List index out of bounds whilst deleting items - delphi

Question

I solved my problem but I need to know why this problem raised to me ?!
I write a project that load file to listBox then delete the strings one by one,
but when I delete listBox strings this exception raised to me!
list index out of bounds (5) !
I type this for loop to read list box and delete strings:
for i := 0 to ListBox3.Count -1 do
begin
ShowMessage(ListBox3.Items[i]);
ListBox3.items.Delete(i);
end;
and my problem solved by do a little change in for-loop statement
for i := ListBox3.Items.Count - 1 downto 0 do
begin
ShowMessage(ListBox3.Items[i]);
ListBox3.items.Delete(i);
end;
Why the first statement raised an exception, and the second one work fine ?

Answer 1

By deleting items moving forward, you're cutting the branch off that you're standing on. :-) The upper bounds of the loop is only evaluated once, before the loop begins, and if you delete items there are now fewer in the list than there were when the bound was calculated.
Loop limit is evaluated (for example, List.Count - 1 = 5). Valid indexes into it are [0..4]
The loop starts, and you retrieve List[0] and delete it. List Count = 4,
bounds is still 5
The index is incremented, you retrieve and delete List[1]. List Count = 3, bounds is still 5
The index is incremented, you retrieve and delete List[2]. List Count = 2, bounds is still 5.
The index is incremented, you retrieve List[3] - Oops! There are only 2 items in the list, now at indexes [0..1] - List index out of bounds(3).
By iterating backwards, even though the bounds is still only calculated at the beginning, you're removing the items from the end and decrementing the count at the same time.
Bounds is 5, and you retrieve List[4] and delete it. Count is now 4, bounds is still 5
Index is decremented, and you retrieve List[3] and delete it. Count is now 3, bounds is still 5
Index is decremented, and you retrieve List[2] and delete it. Count is now 2, bounds is still 5.
Index is decremented, and you retrieve and delete List[1]. Count is now 1, bounds is still 5.
Index is decremented, and you retrieve and delete List[0]. List is now empty, but we've reached the terminating condition of the loop (downto 0) and the loop exits safely.

Answer 2

Each time you delete an item from the list, the list contains one less item. However, the for statement copies the list count at the beginning, and is not updated upon each iteration. Therefore, by the time you get halfway through the list, the counter i becomes larger than the current (new) list count, even though the list no longer contains the original number of items.
As an alternative, you could also do a loop like this:
while ListBox3.Items.Count > 0 do begin
ShowMessage(ListBox3.Items[0]);
ListBox3.items.Delete(0);
end;

Answer 3

There may be situation when you DO NOT delete certain items. Then the generic approach would be
i := 0;
while i < ListBox3.Items.Count do
begin
ShowMessage(ListBox3.Items[i]);
if <wantToDelete> then // some condition there
ListBox3.Items.Delete(i)
else
Inc(i);
end;