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C++ Discussion Thread
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Aug 9th, 2004

Why doesn't this remove the last node in a linked list?

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I've got a singly linked list, and the following code is supposed to delete all nodes that are ranked as "Trainee", it deletes them all, but if the last node is a "Trainee", it will not delete it, why is that?

C++ Syntax (Toggle Plain Text)
void LinkedList::removeLowest()
{
	Soldier * ptrDelete;
	ptrCurrent = ptrHead;
	while(ptrCurrent != NULL)
	{
		if(!strcmp(ptrCurrent->getRank(), "Trainee"))
		{
			ptrCurrent->Show();
			ptrDelete = ptrCurrent;
			ptrCurrent = ptrCurrent->ptrNext;
			delete ptrDelete;
		}
		else
		{
			ptrCurrent = ptrCurrent->ptrNext;
		}
	}
 
	cout << "\nTrainees Sacked!";
void LinkedList::removeLowest()
{
	Soldier * ptrDelete;
	ptrCurrent = ptrHead;
	while(ptrCurrent != NULL)
	{
		if(!strcmp(ptrCurrent->getRank(), "Trainee"))
		{
			ptrCurrent->Show();
			ptrDelete = ptrCurrent;
			ptrCurrent = ptrCurrent->ptrNext;
			delete ptrDelete;
		}
		else
		{
			ptrCurrent = ptrCurrent->ptrNext;
		}
	}

	cout << "\nTrainees Sacked!";

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paynekiller

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Aug 9th, 2004

Re: Why doesn't this remove the last node in a linked list?

Because you don't update ptrHead. If you delete the last node, ptrHead should become NULL.

Chainsaw

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Aug 10th, 2004

Re: Why doesn't this remove the last node in a linked list?

but ptrHead just points to the first node in the list, setting it to NULL would delete that node

paynekiller

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Aug 10th, 2004

Re: Why doesn't this remove the last node in a linked list?

Sorry, I misunderstood; I thought you meant Last as in "last remaining".

You have a list like this:

head-->first-->second-->third-->last

and you want to delete second because she is a trainee.

your code does this:

ptrCurrent points to second
show second
set ptrCurrent to point to third
delete second

Unless the destructor does some work on the entire list, you have this list:

head-->first-->[deleted second]-->third-->last

1) Does the destructor move first's next pointer to point to third (second's next)?

2) If you delete 'first', who updates the head pointer to point to first's next pointer?

3) If you delete 'last' does third get updated to point to null (last's next)? (note this would work fine if (1) works fine, I think)

Chainsaw

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