form1.TileCacheDescriptor).Compare
        If _sortBy = sortOptions.TileURL Then
            Return x.TileURL.CompareTo(y.TileURL)
        ElseIf _sortBy = sortOptions.TileLayer Then
            Return x.TileLayer.CompareTo(y.TileLayer)
        End If
    End Function
 
End Class

[Grimfort]

Thanks Guys but I'm a bit lost now

Should that be ICompare or IComarable?

I read in several places that an arraylist needs to be sorted so that binarysearch can be used. And if it uses binary chop then it must be sorted.

You can use either, but the ICompare does specifically say about the binarysearch (I did not know that ). Can you not use a key to get this information, when and why do you need to sort it I still can't work out. There may be better ways with generic collections as pointed out by .paul.

[T120]

Guys

I'm still confused. All I want to do is create a caches of images in memory to try and save time on hard drive accesses!

If the arraylist.binarysearch uses the binary chop techneque then the array must be sorted.

If I'm "barking up the wrong tree" is there a better way of achieving this?

[Grimfort]

How do you access the images? By name, by a binary version of the image, by a number, its position?

[T120]

I intended accessing them by the originating web site url ie http://tile.openstreetmap.org/mapnik/17/64234/44116.png My original struture contained the URL and the image. But it could also be the locally stored folder/file.