从列表中删除元素

Question

I have a list and I would like to remove some elements from it.

Problem I ran into is that If I want to remove more than one element, then I lose control of what I'm removing.

See, the code `mylist.Removeat(index) does the work just fine, however, if I have a list o let's say 10 elements and I need to remove 6 randomly, I don't seem to find a function that removes them all at once, instead, it removes the first element, then the second and so on and it is not really what I want.

Thanks,

EDIT:

Thanks all for your comments, the best way was doing a loop descending like this:

            int img_count = img.Count;

            for (int i = img_count; i >0; --i)
            {
                if (images_to_delete.Contains(i)) img.RemoveAt(i);
            }

Answer 1

I think your problem is that once you delete an item at a certain index, all the other indexes shift. Like if you had an array of {"a", "b", "c"} , and wanted to remove a and b by passing in indexes = [0,1] . Once you call removeAt(0) , now "b" is at index 0.

One way to handle it is to notice the fact that only higher indexes get shifted. So if you remove the 5th element for example, 0-4 are still going to have the same meaning; only 5 and up are affected.

Because of this, you can just remove the indexes in descending order (highest to lowest)! For example:

var vals = new[]{ "a", "b", "c" };
RemoveMultiple(vals, 0, 1);

and then RemoveMultiple looks like this:

public void RemoveMultiple<T>(List<T> list, params int[] indexes)
{
   // if this next line gives you a compiler error, you need to include
   // linq by putting a "using System.Linq;" at the very top of the file.
   var sortedIndexes = indexes.OrderByDescending(i => i);
   foreach(var indexToRemove in sortedIndexes)
   {
      list.RemoveAt(indexToRemove);
   }
}

That should work!

Answer 2

The question isn't extremely clear, but if you want to remove a specified number of elements at random indexes from the List here is a method that will do that. (This is example is with a list of String objects, but you could change it to use any object of course).

private static List<String> strList = new List<String>();        
        static void Main(string[] args)
        {
            strList.AddRange("A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z".Split(','));
            removeRandomItemsFromList(strList, 8);
            Console.WriteLine(String.Join(",",strList.ToArray()));
            Console.ReadLine();
        }

        private static void removeRandomItemsFromList(List<String> list, int numToRemove){
            Random randGenerator = new Random();
            if(numToRemove >= list.Count()){
                Console.WriteLine("Num to remove is as big or larger than list size, clearing list.");
                list.Clear();
                return;                
            }

            for (int i = 0; i < numToRemove; i++)
            {
                int randIndex = randGenerator.Next(list.Count - 1); //get random index within bounds of list
                list.RemoveAt(randIndex);
            }
        }

When I ran it I got the output: B,C,F,H,I,J,K,M,N,O,P,Q,R,U,V,X,Y,Z
But of course it will usually be different since it's removing random items.