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Groups > comp.lang.java.programmer > #9511

Re: iteration blues

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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 {
> 	static Vector<Particle>  particles = new Vector<Particle>();
>
> 	public static void startfire(float x, float y) {
> 		for (int ctr = 0; ctr<  100; ctr++) {
> 			Particle p = new Particle();
> 			p.x = (float) (x + Math.random()-.5);
> 			p.y = (float) (y + Math.random()-.5);
> 			p.dx = (float) (Math.random()-.5)/4f;
> 			p.dy = (float) (Math.random()-.5)/4f;
> 			p.timeleft = (int) (Math.random() * 50 + 50);
> 			particles.add(p);
> 		}
> 	}
>
> 	public static void burnfire() {
> 		Iterator<Particle>  i = particles.iterator();
> 		Vector<Particle>  removelist = new Vector<Particle>();
> 		while (i.hasNext()) {
> 			Particle p = i.next();
> 			p.move();
> 			p.timeleft--;
> 			if (p.timeleft == 0) removelist.add(p);
>
> 		}
> 		particles.removeAll(removelist);
>
> 	}
>
> 	public static void drawfire(GL10 gl) {
> 		Iterator<Particle>  i = particles.iterator();
> 		while (i.hasNext()) {
> 			Particle p = i.next();
> 			p.draw(gl);
> 		}
> 	}
>
> }
>
> I'm concerned about inefficiency in the burnfire function.  Does
> anyone know how to rewrite this quickly if particles was a linked
> list?  The 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() {
   for (final Iterator<Particle> i = particles.iterator(); i.hasNext();) {
     final Particle p = i.next();
     p.move();
     p.timeleft--;  // Direct access to member, bad!

     if (p.timeleft == 0) {
       iter.remove();
     }
   }
}

This can be used regardless of container type.  Efficiency depends on 
the ratio of removed elements.  If 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.  There is no point in 
using Vector these days any more.

And btw, do not be concerned about performance, measure it.  Results may 
be surprising.

Kind regards

	robert

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iteration blues bob <bob@coolgroups.com> - 2011-11-03 08:37 -0700
  Re: iteration blues Knute Johnson <nospam@knutejohnson.com> - 2011-11-03 08:51 -0700
    Re: iteration blues Lew <lewbloch@gmail.com> - 2011-11-03 09:32 -0700
      Re: iteration blues Arne Vajhøj <arne@vajhoej.dk> - 2011-11-04 21:00 -0400
        Re: iteration blues spk <jhic@speak.invalid> - 2011-11-05 07:48 -0400
  Re: iteration blues Henk van Voorthuijsen <voorth@xs4all.nl> - 2011-11-03 09:31 -0700
    Re: iteration blues Lew <lewbloch@gmail.com> - 2011-11-03 13:50 -0700
      Re: iteration blues Henk van Voorthuijsen <voorth@xs4all.nl> - 2011-11-04 08:07 -0700
    Re: iteration blues Arne Vajhøj <arne@vajhoej.dk> - 2011-11-04 21:02 -0400
  Re: iteration blues Roedy Green <see_website@mindprod.com.invalid> - 2011-11-03 10:08 -0700
  Re: iteration blues Travers Naran <tnaran@gmail.com> - 2011-11-03 22:22 -0700
  Re: iteration blues Robert Klemme <shortcutter@googlemail.com> - 2011-11-04 10:34 +0100
    Re: iteration blues Lew <lewbloch@gmail.com> - 2011-11-04 10:46 -0700
      Re: iteration blues Robert Klemme <shortcutter@googlemail.com> - 2011-11-04 23:55 +0100
        Re: iteration blues Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-11-04 21:06 -0400
        Re: iteration blues Lew <lewbloch@gmail.com> - 2011-11-04 20:30 -0700
        Re: iteration blues bob <bob@coolgroups.com> - 2011-11-05 12:40 -0700
          Re: iteration blues Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-11-05 16:14 -0400
            Re: iteration blues Lew <lewbloch@gmail.com> - 2011-11-05 13:41 -0700
    Re: iteration blues bob <bob@coolgroups.com> - 2011-11-04 13:42 -0700

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