doubly linked list insert at middle - c

Question

Can anyone identify what is happening in my code that is causing the segmentation fault? Please modify/correct the wrong part.
void InsertAtMid (Node *head){
int num,count=0,i;
Node *ptr=head;
Node *newnode=NULL;
Node *newnode2=head;
printf("Enter node to be inserted: ");
scanf("%d", &num);
if (head==NULL){
newnode = head;
newnode=(Node *)malloc(sizeof(Node));
newnode->x=num;
newnode->next=NULL;
newnode->prev=NULL;
} else {
ptr=head->next;
while(ptr->x!=(count/2)){
ptr=ptr->next;
}
newnode->next=ptr->next;
newnode->prev=ptr;
ptr->next->prev=newnode;
ptr->next=newnode;
}
}

Answer 1

So, based on my understanding of your code - the following should [mostly] work:
void InsertAtMid (Node **head){
int num = 0;
int count = 0
int advance = 0;
Node *ptr = *head;
Node *newnode = NULL;
printf("Enter node to be inserted: ");
scanf("%d", &num);
if (*head == NULL) {
*head = (Node *)malloc(sizeof(Node));
ptr = *head;
ptr->x = num;
ptr->next = NULL;
ptr->prev = NULL;
} else {
// *** Count the number of items
ptr = *head;
while (ptr != NULL) {
ptr = ptr->next;
count++;
}
// *** Move to the middle of the list
ptr = *head;
while (advance < (count/2)){
ptr = ptr->next;
advance++;
}
// *** Insert the new value
newnode = (Node *)malloc(sizeof(Node));
newnode->x = num;
newnode->next = ptr->next;
newnode->prev = ptr;
ptr->next->prev = newnode;
ptr->next = newnode;
}
}
The following are the issues I fixed:
You are assigning to head at one point, but since "head" isn't passed in as a reference, the value isn't going to be maintained beyond the first time the function is called. Needless to say you need a pointer to a pointer of type node.
You never calculated the number of items in the list. Often "head" pointer would store this information and you would increment when you add a node, but since you don't have that the only way to determine it is to traverse the list till you find the count.
You never allocated space for the new node to insert except if you were initializing the head pointer. This was also an issue.
Hope that helps some. Best of luck!

Answer 2

int num,count=0,i;
...
ptr=head->next;
while(ptr->x!=(count/2)){
ptr=ptr->next;
count is initialized to 0 and never changed.
So unless you enter "0" for x, that while loop is just going to walk off the end of the list, every time.

Answer 3

Test to work out under what circumstances your code segfaults.
You'll find that it works OK when head == NULL, but fails if head is not null.
So you know that your error is somewhere in the else block.
Step through your running code in a debugger (if you don't know how, it's never too early to learn: whenever you solve a problem with a debugger, you think "why didn't I turn to this sooner?").
Work out what you expect to happen, watch the variables in the debugger, and when the actual values deviate from your expectations, reason about why.
It's not clear to me what you expect to happen in your code, but what will actually happen for a list with one node, is that:
it will execute ptr=head->next; -- so ptr is now NULL.
then for the while condition it will try to dereference ptr->x, and since ptr is NULL, it will segfault.
A quick fix for that would be:
while(ptr != NULL && ptr->x ....) {
But you need to think about whether that's the actual logic you want; and once you get past that, you'll hit other problems (for example, count never changes), which can be sorted out with a debugger in the same way.