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From: bob <bob@coolgroups.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: iteration blues
Date: Fri, 4 Nov 2011 13:42:58 -0700 (PDT)
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On Nov 4, 2:34=A0am, Robert Klemme <shortcut...@googlemail.com> wrote:
> On 11/03/2011 04:37 PM, bob wrote:
>
>
>
>
>
>
>
>
>
> > So, I wrote this code for some particle effects:
>
> > package com.coolfone.particles;
>
> > import java.util.Iterator;
> > import java.util.Vector;
>
> > import javax.microedition.khronos.opengles.GL10;
>
> > public class FireManager {
> > =A0 =A0static Vector<Particle> =A0particles =3D new Vector<Particle>();
>
> > =A0 =A0public static void startfire(float x, float y) {
> > =A0 =A0 =A0 =A0 =A0 =A0for (int ctr =3D 0; ctr< =A0100; ctr++) {
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Particle p =3D new Particle();
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0p.x =3D (float) (x + Math.random=
()-.5);
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0p.y =3D (float) (y + Math.random=
()-.5);
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0p.dx =3D (float) (Math.random()-=
.5)/4f;
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0p.dy =3D (float) (Math.random()-=
.5)/4f;
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0p.timeleft =3D (int) (Math.rando=
m() * 50 + 50);
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0particles.add(p);
> > =A0 =A0 =A0 =A0 =A0 =A0}
> > =A0 =A0}
>
> > =A0 =A0public static void burnfire() {
> > =A0 =A0 =A0 =A0 =A0 =A0Iterator<Particle> =A0i =3D particles.iterator()=
;
> > =A0 =A0 =A0 =A0 =A0 =A0Vector<Particle> =A0removelist =3D new Vector<Pa=
rticle>();
> > =A0 =A0 =A0 =A0 =A0 =A0while (i.hasNext()) {
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Particle p =3D i.next();
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0p.move();
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0p.timeleft--;
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0if (p.timeleft =3D=3D 0) removel=
ist.add(p);
>
> > =A0 =A0 =A0 =A0 =A0 =A0}
> > =A0 =A0 =A0 =A0 =A0 =A0particles.removeAll(removelist);
>
> > =A0 =A0}
>
> > =A0 =A0public static void drawfire(GL10 gl) {
> > =A0 =A0 =A0 =A0 =A0 =A0Iterator<Particle> =A0i =3D particles.iterator()=
;
> > =A0 =A0 =A0 =A0 =A0 =A0while (i.hasNext()) {
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Particle p =3D i.next();
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0p.draw(gl);
> > =A0 =A0 =A0 =A0 =A0 =A0}
> > =A0 =A0}
>
> > }
>
> > I'm concerned about inefficiency in the burnfire function. =A0Does
> > anyone know how to rewrite this quickly if particles was a linked
> > list? =A0The main issue is that I'm not sure if removing items during
> > iteration messes up the iterator.
>
> I'm surprised nobody seems to mention Iterator.remove().
>
> public static void burnfire() {
> =A0 =A0for (final Iterator<Particle> i =3D particles.iterator(); i.hasNex=
t();) {
> =A0 =A0 =A0final Particle p =3D i.next();
> =A0 =A0 =A0p.move();
> =A0 =A0 =A0p.timeleft--; =A0// Direct access to member, bad!
>
> =A0 =A0 =A0if (p.timeleft =3D=3D 0) {
> =A0 =A0 =A0 =A0iter.remove();
> =A0 =A0 =A0}
> =A0 =A0}
>
> }
>
> This can be used regardless of container type. =A0Efficiency depends on
> the ratio of removed elements. =A0If you remove much and do not need
> indexed access (i.e. via List.get(int)) you can use a LinkedList.
> Otherwise use ArrayList as indicated already. =A0There is no point in
> using Vector these days any more.
>
> And btw, do not be concerned about performance, measure it. =A0Results ma=
y
> be surprising.
>
> Kind regards
>
> =A0 =A0 =A0 =A0 robert

Just what I needed.  Thanks.