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Killing AI


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#1 mypel16000   Members   -  Reputation: 46

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Posted 20 January 2013 - 03:31 PM

Hi there,

 

I am making a game of zombies and I am trying to make them die when I hit them. I have tried different methods, but they never work.

I just need some concepts to base my code on.

 

What I thought was:

 

having :          if the bullet hits the zombie, take 10 away from its health;

another if:      if health < 0, make a bool variable "Dead" true;

My zombies are stored in a vector, so:

 

for (int i = 0 ;  i < zombie.size() ; i++ )

{

      if( zombie[ i ].Dead )

     {

          zombie.clear( zombie.begin() + i )

     }

}

 

But this didn't work, I shot one and it died, but when I shot the 2nd one, all of the zombies on the screen died. How can I do this effectively?

 

 

P.S: I have tried simple deleting the object using delete zombies[ i ], but the program just stops responding



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#2 zacaj   Members   -  Reputation: 643

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Posted 20 January 2013 - 03:43 PM

Try using

 

delete zombie[i];

zombie.clear( zombie.begin() + i-- );



#3 mypel16000   Members   -  Reputation: 46

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Posted 20 January 2013 - 03:58 PM

I've tried, but the game "stops responding" when you use delete zombie[ i ]. Has it got something to do with its members or its functions or.... does it need a destructor?



#4 slicer4ever   Crossbones+   -  Reputation: 3981

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Posted 20 January 2013 - 04:14 PM

i suspect the problem is in your code for setting the dead flag.

 

edit: nvm, thought u were using erase, not clear.


Edited by slicer4ever, 20 January 2013 - 06:35 PM.

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#5 mypel16000   Members   -  Reputation: 46

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Posted 20 January 2013 - 04:58 PM

the problem is in how I use the vectors I think, when I try to delete 1. all of the others are deleted,,,



#6 minibutmany   Members   -  Reputation: 1677

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Posted 20 January 2013 - 05:37 PM

I've tried, but the game "stops responding" when you use delete zombie[ i ]

Could your array be going out of bounds?


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#7 TheChubu   Crossbones+   -  Reputation: 4756

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Posted 20 January 2013 - 06:01 PM

Isn't a zombie already dead?

 

(sorry for lame joke :P)

 

Anyway, you'd want to set it up so as soon as the zombie's health reaches 0 (not < 0, but <= 0) the game sets it as "dead".


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#8 zacaj   Members   -  Reputation: 643

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Posted 20 January 2013 - 06:29 PM

Did you note the i--?



#9 Mercile55   Members   -  Reputation: 136

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Posted 20 January 2013 - 06:54 PM

The problem is that you're modifying the very collection that you're iterating through. Every time you delete something from a collection that you're going through, the size will change, but not in the loop, so original size (i.e. 10) will now be one less, (i.e. 9), and when you get to the last original index (i.e. 10), you'll access random location, and usually crash the app.

Save indexes of zombies to delete in another collection, and delete them in one consistent loop after the loop you're doing now.



#10 mypel16000   Members   -  Reputation: 46

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Posted 21 January 2013 - 02:48 AM

This isn't working on my code. I believe I have got to the problem, and it is the bool "dead". I can't seem to fix what is wrong though.

 

What I first do is check to see if health is less than zero by looping through the objects in my vector:       zombies[ i ].checkHEalth();

If it returns a 0 or less, i change a public bool in the object ( dead ) to true. Then I loop through them again and zombies.clear() the ones that return true for dead. The problem is that on debugging the code, i find that when one dies all of the bools in the loop turn to true, and therefore they are all deleted.

 

Any more ideas about this



#11 Khatharr   Crossbones+   -  Reputation: 3038

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Posted 21 January 2013 - 03:44 AM

Could you please describe what steps you've taken based on the information you've been given so far?


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#12 stillLearning()   Members   -  Reputation: 212

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Posted 21 January 2013 - 04:27 AM

 

for (vector<zombiesClass*>::iterator i = zombies.begin() ; 
i != zombies.end() ;)
{
if( (*i)->Dead )
{
delete zombies(i);
zombies.erase(i);
}
else ++i;
}

 

 

Perhaps something like this? (argh, cant get rid of italic text..)

 

Im not to sure this works thou. It deallocated the object and removes it from the vector without invalidizing the vector. Atleast that's what I intent to do with this code.


If you don't understand the stuff written here, please sharpen your C++ skills.


#13 mypel16000   Members   -  Reputation: 46

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Posted 21 January 2013 - 04:57 AM

|error: 'zombieClass' cannot appear in a constant-expression|


#14 Khatharr   Crossbones+   -  Reputation: 3038

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Posted 21 January 2013 - 06:53 AM

 
for (vector<zombiesClass*>::iterator i = zombies.begin() ; 
i != zombies.end() ;)
{
if( (*i)->Dead )
{
delete zombies(i);
zombies.erase(i);
}
else ++i;
}

 

 

Perhaps something like this? (argh, cant get rid of italic text..)

 

Im not to sure this works thou. It deallocated the object and removes it from the vector without invalidizing the vector. Atleast that's what I intent to do with this code.

 

Closer, but incorrect for... several reasons. At this point I'm thinking that handing him working code would set a bad precedent, though.


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#15 mypel16000   Members   -  Reputation: 46

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Posted 21 January 2013 - 07:45 AM

Any suggestions?



