Linked list- getting segmentation fault - c

Question

void AlternatingSplit(struct Node* source, struct Node** aRef,
struct Node** bRef)
{
/* split the nodes of source to these 'a' and 'b' lists */
struct Node* a ;
struct Node* b;
struct Node* current = source;
if(current){
a=current;
b=current->next;
current=b->next;
a->next=NULL;
b->next=NULL;
}
while(current) {
a->next=current;
b->next=current->next;
if(b)
current=b->next;
b=b->next;
a=a->next;
}
*aRef = a;
*bRef = b;
}
I am getting segmentaton fault here i dont know why pls help.
This question is to alternating split linkedlist nodes. I m using two pointers a and b and adding to it alternatingly but its giving error . pls help me

Answer 1

Like most linked-list rearrangement exercises, pointers to pointers make the job much, much easier. The point of this exercise it to flex your ability to change the next pointers without ever changing the data values of said-same. Pointers to pointers are an excellent way to do that in C.
This is especially trivial because you were already provided the target pointer-to-pointer arguments that we can reuse for building each list. How that works is best understood by demonstrating a technique for building a forward-chained linked list using a single head pointer and a pointer to pointer p:
struct Node *head, **pp = &head;
for (int i = 1; i <= 20; ++i)
{
*pp = malloc(sizeof **pp);
(*pp)->data = i;
pp = &(*pp)->next;
}
*pp = NULL;
Yes, it needs error checking, but the algorithm is what to focus on here. This code uses only pp to build the actual list. The rule is this: pp is a pointer to pointer to Node, and always holds the address of the next pointer to Node to populate. That's what pointers to pointers do: hold addresses of pointers. In this case pp initially holds the address of the head pointer. With each new node added pp takes the address of the next pointer of the previously just-added node. Makes sense, right? That will be the next pointer where we want to hang the next node. This process continues until we finish the loop. At that pointer pp holds the address of the last node's next pointer, which we set to NULL to terminate the list.
Now, knowing what we learned above, consider this:
void AlternatingSplit(struct Node* source, struct Node** a, struct Node** b)
{
while (source)
{
*a = source;
a = &(*a)->next;
source = source->next;
if (source)
{
*b = source;
b = &(*b)->next;
source = source->next;
}
}
*a = *b = NULL;
}
Example
A short example using both the forward-chaining build algorithm I showed first, and the split algorithm I showed after, appears below. Some utility functions for printing the list are included. I leave freeing the lists (there are two now, remember to walk both and free each node) as an exercise for you:
#include <stdio.h>
#include <stdlib.h>
struct Node
{
int data;
struct Node *next;
};
void AlternatingSplit(struct Node* source, struct Node** a, struct Node** b)
{
while (source)
{
*a = source;
a = &(*a)->next;
if ((source = source->next))
{
*b = source;
b = &(*b)->next;
source = source->next;
}
}
*a = *b = NULL;
}
void PrintList(struct Node const *p)
{
while (p)
{
printf("%d ", p->data);
p = p->next;
}
fputc('\n', stdout);
}
int main(void)
{
struct Node *head, **pp = &head;
for (int i = 1; i <= 20; ++i)
{
*pp = malloc(sizeof **pp);
(*pp)->data = i;
pp = &(*pp)->next;
}
*pp = NULL;
PrintList(head);
struct Node *a = NULL, *b = NULL;
AlternatingSplit(head, &a, &b);
PrintList(a);
PrintList(b);
return EXIT_SUCCESS;
}
Output
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1 3 5 7 9 11 13 15 17 19
2 4 6 8 10 12 14 16 18 20

Answer 2

There are few errors in your code -
Trying to access node->next , without checking whether node exists or not .
Not tackling the corner cases depending on the length of linked list (i.e. if length (linked list) < 3 )
And then comes the blunder , you are trying to make the new linked lists and then in the end aRef and bRef is assigned to the last node in their respective linked lists.
Try to deal with these problems and for reference you can see the code below.
void AlternatingSplit(struct Node* source, struct Node** aRef,
struct Node** bRef)
{
struct Node* a,b;
struct Node* current = source;
if(current){
a=current;
b=current->next;
// moving 'current' one step at a time will secure the code from crashing for corner cases
current = current->next;
if(b)
current=b->next;
a->next=NULL;
b->next=NULL;
//link aRef bRef right here
*aRef = a;
*bRef = b;
}
else {
*aRef = source; // Null
*bRef = source; // Null
return;
}
while(current)
{
a->next=current;
a=a->next;
b->next=current->next;
b=b->next;
current=current->next;
if(b){
current = b->next;
}
}
b->next = NULL;
a->next = NULL;
}
Hope this will help .
Keep asking , keep growing :)