从arrayList中删除特殊元素

Question

Hi I have written this part of code .before the for loop I have this arraylist :

[X :49.0 Y: 113.0 , angle :0.0, X :141.0 Y: 106.0 , angle :0.0, X :110.0 Y: 185.0 , angle: 1.0768211289482128, X :99.0 Y: 139.0 , angle: 1.9961041242180873, X :103.0 Y: 126.0 , angle : 2.4208694638343324] which show "x" and "y" and "angle" of some points. but in the for loop I want to remove those elements that are more than

X :110.0 Y: 185.0 , angle: 1.0768211289482128

BUT it print this array for me :

[X :49.0 Y: 113.0angle0.0, X :141.0 Y: 106.0 , angle:0.0, X :110.0 Y: 185.0 , angle: 1.0768211289482128, X :103.0 Y: 126.0 , angle: 2.4208694638343324] which is incorrect

 int size  = list .size();
    for (int i=0;i<size;i++) {
        if (list.get(i).getAngle() > pivot.getAngle() ) {
            list.remove(i);
            size--;

        }
    }

please help me thanks

Answer 1

I recommend you to get an Iterator from the list and use iter.hasNext() , iter.next() and iter.remove() , like this:

 Iterator<Point> iter = list.iterator();
 while (iter.hasNext())
     if (iter.next().getAngle() > pivot.getAngle())
         iter.remove();

---

Another option would be to put them in a SortedMap , let the angles represent the keys, and use tailMap(pivot.getAngle()) to get those points with a larger angle than the pivot.

Answer 2

Whenever you remove an element it shifts the indices of the following elements down by 1. The simplest fix is to go in reverse:

for (int i  = list.size() - 1; i >= 0; i--) {
    if (list.get(i).getAngle() > pivot.getAngle() ) {
        list.remove(i);
    }
}

Answer 3

When you remove the element, you should also lower your index value ( i ) as otherwise it will skip the next value - because the values will have shifted. For example, imagine you originally had values { a, b, c, d, e } . On the iteration where i==2 , you'd look at c . If you then remove c you're left with { a, b, d, e } ... when i is incremented to 3 in the iteration part of the for statement, that means list.get(i) would return e - so you'd never look at d .

You can fix this by adding an i--; statement inside your if block... but a simpler approach is to work backwards from the end:

for (int i = list.size() - 1; i >= 0; i--) {
    if (list.get(i).getAngle() > pivot.getAngle()) {
        list.remove(i);
    }
}

Alternatively, you might want to consider creating a new list which only contains the elements you do want to keep:

List<Foo> validValues = new ArrayList<Foo>();
for (Foo foo : list) {
    if (foo.getAngle() <= pivot.getAngle()) {
        validValues.add(foo);
    }
}