How can I restart a double linked list? - c

Question

I'm coding a program to multiply two polynomials. I need some advice as to how I can restart the double linked list in the nested while loop in the polyProduct function. I mean, at this point I need to go back to the first position of the list. This is the code I have so far:
#include "DLListInterface.h"
#include <stdio.h>
#include <stdlib.h>
int numRead(DLnode*,int);
void printList(DLnode*);
DLnode *polyProduct(DLnode*,DLnode*);
int main()
{
DLnode *number1;
DLnode *number2;
DLnode *result;
int readval;
int numbterms;
int i;
scanf("%d",&numbterms);
scanf("%d",&readval);
/* initial DLnode of number1*/
number1=(DLnode*)malloc(sizeof(DLnode));
number1->value=readval;
number1->next=NULL;
number1->prev=NULL;
readval=numRead(number1,numbterms);
scanf("%d",&numbterms);
scanf("%d",&readval);
number2=(DLnode*)malloc(sizeof(DLnode));
number2->value=readval;
number2->next=NULL;
number2->prev=NULL;
readval=numRead(number2,numbterms);
printf("\n");
printList(number1);
printList(number2);
result=polyProduct(number1,number2);
printList(result);
getch();
return 0;
}
int numRead(DLnode*number,int numbterms)
{
int readval,i;
DLnode *anothernode;
for(i=0;i<(numbterms*2)-1;i++)
{
scanf("%d",&readval);
anothernode=(DLnode*)malloc(sizeof(DLnode));
anothernode->value=readval;
anothernode->next=NULL;
anothernode->prev=Dlast(number);
Dlast(number)->next=anothernode;
}
return readval;
}
DLnode *polyProduct(DLnode*poly1,DLnode*poly2)
{
DLnode *result;
DLnode *newnode;
DLnode *first;
first=poly2;
newnode=(DLnode*)malloc(sizeof(DLnode));
newnode->value=(poly1->value)*(poly2->value);
newnode->next=NULL;
newnode->prev=NULL;
result=newnode;
poly1=poly1->next;
poly2=poly2->next;
newnode=(DLnode*)malloc(sizeof(DLnode));
newnode->value=poly1->value+poly2->value;
newnode->next=NULL;
newnode->prev=Dlast(result);
Dlast(result)->next=newnode;
poly1=poly1->prev;
poly2=poly2->next;
while(poly1!=NULL)
{
while(poly2!=NULL)
{
newnode=(DLnode*)malloc(sizeof(DLnode));
newnode->value=(poly1->value)*(poly2->value);
newnode->next=NULL;
newnode->prev=Dlast(result);
Dlast(result)->next=newnode;
poly1=poly1->next;
poly2=poly2->next;
newnode=(DLnode*)malloc(sizeof(DLnode));
newnode->value=poly1->value+poly2->value;
newnode->next=NULL;
newnode->prev=Dlast(result);
Dlast(result)->next=newnode;
poly1=poly1->prev;
poly2=poly2->next;
}
poly1=poly1->next;
poly2=first; //restart poly 2
}
return result;
}
void printList(DLnode*number)
{
while (number!=NULL)
{
printf("%d ",number->value);
number=number->next;
}
printf("\n");
return;
}
//Declaration of DLnode in DLList.h file
typedef struct Dcontainer
{
int value;
struct Dcontainer *next;
struct Dcontainer *prev;
} DLnode;

Answer 1

One way of going to the start of the list is to manipulate the pointer to move to the start of the list
void seekstart(DLNode* number)
{
while(number->prev != null)
{
number = number->prev;
}
}
The other option is use temporary nodes to store the Starting node of the list before processing.
DLnode *temp_num1 = number1;
DLnode *temp_num2 = number2;
result=polyProduct(number1,number2);
Or You keep a pointer to the root in the structure of the node itself. Although this might not be an efficient solution. It is quite helpful if you need to access the starting of the list quite frequently
//Declaration of DLnode in DLList.h file
typedef struct Dcontainer
{
int value;
struct Dcontainer *next;
struct Dcontainer *prev;
struct Dcontainer *root; //root is the starting of the list
} DLnode;