Remove list elements at given indices

Question

I have a list which contains some items of type string.

List lstOriginal;

I have another list which contains idices which sh

Answer 1

My in-place deleting of given indices as handy extension method. It copies all items only once so it is much more performant if large amount of indicies is to be removed.

It also throws ArgumentOutOfRangeException in case where index to remove is out of bounds.

 public static class ListExtensions 
 {
    public static void RemoveAllIndices<T>(this List<T> list, IEnumerable<int> indices)
    {
        //do not remove Distinct() call here, it's important
        var indicesOrdered = indices.Distinct().ToArray();
        if(indicesOrdered.Length == 0)
            return;

        Array.Sort(indicesOrdered);

        if (indicesOrdered[0] < 0 || indicesOrdered[indicesOrdered.Length - 1] >= list.Count)
            throw new ArgumentOutOfRangeException();

        int indexToRemove = 0;
        int newIdx = 0;

        for (int originalIdx = 0; originalIdx < list.Count; originalIdx++)
        {
            if(indexToRemove < indicesOrdered.Length && indicesOrdered[indexToRemove] == originalIdx)
            {
                indexToRemove++;
            }
            else
            {
                list[newIdx++] = list[originalIdx];
            }
        }

        list.RemoveRange(newIdx, list.Count - newIdx);
    }
}

Answer 2

        var array = lstOriginal.ConvertAll(item => new int?(item)).ToArray();
        lstIndices.ForEach(index => array[index] = null);
        lstOriginal = array.Where(item => item.HasValue).Select(item => item.Value).ToList();

Answer 3

How are you populating the list of indices? There's a much more efficient RemoveAll method that you might be able to use. For example, instead of this:

var indices = new List<int>();
int index = 0;
foreach (var item in data)
    if (SomeFunction(data))
        indices.Add(index++);

//then some logic to remove the items

you could do this:

data.RemoveAll(item => SomeFunction(item));

This minimizes the copying of items to new positions in the array; each item is copied only once.

You could also use a method group conversion in the above example, instead of a lambda:

data.RemoveAll(SomeFunction);

Answer 4

 lstIndices.OrderByDescending(p => p).ToList().ForEach(p => lstOriginal.RemoveAt((int)p));

As a side note, in foreach statements, it is better not to modify the Ienumerable on which foreach is running. The out of range error is probably as a result of this situation.

Answer 5

The reason this is happening is because when you remove an item from the list, the index of each item after it effectively decreases by one, so if you remove them in increasing index order and some items near the end of the original list were to be removed, those indices are now invalid because the list becomes shorter as the earlier items are removed.

The easiest solution is to sort your index list in decreasing order (highest index first) and then iterate across that.

Answer 6

for (int i = 0; i < indices.Count; i++)
{
    items.RemoveAt(indices[i] - i);
}