How would I be able to use a for loop to create a 5x10 set of a specified actor in the prepare() of myWorld? An example of code and explanation would be greatly appreciated.
Mattzh wrote ...
2020/6/2
Help with for loops
Mattzh wrote...
How would I be able to use a for loop to create a 5x10 set of a specified actor in the prepare() of myWorld? An example of code and explanation would be greatly appreciated.As of now, I have 5x6 (height x length) set of actors located at the top that are not centered. I am attempting to create a 5x10 set of actors that are centered at the top. I repeated the code 5 times at different y-values, but I believe I am doing something wrong as I am thinking that there is a shorter way to do it.
private void prepare()
{
spaceship spaceship = new spaceship();
addObject(spaceship,300,550);
for(int i=4; i<=9; i++)
{
alien alien = new alien();
addObject(alien,30+40*i, 15);
}
for(int i=4; i<=9; i++)
{
alien alien = new alien();
addObject(alien,30+40*i, 45);
}
for(int i=4; i<=9; i++)
{
alien alien = new alien();
addObject(alien,30+40*i, 75);
}
for(int i=4; i<=9; i++)
{
alien alien = new alien();
addObject(alien,30+40*i, 105);
}
for(int i=4; i<=9; i++)
{
alien alien = new alien();
addObject(alien,30+40*i, 135);
}
}With the following variables:
you can compute the x-coordinate of the left-most alien in a row:
Then a double loop can be constructed to add the aliens in the world, centered:
int apr = 10; aliens per row int gap = 40; // distance between aliens along a row int midX = getWorld()/2; // mid-world x-coordinate
int x0 = midX-(apr-1)*gap/2;
for (int row=0; row<5; row++) // for each row
{
for (int n=0; n<apr; n++) // for each alien in a row
{
addObject(new alien(), x0+gap*n, 15+30*row);
}
}It works, thank you!
While testing the game, I found an error for a part that is unrelated to the for loops, and was wondering how I could fix these.
The error says
"
java.lang.NullPointerException
at alien.displayScore(alien.java:50) corresponds to line 42 below
at alien.act(alien.java:20) corresponds to line 12 below
at greenfoot.core.Simulation.actActor(Simulation.java:567)
at greenfoot.core.Simulation.runOneLoop(Simulation.java:530)
at greenfoot.core.Simulation.runContent(Simulation.java:193)
at greenfoot.core.Simulation.run(Simulation.java:183)
"
public class alien extends Actor
{
public static int score = 0;
/**
* Act - do whatever the alien wants to do. This method is called whenever
* the 'Act' or 'Run' button gets pressed in the environment.
*/
public void act()
{
alienShoot();
destroyAlien();
displayScore();
}
public boolean canSee(Class otherActor)//Actor detects other actors
{
Actor actor = getOneObjectAtOffset(0,0,otherActor);
if(actor != null)
{
return true;
}
else
{
return false;
}
}
public void destroy(Class otherActor)//Actor deletes the actor from the map
{
Actor actor = getOneObjectAtOffset(0,0,otherActor);
if(actor != null)
{
getWorld().removeObject(actor);
}
}
public void displayScore() //Score is shown
{
getWorld().showText("Aliens Destroyed: "+score, 125,30);
}
public void destroyAlien()
{
if(canSee(laser1.class))
{
removeTouching(laser1.class);
getWorld().removeObject(this);
score++;
}
}
private void alienShoot()
{
if (Greenfoot.getRandomNumber(1000)>=999) //1 in 1000 chance that an alien will generate and shoot a laser
{
laser2 laser2 = new laser2();
getWorld().addObject(laser2, getX(),getY());
}
}
}
Remove line 12. Move to and modify the displayScore method to be in your World subclass. Call it from act method in that class.
I am having trouble doing what you had suggested - I'm getting an error with the +score part of showText("Aliens Destroyed: "+score, 125,30); in myWorld, because the score variable is located in the alien class. I'm not sure how to fix this.
Mattzh wrote...
I am having trouble doing what you had suggested - I'm getting an error with the +score part of showText("Aliens Destroyed: "+score, 125,30); in myWorld, because the score variable is located in the alien class. I'm not sure how to fix this.You need to login to post a reply.