Thread: Why Do i get NULL pointer assignment In this program?

Hello,
i created a program for polynomial addition which adds 2 polynomials by way of link list. But when i create 2 linklist and add them everything goes right and then when i press 4 for exit i keep getting "NULL pointer assignment" i have deleted all the memory that i created with new but still why??Please help me. Here is the source code

Code:

#include <iostream.h>
#include <stdio.h>
class poly
{
  struct node
  {
	float co;
	int power;
	struct node *next;
  }*first;
  public:
	poly()
	{

		first=NULL;
	}
	void getdata();
	void putdata();
	void add(poly );
	void del();
};
void poly::del()
{
  struct node *q;
  while(first!=NULL)
  {
	q=first;
	first=first->next;
	delete q;
  }

}
void poly::add(poly b)
{
 struct node *q=first,*t=b.first,*ans=NULL,*m,*newnode;
 while(q!=NULL && t!=NULL)
 {
   newnode=new node;
   newnode->next=NULL;
   if(q->power>t->power)
   {
	newnode->co=q->co;
	newnode->power=q->power;
	q=q->next;
   }
   else if(q->power<t->power)
   {
	newnode->co=t->co;
	newnode->power=t->power;
	t=t->next;
   }
   else
   {
	newnode->co=t->co+q->co;
	newnode->power=t->power;
	t=t->next;
	q=q->next;
   }
   if(ans==NULL)
	ans=newnode;
   else
	m->next=newnode;
	m=newnode;
 }
 while(q!=NULL)
 {
	newnode=new node;
	newnode->co=q->co;
	newnode->power=q->power;
	newnode->next=NULL;
	q=q->next;
 }
 while(t!=NULL)
 {
	newnode=new node;
	newnode->co=t->co;
	newnode->power=t->power;
	newnode->next=NULL;
	t=t->next;
 }
 q=ans;
 cout<<endl;
 while(q!=NULL)
 {
	cout<<q->co<<" X"<<q->power<<" +";
	q=q->next;
 }
 cout<<"\b";
 cout<<NULL;

 while(ans!=NULL)
 {
            q=ans;
	ans=ans->next;
	delete q;
 }
}
void poly::getdata()
{
 struct node *q=first,*newnode;
 float tco;
 int tpower;
 while(first!=NULL)
 {
	first=first->next;
	delete q;
 }
 cout<<"\n\nEnter Co-ordinate (Enter -1 To Exit) :";
 cin>>tco;
 while(tco!=-1)
 {
	cout<<"\n\nEnter Power :";
	cin>>tpower;
	newnode=new node;
	newnode->co=tco;
	newnode->power=tpower;
	newnode->next=NULL;
	if(first==NULL || newnode->power>first->power)
	{
		newnode->next=first;
		first=newnode;
	}
	else
	{
		q=first;
		while(q!=NULL)
		{
			if(q->power>=newnode->power && (q->next->power<newnode->power || q->next==NULL))
			{
				newnode->next=q->next;
				q->next=newnode;
				break;
			}
			q=q->next;
		}
	}
		 cout<<"\n\nEnter Co-ordinate (Enter -1 To Exit) :";
		 cin>>tco;

 }
}
void poly::putdata()
{
 struct node *q=first;
 while(q!=NULL)
 {
	cout<<q->co<<"X"<<q->power<<" +";
	q=q->next;
 }
 cout<<"\b";
 putch(NULL);
}

int main(void)
{
 poly a,b;
 int ch;
do
{
 cout<<"\n\nAddition Of 2 Polynomials";
 cout<<"\n\n1 :Enter 1st Polynomial";
 cout<<"\n\n2 :Enter 2nd Polynomial";
 cout<<"\n\n3 :Addition Of 2 Polynomials";
 cout<<"\n\n4 :Exit";
 cout<<"\n\nEnter Your Choice :";
 cin>>ch;
 switch(ch)
 {
	case 1:
	a.getdata();
	a.putdata();
	break;
	case 2:
	b.getdata();
	b.putdata();
	break;
	case 3:
	a.add(b);
	break;
	case 4:
		cout<<"\n\nBye-Bye";
	break;
	default:
		cout<<"\n\nWrong Choice";
 }

}while(ch!=4);
a.del();
b.del();

 return 0;
}