How to iterate through a list that is being modified?

Question

I have a list of rows from a dataset that I need to iterate through.

The problem is that the processing in the iteration may delete one or more rows from the list.

Since the list is being modified, I can't use a foreach() loop.

But since it is possible some of the deletions may occur at elements BEFORE the one I'm processing, I also can't use a for() loop (i.e, if I'm processing element , and that results in the deletion of element and also other elements , I can't think of a way to adjust i to correctly point to the element following the one that I was processing).

How would you tackle this problem? My current thought it is to always process the first element in the list. If it gets deleted, process the new first element. If it doesn't get deleted, the move it to an "alreadyProcessed" list, and process the new first element.

Is there an easier way?

Answer 1

Typically this is done with a reverse loop:

List<string> Items = ...
for(int i = Items.Count - 1; i >= 0; i--)
{
   if(Items[i] == "DELETE ME")
   {
      Items.RemoveAt(i);
   }
}

This causes the items to be processed in reverse order, so if you delete an item, it does not affect the position of any items still to be processed.

Answer 2

for(int i = list.Length -1, i >= 0; i--)
{
   // process and delete if you want
}

Answer 3

When modifying a list I'm iterating through, I always find it easiest to build a new list with the items I want to keep, and then use the new list to do whatever it was I was going to do.

It really depends on what you're doing with the data when you're done, I suppose.

Answer 4

int i = 0;
while (i < dataSet.Tables[0].Rows.Count) {
    if (some_condition) {
       dataSet.Tables[0].Rows.RemoveAt(i);
       continue;
    }
    i++;
}

Answer 5

If you can get your data into a linked list, you're golden.