What's the best algorithm to remove items from a list that is reindexed when an item is removed?

Question

I'm dealing with a probably common situation that I don't even know how to formulate in a search.

I decided to ask for guidance, not necessarily to get any piece of code but to ask about what is the best approach for this kind of situation.

I have a list named arrIntIndexesOfJobsThatWillBeKilled containing the original indexes of the items that must be removed from another 5 lists.

However, when I remove an item from the other 5 lists, they are reindexed, and the indexes I once had in arrIntIndexesOfJobsThatWillBeKilled are now invalid.

I want to remove only the items linked to the original indexes...

What is the best algorithm to handle this situation?

arrIntIndexesOfJobsThatWillBeKilled = arrIntIndexesOfJobsThatWillBeKilled.Distinct.ToList

If arrIntIndexesOfJobsThatWillBeKilled.Count > 0 Then

    For Each Job As Integer In arrIntIndexesOfJobsThatWillBeKilled

        arrStrJobDescription2.RemoveAt(Job)
        arrStrPNDirectories2.RemoveAt(Job)
        arrIntJobIDDirectories2.RemoveAt(Job)
        arrStrSubDirectories2.RemoveAt(Job)
        arrStrNCFile2.RemoveAt(Job)

    Next

End If

Does this answer your question? [How to remove elements from a generic list while iterating over it?](https://stackoverflow.com/questions/1582285/how-to-remove-elements-from-a-generic-list-while-iterating-over-it) — Charlieface, May 22 '23 at 10:14

Guru Stron · Accepted Answer · 2023-05-22T15:46:23.397

3

You can change the order of removal to reverse one - delete bigger indexes first. Not fluent with VB, but in C# it will look like:

arrIntIndexesOfJobsThatWillBeKilled = arrIntIndexesOfJobsThatWillBeKilled
    .OrderByDescending(i => i) // assuming list of integers here, or .OrderDescending() for later framework
    .Distinct()
    .ToList();

And then proceed with deletion.

P.S.

If arrIntIndexesOfJobsThatWillBeKilled.Count > 0 Then check is redundant - foreach handles empty collections just fine.

edited May 22 '23 at 15:46

answered May 22 '23 at 07:47

Guru Stron

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3

The only change for VB.NET would be to replace `(i => i)` with `(Function(i) i)`. – Andrew Morton May 22 '23 at 13:40
Unfortunately the indexes are not ordered... So starting backwards won't solve this... I'll keep trying... – Daniel Santos May 22 '23 at 15:16
1

@DanielSantos what do you mean that they are not ordered? What stops you from ordering them? The code is provided in the answer. – Guru Stron May 22 '23 at 15:44
1

My bad @GuruStron - I jumped to a conclusion before testing it. It works! Thanks! – Daniel Santos May 22 '23 at 16:14

score 0 · Answer 2 · answered May 22 '23 at 16:19

After the guidance of the collaborators here, this is the code that works:

If arrIntIndexesOfJobsThatWillBeKilled.Count > 0 Then

arrIntIndexesOfJobsThatWillBeKilled = arrIntIndexesOfJobsThatWillBeKilled.OrderByDescending(Function(i) i).Distinct.ToList

   For Each Job As Integer In arrIntIndexesOfJobsThatWillBeKilled

       arrStrJobDescription2.RemoveAt(Job)
       arrStrPNDirectories2.RemoveAt(Job)
       arrIntJobIDDirectories2.RemoveAt(Job)
       arrStrSubDirectories2.RemoveAt(Job)
       arrStrNCFile2.RemoveAt(Job)

   Next

End If

Many thanks for once again educating this mediocre coder...

What's the best algorithm to remove items from a list that is reindexed when an item is removed?

2 Answers2