vector::erase Questions

[Aaron Vienneau]

Hello,

It appears that the vector::erase method isn't working for me. I have a main play() function for this simulation that looks like this:

void BunnyGraduation::play()
{
	// Count each year that progresses
	int yearsAlive = 0;

	init(); // Initialize the colony with 5 bunnies
	printColony(); // Print the initial colony

	while (!gameOver()) // While game is not over. . .
	{
		printColony(); // Print the colony
		breed(); // Breed the bunnies
		infect(); // Infect bunnies.
		starving(); // Determine if the new colony is starving because of births
		ageColony(); // Age the colony
		yearsAlive++; // Increment years alive
	}
	// When loop ends, print how many years they're alive
	cout << "Years survived: " << yearsAlive << endl;

}

In the "starving" method - it checks if the colony size is >= 1000 and is supposed to kill half of the colony, chosen randomly. Starving method looks like this:

void BunnyGraduation::starving()
{
	if (colony.size() >= 1000) // If the colony is greater than 1000 bunnies
	{
		int randomNumber = 0;
		// Output that they're starving
		cout << "\nThe colony is starving! Colony size: " << colony.size() << endl;

		for (int i = 0; i < colony.size() / 2; i++) // While i is less than half of the colony size
		{
			randomNumber = rand() % colony.size();
			kill(randomNumber); // kill a random bunny
		}
	}
}

and the "Kill()" function looks like this:

void BunnyGraduation::kill( int suppliedIndex )
{
	// Output that a bunny has died.
	//cout << endl << colony.at(suppliedIndex).getName() << " died!" << endl;
	// Erase that bunny from the vector
	colony.erase(colony.begin() + suppliedIndex);
}

But for some reason, half the colony is not being killed properly when the starving function is called because the colony size keeps getting exponentially larger.

[kbw]

http://en.wikipedia.org/wiki/Erase-remove_idiom

[JLBorges]

Or to remove just one element (without preserving the original order):

void BunnyGraduation::kill( int suppliedIndex )
{
	// Output that a bunny has died.
	//cout << endl << colony.at(suppliedIndex).getName() << " died!" << endl;
	// Erase that bunny from the vector
        std::swap( colony[suppliedIndex], colony.back() ) ;
	colony.pop_back() ;
}

Each time a bunny is killed, the size() decreases. The loop should probably be:

const auto original_size =  colony.size() ;
for (int i = 0; i < original_size / 2; ++i )
    kill( rand() % colony.size() ); // kill a random bunny 

Or simpler:

std::random_shuffle( colony.begin(), colony.end() ) ;
colony.resize( colony.size() / 2 ) ;

[Aaron Vienneau]

std::random_shuffle( colony.begin(), colony.end() ) ;
colony.resize( colony.size() / 2 ) ;

I like this. Thank you :)