locking Array Indexes

jozze

The script below represents a matrix of random PVectors.

I want to lock some indexes from that matrix. (4 of them, one at each corner):

I tried to concat() 2 different arrayLists for such end (the random values matrix + another one with the fixed values) but didn´t work. I also tried to append() the values independently but got same result.

Now I´m trying to use the set() method, so I could eventually replace the required item in the specified position in the list, but a NullPointerException warning is thrown (line 143). I wonder if the use of the arrayList() method is the correct here. thx

int elements=10; 
PVector centro;
ArrayList <Agent> a = new ArrayList <Agent>();
Repeler rep;
//ArrayList <FixedPoint> fp = new ArrayList <FixedPoint>();
Agent ag;

/**                                                       //list of PVectors to lock in the arrayList
    loc1= new PVector(100, 100);
 loc2= new PVector(100, 700);
 loc3= new PVector(1300, 100);
 loc4= new PVector(1300, 700);
 */

PVector loc1= new PVector(100, 100);
Agent vec1= new Agent (loc1);// 2nd constructor


void setup() {//______________________________________________________________
  size(1400, 800);

  frameRate(30);
  smooth();

  centro= new PVector(width/2, height/2);
  rep = new Repeler(centro);

  //a.add(vec1);

  for (int i=0; i<elements; i++) {
    a.add(new Agent());
    println(a.size());
  }
  a.set(0, vec1);
}

void draw() {//_______________________________________________________________
  background(99);

  for (int i=0; i<a.size(); i++) {
    ag= a.get(i); 

    PVector repulse = rep.attract(ag);
    ag.update(repulse);
    ag.run();
  }

  rep.display();
}


class Agent {//_______________________________________________________________
  PVector loc, vel, acc;
  PVector lol;

  Agent() {//1st constructor

    loc= new PVector(random(width), random(height));
    vel= new PVector (0, 0);
    acc= new PVector (random (-.01, .01), random (-.01, .01));
  }

  Agent (PVector _lol) {//2nd constructor for set() mode
    lol=_lol;
  }

  void run() {
    move();
    display();
    limits();
    drawLine();
  }

  void update(PVector force) {  
    vel.add(force);
  }

  void drawLine() {
    for (int i=0; i<a.size(); i++ ) {
      Agent other= a.get(i);
      stroke(255, 0, 0);
      line(loc.x, loc.y, other.loc.x, other.loc.y);
    }
  }

  void move() {
    vel.add(acc);
    loc.add(vel);
    acc.mult(0);
  }

  void limits() {
    if (loc.x > width-10 || loc.x < 10 ) {
      vel.x = -vel.x ;
    } else if (loc.y > height-10 || loc.y < 10) {
      vel.y = -vel.y;
    }
  }

  void display() {
    stroke(222);
    noFill();
    ellipse(loc.x, loc.y, 5, 5);
  }
}

/**
class FixedPoint extends Agent {//____________________________________________

 PVector loc1, loc2, loc3, loc4;


 FixedPoint() {

 loc1= new PVector(100, 100);
 loc2= new PVector(100, 700);
 loc3= new PVector(1300, 100);
 loc4= new PVector(1300, 700);
 }

 void run() {
 display();
 }

 void display() {
 fill(0, 0, 255);
 ellipse(loc1.x, loc1.y, 5, 5); 
 ellipse(loc2.x, loc2.y, 5, 5); 
 ellipse(loc3.x, loc3.y, 5, 5); 
 ellipse(loc4.x, loc4.y, 5, 5);
 }
 }
 */

class Repeler {//_____________________________________________________________
  PVector rep;

  Repeler(PVector _rep) {
    rep= _rep;
  }

  PVector attract(Agent ag) {
    PVector force= PVector.sub(rep, ag.loc);// here is where NPE warning is thrown
    float dist=force.mag();

    dist=constrain(dist, 1., 2.);

    force.normalize();
    //force.mult(-1); 
    return force;
  }


  void display() {
    noStroke();
    fill(65, 111);
    ellipse(rep.x, rep.y, 50, 50);
  }
}

giorgos123

Hello hope am not late, idk what your problem is but i know why the NPE occurs: on this line:

PVector force= PVector.sub(rep, ag.loc);

the loc property of ag is null, this is because at line 34:

a.set(0, vec1);

u put vec1 in first index, but

Agent vec1= new Agent (loc1);// 2nd constructor

and

  Agent (PVector _lol) {//2nd constructor for set() mode
    lol=_lol;
  }

You dont initialize the loc PVector in vec1, so just use the other constructor for vec1:

Agent vec1= new Agent ();// 1st constructor

jozze

@giorgos123. U r right, actually I needed to create a subclass to cast the updated PVectors to make the system work. Anyway, once this was done, the problem was solved, thx!