How to get enemy that is being hit to disappear - processing

Question

I'm currently working on a school project and I'm trying to make a game similar to galaga. My problem is that whenever the bullet hits the enemy, another enemy would disappear. I believe this is because of the fact that I'm using a for loop and the enemies are being deleted in sequential order. Another problem I am having is with the bullets, I do not know why it slows down and eventually disappears as the number of the enemies decreases. Any help is appreciated.
ArrayList<enemy> e = new ArrayList<enemy>();
ArrayList<bullet> b = new ArrayList<bullet>();
boolean shoot = false;
void setup() {
fullScreen();
//enemy
for (int i = 0; i<5; i++) {
e.add(new enemy(50, 50));
}
//bullet
for (int i = 0; i<5; i++) {
b.add(new bullet(mouseX, mouseY));
}
}
void draw() {
background(255);
for (int p = 0; p<e.size(); p++) {
for (int i = 0; i<b.size(); i++) {
bullet a = b.get(i);
enemy o = e.get(p);
if (a.update()) {
b.remove(i);
}
if (o.col()) {
b.remove(i);
e.remove(i);
}
}
}
//enemy
for (int i = 0; i<e.size(); i++) {
enemy a = e.get(i);
a.display();
}
}
void mouseReleased() {
shoot = true;
b.add(new bullet(mouseX, mouseY));
}
class enemy {
int x, y, w, h;
int enemyX = int(random(width));
int enemyY = int(random(200));
public enemy(int tempenemyW, int tempenemyH) {
int tempenemyX = enemyX;
int tempenemyY = enemyY;
this.x = tempenemyX;
this.y = tempenemyY;
this.w = tempenemyW;
this.h = tempenemyH;
}
void display() {
fill(255, 0, 0);
rect(this.x, this.y, this.w, this.h);
}
boolean col() {
for (int i = 0; i<b.size(); i++) {
bullet a = b.get(i);
if (a.x+a.w>this.x && a.x<this.x+this.w && a.y+a.h+a.bulletSpeed>this.y && a.y+a.bulletSpeed<this.y+this.h) {
return true;
}
}
return false;
}
}
class bullet {
int x, y, w, h;
int bulletSpeed = 10;
public bullet(int tempx, int tempy) {
int tempw = 3;
int temph = 20;
this.x = tempx;
this.y = tempy;
this.w = tempw;
this.h = temph;
}
boolean update() {
this.y -= bulletSpeed;
fill(0, 255, 0);
rect(this.x, this.y, this.w, this.h, 100, 100, 100, 100);
if (x<0 || x>width || y<0 || y>height) {
return true;
} else {
return false;
}
}
}

Answer 1

Your removing the enemy that lives at the index of the bullet instead of the enemy at the index of the enemy. To fix this, try changing the line
e.remove(i);
to
e.remove(p);
I also recommend that you give better variable names. The reason that this was confusing for you is because "i" and "p" aren't very descriptive or helpful. It would have been easier to spot the mistake if the middle of the draw function looked like this:
for (int enemyIndex = 0; enemyIndex<e.size(); enemyIndex++) {
for (int bulletIndex = 0; bulletIndex<b.size(); bulletIndex++) {
bullet currentBullet = b.get(bulletIndex);
enemy currentEnemy = e.get(enemyIndex);
if (a.update()) {
b.remove(bulletIndex);
}
if (currentEnemy.col()) {
b.remove(bulletIndex);
e.remove(bulletIndex); // Clearly the wrong index!
}
}
}
Then you could go further by changing the names of your lists to "bullets" and "enemies" instead of just "b" and "e" :p

Answer 2

Teddy's answer is half the story. The other half of the story is that you're removing items from the list as you loop over it.
Let's say you have a loop like this:
for(int index = 0; index < list.size(); index++){
if(list.get(index).contains("remove me")){
list.remove(index);
}
}
And let's say your list looks like this:
0: "keep"
1: "remove me one"
2: "remove me two"
3: "keep"
Go through the loop with this list in mind. When index is 0, it sees "keep" and doesn't do anything. Now index is 1 and it sees "remove me one" so it removes the item at index 1 and every index after that shifts down one, making the list this:
0: "keep"
1: "remove me two"
2: "keep"
But index is still 1 at the end of the loop, so it increases to 2, and the next iteration sees "keep".
In other words, we skip over "remove me two" because we never check that index when it's shifted down by one.
The solution to this problem is to either use an Iterator or to loop backwards over the list.
Shameless self-promotion: I wrote a tutorial on using ArrayLists in Processing available here. See the section on removing items from an ArrayList.