从List中删除特定的字符串值

Question

I have a List of string values and and some of the values contains xxx or XXX in front.

xxxRed
xxxYellow
xxxxCareful with that axe Eugene!
xxxxxxdedicum aceasta frumoasa Melodia
xxxxLeaders
xxxxWorking Around - titles
XXXXXNothing To Fear
xxxxAvoiding standards
xxxFirst Aid

List<string> lstTitles = new List<string>();

This is what I have tried

for (int i=0; i < lstTitles.Count; i++)
            {
                string title = lstTitles[i].ToLower().Trim();
                if (title[0] == 'x')
                {
                    lstTitles.Remove(lstTitles[i]);

                }
            }

Problem I have is that only some of the values are removed but not all of them.

Is there perhaps a better way of removing these values?

Answer 1

Use RemoveAll method

lstTitles.RemoveAll(s => s[0] == 'x' || s[0] == 'X');

and you may want to use StartsWith instead of comparing first char.

lstTitles.RemoveAll(s => s.StartsWith("x",StringComparison.InvariantCultureIgnoreCase));

Answer 2

Problem I have is that Only some of the values are removed but not all of them.

Because you're skipping items. When you call Remove() , the next item will be at index i , but you'll increase i in the next loop.

It can be solved by iterating over a copy of the list, and removing unwanted items in the original:

foreach (var item in lstTitles.ToList())
{
    if (item.StartsWith("x", StringComparison.InvariantCultureIgnoreCase))
    {
        lstTitles.Remove(item);
    }
}

Though this involves creating a copy of the list, which isn't really useful, as well as calling Remove() which itself is far from performant.

So you could invert your for-loop, to remove the last items first which doesn't change the indexing for unprocessed items:

for (int i = lstTitles.Count - 1; i > 0; i--)
{
    if (lstTitles[i].StartsWith("x", StringComparison.InvariantCultureIgnoreCase))
    {
        lstTitles.RemoveAt(i);
    }
}

But as @I4V points out, all of this logic already is in List<T>.RemoveAll() , which is nicer to read and probably optimized for some edge cases, so there's little use to hand-code it again.

Answer 3

That's because your skipping values.

Suppose your list contains ['xVal1', 'xVal2', 'val3', 'xVal4', 'val5']. At first your i is 0, and you look at list[0], which is 'xVal1', so you remove it.

Now your list contains ['xVal2', 'val3', 'xVal4', 'val5'], and your i is 1. So you look at list[1] which is 'val3'. You ignored xVal2 !

You can start at the back of the list and go to the front, although you will still have a potential bug in case there are identical values you remove.

A shorter way would be to use LINQ:

var newList = lstTitles.Where(title=>!title.StartsWith('xxx'))

Answer 4

Instead of ToLower you should use the overload of StartsWith which allows to pass a StringComparison.OrdinalIgnoreCase .

Then use List.RemoveAll which is the most readable, most efficient and shortest approach:

lstTitles.RemoveAll(s => s.TrimStart().StartsWith("x", StringComparison.OrdinalIgnoreCase));

Demo

Answer 5

I think, you'd better just create a new list this way

list = list
    .Where(i => ! i.StartsWith("xxx", StringComparison.InvariantCultureIgnoreCase))
    .ToList();

It would have a O(n) complexity whereas, trying to remove then 1 by 1 would be in O(n^2).

Answer 6

This could work also :

list.RemoveAll(i => i.StartsWith("xxx", StringComparison.InvariantCultureIgnoreCase));

Handles all cases and without a second list.