Clear all array list data

Why doesn't the code below clear all array list data?

        Console.WriteLine("Before cleaning:" + Convert.ToString(ID.Count));
        //ID.Count = 20
        for (int i = 0; i < ID.Count; i++)
        {
            ID.RemoveAt(i);
        }
        Console.WriteLine("After cleaning:" + Convert.ToString(ID.Count));
        //ID.Count = 10

Why is 10 printed to the screen?

Maybe there is another speci开发者_StackOverflow社区al function, which deletes everything?

You're only actually calling RemoveAt 10 times. When i reaches 10, ID.Count will be 10 as well. You could fix this by doing:

int count = ID.Count;
for (int i = 0; i < originalCount; i++)
{
    ID.RemoveAt(0);
}

This is an O(n²) operation though, as removing an entry from the start of the list involves copying everything else.

More efficiently (O(n)):

int count = ID.Count;
for (int i = 0; i < originalCount; i++)
{
    ID.RemoveAt(ID.Count - 1);
}

or equivalent but simpler:

while (ID.Count > 0)
{
    ID.RemoveAt(ID.Count - 1);
}

But using ID.Clear() is probably more efficient than all of these, even though it is also O(n).

`Array.Clear()` 

removes all items in the array.

`Array.RemoveAt(i)` 

removes the element of ith index in the array.

ArrayList.Clear Method
Removes all elements from the ArrayList.

for more detail : http://msdn.microsoft.com/en-us/library/system.collections.arraylist.clear.aspx

After removing 10 items, ID.Count() == 10 and i == 10 so the loop stops.

Use ID.Clear() to remove all items in the array list.

Use the clear() Method

or

change ID.RemoveAt(i); to ID.RemoveAt(0);

Whenever an element is removed from the collection, its index also changes. Hence when you say ID.RemoveAt(0); the element at index 1 now will be moved to index 0. So again you've to remove the same element (like dequeuing). until you reach the last element. However if you want to remove all the elements at once you can better use the Clear() method.

Your code does:

ID.RemoveAt(0);
...
ID.RemoveAt(9);
ID.RemoveAt(10); \\ at this point you have already removed 10 
                 \\ items so there is nothing left on 10- 19, but you are left with 
                 \\ the 'first' 10 elements
...
ID.RemoveAt(19);

Generally speaking your method removes every second element from the list..

Use ArrayList.Clear instead as other have mentioned.