Sort function for Linked list program

[Mchubbs]

I'm having trouble getting my sort function to work,

void Linkedlist::sort(int num)
{
	node *q;
	node *a;
	int input=num;
	
	
	a = new node;
	a->data=input;
	a->link=NULL;
	
	q=head;
	
	while(q->link !=NULL)
	{
		int temp=0;
		temp = q->link->data - input;
		if(temp>0)
		{
			a->link=q->link;
			q->link=a;
			break;
		}
		
	}
	
	q=q->link;

	if (q->link = NULL)
	{

	q->link = a;
	}
}

I get a crash and I've narrowed it down to the last if statement. Also this function is meant to handle new members after 1 member has been added. Where am I going wrong?

[Smac89]

What sorting algorithm are you trying to implement?

Line 29:
- You assign q->link to NULL then line 32 you try to dereference a null pointer
- Either line 27 or 29 could crash the program as soon as line 32 is executed - look at the condition of your while loop

[Mchubbs]

I was trying to make it sort so that if node *a was less than the element node *q was pointing to, it would point a->link=q->link and q->link=a and swap the a and q's positions.

[Mchubbs]

So I realized my function was pretty screwy, heres my new attempt, but it is still crashing...

void Linkedlist::sort(int num)
{
	node *q;
	node *a;
	int input=num;
	
	
	a = new node;
	a->data=input;
	a->link=NULL;
	
	q=head;
	
	if(input < q->data)
	{
		a->link=q;
	}
	else
	{
		int temp=0;
		temp = q->link->data - input;
		if(temp>0)
		{
			a->link=q->link;
			q->link=a;
		}


	}
}

[Smac89]

Line 21, how are you making sure that q->link is not a null reference?

[Mchubbs]

okay I almost fixed it!

void Linkedlist::sort(int num)
{
	node *q;
	node *a;
	int input=num;
	
	
	a = new node;
	a->data=input;
	a->link=NULL;
	
	q=head;
	
	if(input < q->data)
	{
		a->link=q;
		a=head;
	}
	else
	{
		int temp=0;
		if(q->link!=NULL)
		{
			temp = q->link->data - input;
			if(temp>0)
			{
				a->link=q->link;
				q->link=a;
				q=head;
			}
		}
		else
		{
			q->link=a;
			a->link=NULL;
			q=head;
		}

	}
}

Okay so now I have it working, but im only getting certain numbers on the output:

Enter 10 numbers, or -1 to quit:
6
3
8
7
1
9
8
4
3
2
Linkedlist Members in sorted order: 6 7 8
Press any key to continue . . .

[Smac89]

You only did one set of operations on the list which is why you obtained the numbers you get. What does your insert function look like?

[Mchubbs]

I fixed it!
thanks for the help but i got a friend to help me out

void Linkedlist::sort(int num)
{
	node *q;
	node *a;
	int input=num;
	
	
	a = new node;
	a->data=input;
	a->link=NULL;
	
	q=head;
	
	if(a->data < q->data)
	{
		a->link=q;
		head=a;
	}
	else
	{
		
		while(q!=NULL)
		{   
			
			if(q->link!=NULL)
			{  
				int temp=0;
				temp = q->link->data - input;
			
				if(temp>0)
				{
					a->link=q->link;
					q->link=a;
					break;
				}
			}
			else
			{
					q->link=a;
					a->link=NULL;
					break;
			}
		q=q->link;
		}
	}
}