How to make a proper nested for loop for checking each item in an array?

Processing.py
1

im recreating the asteroids game that the coding train did in processing.py. im in the part of the video when daniel is coding the hit detection of the laser ( i named them bullets) to the asteroids. but when i try to do a nested for in range loop in the draw function so that each bullet checks if it hits any asteroids i get a index out of range error in the console.

here is the segment of code im having trouble with (in the draw function)

for x in range(len(bullet)):
        bullet[x].renderbullet()
        bullet[x].update()
        for i in range(len(asteroids)):
            if (bullet[x].hits(asteroids[i])):
                del asteroids[i]

to test if the nested for loop works properly, i’ve planned that if a bullet impacts an asteroid, it would be deleted.

here is the whole code ( i didnt open any tabs)

class Ship:

  def __init__(self):
      self.ssize = 10
      self.facing = 0
      self.rotation = 0
      self.position = PVector(1000/2, 500/2)
      self.velocity = PVector(0, 0)
      self.isthrusting = False
      
      
  def display(self):
      pushMatrix()
      stroke(255)
      fill(0)
      translate(self.position.x ,self.position.y)
      rotate(self.facing + PI/2 )
      triangle(-self.ssize, self.ssize, self.ssize, self.ssize, 0, -self.ssize)
      popMatrix()
      
  def turn(self):
      self.facing  += self.rotation

  def putrotation(self, a):
      self.rotation = a    
      
  def move(self):
      if self.isthrusting == True:
          self.thrust()
      self.position.add(self.velocity)
      self.velocity.mult(0.95)
      
      
  def thrusting(self, b):
      self.isthrusting = b
          
      
  def thrust(self):
      force = PVector.fromAngle(self.facing)
      self.velocity.add(force)    

  def edges(self):
      if (self.position.x > width + self.ssize):
              self.position.x = -self.ssize
      elif (self.position.x < -self.ssize):
              self.position.x = width + self.ssize
      elif (self.position.y > height + self.ssize):
              self.position.y = - self.ssize
      elif (self.position.y < -self.ssize):
              self.position.y = height + self.ssize                          

class Asteroids:
    
    def __init__(self):
        self.pos = PVector(random(1000), random(500))
        self.vel = PVector.random2D()
        self.r = random(5, 55)
        self.reso = floor(random(5, 10))
        self.offset = []
        for i in range(0, self.reso ):
            self.offset.append(random(-12, 6))
            
    def render(self):
        pushMatrix()
        translate(self.pos.x, self.pos.y)
        stroke(255)
        noFill()
        beginShape()
        for i in range(0, self.reso):
           angle = map(i, 0 , self.reso, 0, TWO_PI)
           self.spike = self.r + self.offset[i]
           x = self.spike*cos(angle)
           y = self.spike*sin(angle)
           vertex(x,y) 
        endShape(CLOSE)       
        popMatrix()
        
    def edges(self):
      if (self.pos.x > width + self.r):
              self.pos.x = -self.r
      elif (self.pos.x < -self.r):
              self.pos.x = width + self.r
      elif (self.pos.y > height + self.r):
              self.pos.y = - self.r
      elif (self.pos.y < -self.r):
              self.pos.y = height + self.r    
              
    def update(self):
          self.pos.add(self.vel)
                  

class Bullet:
      def __init__(self, spos, svel):
          self.pos = PVector(spos.x, spos.y)
          self.vel = PVector.fromAngle(svel)
          self.vel.mult(20)
          
      def update(self):
          self.pos.add(self.vel)
          
      def renderbullet(self):
          pushStyle()
          pushMatrix()
          stroke(255)
          strokeWeight(4)
          point(self.pos.x, self.pos.y)
          popMatrix()
          popStyle()      
    
      def hits(self, asteroids):
          d = dist(self.pos.x, self.pos.y, asteroids.pos.x, asteroids.pos.y)
          if ( d < asteroids.r):
              return True
          else:
              return False
          
    
    
                                                                    
    
asteroids = [] 
for i in range (0, 8):       
    asteroids.append(Asteroids())
ship = Ship()
bullet = []

def keyPressed():
    if key == ' ':
        bullet.append(Bullet(ship.position, ship.facing))
    elif keyCode == RIGHT:
        ship.putrotation(0.1)
    elif keyCode == LEFT:
        ship.putrotation(-0.1)
    elif keyCode == UP:
        ship.thrusting(True)
        

   
    
def keyReleased():
    ship.putrotation(0)    
    ship.thrusting(False)                            

def setup():
    size(1000,500)
    
def draw():
    background(0)
    for x in range(len(bullet)):
        bullet[x].renderbullet()
        bullet[x].update()
        for i in range(len(asteroids)):
            if (bullet[x].hits(asteroids[i])):
                del asteroids[i]
                                     
             
      

                            
    
    ship.display()
    ship.turn()
    ship.move()
    ship.edges()
    for i in range(0,len(asteroids)):
        asteroids[i].render()
        asteroids[i].update()
        asteroids[i].edges()

here is the coding train video im basing myself on

2

Hi @nothome123,

Does the following snippet solve your issue ?

for b in bullet:
        b.renderbullet()
        b.update()
        for i, a in enumerate(asteroids): 
            if b.hits(a): del asteroids[i]
3

YES, WOW THANK YOU SOOOOOOO MUCH. may i ask you a favor? could you please explain the code you’ve made? i’m in the process of learning python and i would like to learn as much as i can.

4

Glad you got it working.

for b in bullet:
        b.renderbullet()
        b.update()

I’m just referring to the objects in the bullet and asteroids array list directly instead of manipulating them with their indices (b instead of bullet[x])

for i, a in enumerate(asteroids)

  • enumerate() is a builtin function that adds a counter to an iterable.
  • a is each Asteroids() object contained in the asteroids array list and i is its index.

It’s helpful because you can loop over asteroids and have the corresponding index counter available at the same time.

1 Like
5

you’re a genius. thank you so much.

6

The real reason for i in range(len(asteroids)): fails when you use del asteroids[i] is b/c the len() of the asteroids[] list is decreased, and the loop still “thinks” it has the same len() as when the loop started! :open_mouth:

It iterates over the sequence created by the range() function. It isn’t directly aware of asteroids[] at all! :expressionless:

Now, when we iterate over the target container, be it directly or via the function enumerate(), its for loop becomes aware of the current len() of it. :cowboy_hat_face:

Therefore, when we issue a del command this time, it immediately knows its len() had decreased, and it adjusts the number of times it iterates over it accordingly. :ok_hand:

7

There’s still a small issue here: Is 1 Bullet object supposed to destroy more than 1 Asteroid? :thinking:

Even after using del once, that loop goes on, checking whether the same Bullet may have destroyed the next Asteroid in the list! :flushed:

If that’s an undesirable behavior, we also need to issue a break statement in order to stop that loop to continue after we had used del: :stop_sign:
https://Docs.Python.org/2.7/reference/simple_stmts.html#break

for i,a in enumerate(asteroids):
    if b.hits(a):
        del asteroids[i]
        break

Coincidentally, break woulda avoided the original for loop w/ range() to crash! :rofl:

And while we’re at it, shouldn’t the Bullet which hits() an Asteroid be del along as well? :smiling_imp:

for i,b in enumerate(bullets):
    b.update()
    b.render()

    for j,a in enumerate(asteroids):
        if b.hits(a):
            del asteroids[j], bullets[i]
            break