Swap function problem

[flyingEagle]

Hi there, I am having a problem with one of my labs. Within this lab I have a function called swapArrayElements. When I test this function, it outputs what I want but then it crashes. Here is the code:

void ArraySorter::swapArrayElements(){
	
	//DO 
	const int SIZE = 5;
	int myArray[SIZE] = { 20, 50, 40, 10, 30 };
	cout << "Array myArray after swapper 0 to 1 using swap" << endl;

	for (int i = 0; i < SIZE; i++)
	{

	ostream_iterator< int > output(cout, " ");
	copy( myArray, myArray + SIZE, output);
	swap(myArray[i], myArray[i+1]);


	}

	
}

It outputs: 20 50 40 10 30 50 20 40 10 30 50 40 20 10 30 50 40 10 20 30 50 40 10 30 20

Thank you in advance for any help! It is greatly appreciated!

flyingEagle

[kbw]

You'll need to copy into a back inserter, not directly into the iterator. There's an example in Stroustrup's book.

[vlad from moscow]

The problem is when i is equal to SIZE - 1 you are trying to swap elements with indexes SIZE - 1 and SIZE. However there is no element with index SIZE in the array.

You could rewrite your loop for example the following way

	for (int i = 0; i < SIZE && ++i < SIZE; /* empty */)
	{

	ostream_iterator< int > output(cout, " ");
	copy( myArray, myArray + SIZE, output);
	std::cout << std::endl;
	swap(myArray[i], myArray[i-1]);
	}

[KRAkatau]

The reason for crash is that you access the element with index SIZE (in line 13 - ... myArray[i+1] for i == SIZE - 1). You need to run the for loop until SIZE - 1.

@kbw
I don't see why you need back inserter here, for ostream_iterator.

[KRAkatau]

@vlad
Well, your code prevents the potential problems with unsigned int SIZE = 0, but IMO, the code is a bit more obscure than simple
for (int i = 0; i < SIZE - 1; ++i) {...}. Well, I guess it's a matter of taste, but I wanted to show to the original poster why you did it the way you did it (btw, is this the reason why? :))

[flyingEagle]

Sorry guys, I actually read the instruction for this function wrong :( I need to use another swap function in my program to swap elements in my array.

Here is my regular swap AND my swapArrayElements function(one I am having problems with).

void ArraySorter::swapArrayElements(){
	
	
	for (int i = 0; i < getSize() && ++i < getSize();){
		swapIntsByRef(anArray[i], anArray[i-1]);
		print();
	}

	
}

void ArraySorter::swapIntsByRef(int &num1, int &num2)
{
	
	cout << "testSwapIntsByRef()---------------" << endl;
	cout << "before swap: x = " << num1 << "," << "y = " << num2 << endl;
	swap(num1, num2);
	cout << "after swap: x = " << num1 << "," << "y = " << num2 << endl;
}

Edit: It works! Thanks again guys, really appreciate it!

[vlad from moscow]

@KRAkatau

All standard containers define type size_type for the number of elements that is typedef for some unsigned integer type

So this statement

for (int i = 0; i < SIZE - 1; ++i)

can give an invalid result for SIZE equal to 0 as you pointed out correctly. It is always better to use the general approach.