Remove items from a list during For Each Loop

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I'm trying to reduce a List during looping through it without success. From other posts I realise that this isn't possible as it causes an exception to be thrown. I cant seem to get other solutions offered working for my situation.

I cant run backwards as the data is order sensitive.

I think (from reading other solutions) that using a for i = 0 to might work instead of For Each XData as RangePoints, but I'm unsure how to do this as I can't also figure out to retain the ability to get access to the properties of XData used inside the loop in question.

As I'm inside a second loop will this cause difficulties?

....been struggling with this all day... it's driving me crazy so any help appreciated.

VB

For Each TData As RangePoints In DataList

    For Each XData As RangePoints In RemainingXPoints.Skip(1)
        XTRange = XData.Range - TData.Range
        myAngle = If(XTRange < Thresh, NearAngle, FarAngle)
        Dim Tj = New Double() {TData.Tx, TData.Ty, TData.Tz}
        Dim Bj = New Double() {TData.Bx, TData.By, TData.Bz}
        Dim Xk = New Double() {XData.Tx, XData.Ty, XData.Tz}
        If isLyingInCone(Xk, Tj, Bj, myAngle) = True Then
            ShadowList1.Add(CInt(XData.Tx) & "," & CInt(XData.Ty) & "," & CInt(XData.Tz))
        Else
            'Redefine RemainingXPoints to reduce it somehow?
        End If
    Next XData 'K

Next TData

Posted 25-Oct-15 6:03am

SheepRustler

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3 solutions

Solution 2

I'd try storing the indices of the items to be removed in a Stack. Then Pop each index off the Stack and remove that item from the List.

Posted 25-Oct-15 6:40am

PIEBALDconsult

v2

Comments

SheepRustler 25-Oct-15 13:35pm

OK, A Stack. New to me but looks straightforward. But isnt the problem the same when then removing that item from the list - just modifying the list again?

PIEBALDconsult 25-Oct-15 15:20pm

No. As with the other solutions, you remove the items after the foreach completes.

Solution 3

I guess that this is a follow up from previous question.
Your code is not complete since we don't know what is RemainingXPoints.
I will assume that all points are in DataList
Here is a piece of code that check if a point is in cone and gives you the list of points not in cone.

VB

Points= 0
For Scan= 1 to DataList.GetUpperBound(0)
    Visible= True
    For Check= 0 To Points
        XTRange = DataList(Check).Range - DataList(Scan).Range
        myAngle = If(XTRange < Thresh, NearAngle, FarAngle)
        Dim Tj = New Double() {DataList(Scan).Tx, DataList(Scan).Ty, DataList(Scan).Tz}
        Dim Bj = New Double() {DataList(Scan).Bx, DataList(Scan).By, DataList(Scan).Bz}
        Dim Xk = New Double() {DataList(Check).Tx, DataList(Check).Ty, DataList(Check).Tz}
        If isLyingInCone(Xk, Tj, Bj, myAngle) = True Then
            Visible= False
            Exit for
        End If
    Next
    If Visible Then
        Points= Points + 1
        DataList(Points) = DataList(Scan)
    End If
Next scan
' all valid points are between 0 and Points
For Scan= 0 to Points
Next Scan

The code is untested

Posted 25-Oct-15 16:34pm

Patrice T

Updated 25-Oct-15 16:50pm

v2

Comments

SheepRustler 26-Oct-15 9:30am

Hello ppolymorphe & thanks for your post,
Yes this is a follow up. To reduce the time taken I've now split up my data into 4 data sets (one for each 2D quadrant) and run each one a separate BackgroundWorker. This has speeded things up dramatically but more importantly reduced the memory consumption as the number of comparisons is much less.
I'm currently 12 hours into a run through a 1.3 million point data set on my PC so I can't test your code on some real data, but I will do as soon as its finished and I've cooked my breakfast on the processor...........
Your help is appreciated.

Patrice T 26-Oct-15 10:33am

I think more dramatic improvements are possible, but I think the whole proces must be reviewed.

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bjoho · Accepted Answer · 2015-10-25T06:14:00

Solution 1

You should collect all the items you want to remove from list1 into another list e.g called toBeRemoved. Then you can iterate over toBeRemoved and remove each item from list1. toBeRemoved should be local and will vanish as soon as it loses scope.

Posted 25-Oct-15 6:14am

bjoho

Comments

SheepRustler 25-Oct-15 12:22pm

Hello bjoho,
I think I tried this with the code below, but I still get an exception. Maybe because I'm in a loop in a loop...?
Or have I misinterpreted your suggestion?

For Each TData As RangePoints In DataList
For Each XData As RangePoints In RemainingXPoints.Skip(1)
XTRange = XData.Range - TData.Range
myAngle = If(XTRange < Thresh, NearAngle, FarAngle)
Dim Tj = New Double() {TData.Tx, TData.Ty, TData.Tz}
Dim Bj = New Double() {TData.Bx, TData.By, TData.Bz}
Dim Xk = New Double() {XData.Tx, XData.Ty, XData.Tz}
If isLyingInCone(Xk, Tj, Bj, myAngle) = True Then
ShadowList1.Add(CInt(XData.Tx) & "," & CInt(XData.Ty) & "," & CInt(XData.Tz))
Else
NotShadowedYet.Add(XData) 'Collect Remaining Points?
End If
Next XData 'K
RemainingXPoints = NotShadowedYet
Next TData

bjoho 25-Oct-15 12:44pm

I think it is that RemainingXPoints and NotShadowedYet is the same reference (object) and the statement "NotShadowedYet.Add(XData)" in the inner loop therefore changes the collection that controls the inner loop.

SheepRustler 25-Oct-15 13:40pm

Yes, I guess so as the exception thrown is the same even though RemainingXPoints = NotShadowedYet is outside the inner RemainingXPoints loop..... must be something to do with having two loops, so I'm no nearer.

bjoho 25-Oct-15 14:35pm

Think of it like this; At the outset you have all points in one collection NotShadowed, WHILE there are still some points in that collection (the outer loop) iterate over those and find the ones (toBeRemoved) that are now shadowed under a new angle. Outside (after) the inner loop, remove the new shadowed points from the NotShadowed collection. This goes on WHILE there are still some points left in NotShadowed. BREAK the outer loop on some criteria if you want.

SheepRustler 25-Oct-15 15:16pm

Thanks bjoho,
I got the hint I think! :) I'll experiment for a WHILE....
MeanWHILE I think I found a LINQ solution that seems to work below:

Thanks for your help, its very much appreciated !

Dim ToDeleteList As New List(Of RangePoints)

For Each TData As RangePoints In DataList
For Each XData As RangePoints In RemainingXPoints.Skip(1).ToList
XTRange = XData.Range - TData.Range
myAngle = If(XTRange < Thresh, NearAngle, FarAngle)
Dim Tj = New Double() {TData.Tx, TData.Ty, TData.Tz}
Dim Bj = New Double() {TData.Bx, TData.By, TData.Bz}
Dim Xk = New Double() {XData.Tx, XData.Ty, XData.Tz}
If isLyingInCone(Xk, Tj, Bj, myAngle) = True Then
ShadowList1.Add(CInt(XData.Tx) & "," & CInt(XData.Ty) & "," & CInt(XData.Tz))
ToDeleteList.Add(XData)
End If
Next XData
RemainingXPoints = RemainingXPoints.Except(ToDeleteList).ToList
Next TData

bjoho 25-Oct-15 15:33pm

Good. Same same. It's btw not linq that helped you. It's rather the fact that RemainingXPoints and NotShadowed is no longer the same object. Isn't it a bit funny that solution 2 has got five stars?