Collection size incosistency

[Alex Teslin]

Hi,

I am doing some tests on collectons and find some unexpected results.

eg, with ArrayList i am adding values to ArrayList timing that and immediately removing those values.

Part of the code:

for(int i = 0; i < SIZE; i++) {
values.add(new Integer(nextValue()));
}
for(int i = 0; i < values.size(); i++) {
values.remove(i);
}

Now, after removing values when i print the size of the collection, there still exactly half of the values left.

Can any one please tell me why is it half empty? I am looping every index until the values.size() to remove them. I just don't undertand?

[Alex Teslin]

I think because ArrayList doubles the initial size once the size is full (I think)

[Reid M. Pinchback]

Take a very close look at the loop condition in the second loop. Compare it to what is going on in the body of that second loop. Your eyes are telling your brain something that isn't true. You'll laugh when you figure it out. Everybody probably goes through it once.
[ February 16, 2006: Message edited by: Reid M. Pinchback ]

[Alex Teslin]

Is it because the value.size decrements and the i loop increments the same time?

[Reid M. Pinchback]

Yup. You are calling a method on an object that you are mutating. Every time you ask it for the size, it gives it to you. Just not the same size.

[Alex Teslin]

Thank you for your tip

[Stefan Wagner]

I would say, asking for the size isn't wrong, but incrementing, since remove (i) will make element i diappear, and element (i+1) is becoming i.

int SIZE = 20;
List <Integer> values = new ArrayList <Integer> (SIZE);
for(int i = 0; i < SIZE; i++)
{
values.add(new Integer(i));
}
for(int i = 0; i < values.size(); i++)
{
values.remove(i);
}
for (Integer inti : values)
System.out.println (inti.toString ());

will show what's happening, and

for(int i = 0; i < values.size (); )
{
values.remove (i);
}

will solve the problem.

[Reid M. Pinchback]

One thing to note (since this is the performance forum) is that for array-backed lists all these variations are likely sub-optimal. Lots of array copying going on. Wouldn't be an issue for elements deleted from the end, and the original problem Alex had wouldn't have even shown up in that case.
[ February 16, 2006: Message edited by: Reid M. Pinchback ]

[Vlado Zajac]

You can also remove all values in one method call

values.clear();