#16 sheep19   Members   -  Reputation: 413

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Posted 21 January 2013 - 11:25 AM

Try using a debugger to see what actually happens.



#17 Servant of the Lord   Crossbones+   -  Reputation: 20999

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Posted 21 January 2013 - 11:52 AM

Are you declaring your vector like this:
 
std::vector<Zombie> zombies;
 
Or like this:
std::vector<Zombie*> zombies;
 
 
P.S: I have tried simple deleting the object using delete zombies[ i ], but the program just stops responding
 
You can only 'delete' objects you have 'new'd. If you don't know what new or delete do, just don't touch them at all until you thoroughly research the issue.
new and delete are dangerous to your code (but won't do any harm to your computer, so feel free to experiment) if used improperly.
 
std::vectors are made for very fast access. It's not good for fast removals or fast inserts, unless you remove or insert from the end of the list with push_back() or pop_back(). You probably want a std::list, which is designed for inserting and erasing from in the very middle of the container.
 
std::list<Zombie> zombies;
typedef std::list<Zombie>::iterator ZombieIterator;
for(ZombieIterator it = zombies.begin(); it !- zombies.end(); it++)
{
	if(it->dead)
	{
		it = zombies.erase(it); //erase() removes the element at the position given, and returns the next element.
		it--; //Take a step back, so the for-loops 'it++' doesn't skip an element.
	}
}
 
Alternatively, you could use the "swap and pop" method. Swaps are fast. Pops are fast. You swap the dead zombie with the live zombie at the end of the vector (messing up ordering, but you might not care about the order in this situation), and pop back the vector erasing the dead zombie.
std::vector<Zombie> zombies;
typedef d::vector<Zombie>::iterator ZombieIterator;

for(ZombieIterator it = zombies.begin(); it != zombies.end(); it++)
{
	if(it->dead)
	{
		//Swap the value of the current zombie with the zombie at the back of the container.
		std::swap(*it, zombies.back());
		//Pop the back of the container.
		zombies.pop_back();
	}
}

A third method is to use a standard algorithm like std::remove_if() to do this for you. You pass a range of iterators to std::remove_if(), and you also pass a callback function or functor to say which elements to remove.
bool ZombieIsDead(const Zombie &zombie)
{
	return zombie.dead;
}

std::vector<Zombie> zombies;
typedef d::vector<Zombie>::iterator ZombieIterator;

ZombieIterator newEnd = std::remove_if(zombies.begin(), zombies.end(), ZombieIsDead);
zombies.erase(newEnd, zombies.end());

If using C++11, you can use a lambda as a callback function.
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#18 EWClay   Members   -  Reputation: 659

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Posted 21 January 2013 - 02:45 PM

Two of those methods are broken.

 

In the first method, if you erase the first element, the iterator will be pointing to the first element still when it is decremented, which is undefined behaviour.

 

In the second method, if you erase the last element, the iterator will be pointing at the end when it is incremented, which undefined behaviour again (and probably a crash).

 

The correct method would be to move the ++it inside the body of the for loop, and only call it if erase was not called, otherwise set the iterator to the return value of erase.



#19 Servant of the Lord   Crossbones+   -  Reputation: 20999

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Posted 21 January 2013 - 04:40 PM

I knew I should've tested the snippets before posting them. laugh.png
 
So the correct method to 'swap and pop' is:

std::vector<Zombie> zombies;
typedef d::vector<Zombie>::iterator ZombieIterator;
 
for(ZombieIterator it = zombies.begin(); it != zombies.end();)
{
	if(it->dead)
	{
		//Swap the value of the current zombie with the zombie at the back of the container.
		std::swap(*it, zombies.back());
		//Pop the back of the container.
		zombies.pop_back();
	}
	else
	{
		++it;
	}
}

Looking over my previous version, it'd also skip whatever elements were at the end before being swapped. rolleyes.gif

 

But the correct choice in most cases is the third solution I posted, using the heavily optimized and standardized std::remove_if() algorithm.


It's perfectly fine to abbreviate my username to 'Servant' rather than copy+pasting it all the time.
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#20 Mercile55   Members   -  Reputation: 136

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Posted 21 January 2013 - 05:13 PM

I knew I should've tested the snippets before posting them. laugh.png

So the correct method to 'swap and pop' is:
std::vector<Zombie> zombies;typedef d::vector<Zombie>::iterator ZombieIterator; for(ZombieIterator it = zombies.begin(); it != zombies.end();){	if(it->dead)	{		//Swap the value of the current zombie with the zombie at the back of the container.		std::swap(*it, zombies.back());		//Pop the back of the container.		zombies.pop_back();	}	else	{		++it;	}}
Looking over my previous version, it'd also skip whatever elements were at the end before being swapped. rolleyes.gif

But the correct choice in most cases is the third solution I posted, using the heavily optimized and standardized std::remove_if() algorithm.
This isn't right man, try this when all Zombies are dead, and you'll end up with a crash, as when you pop_back the very first element, the iterator becomes invalid. It will be ok, as long as there's at least one element left though.
It's always good idea to make sure what does and what doesn't invalidate your iterator, and avoid any such cases if possible. It saves time in the future.

remove_if() does the trick for most cases, although it may not be as fast, due to additional calls overhead.

Edited by Mercile55, 21 January 2013 - 05:15 PM.





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