Deleting Nodes in a Linked List

Question

After a lot of effort, I've managed to piece together a function that deletes some node from my linked list. But, out of sheer interest, I would like to find out how you can go about deleting the first node from the list, ie the head.

My program asks for a letter to delete, so for example. Hello is stored in the list, the user inputs H for deletion, so that now the list is ello At the moment with my code, the program crashes obviously as if H is deleted, there is no head, and the program doesn't know where to go to look for the list.

Below is my current implementation, any clues or hints on how to modify this code( I would like to keep it similar to how I have) to allow Head Node Deletion would be much appreciated!.

EDIT: In response to below

FullList DeleteNode(FullList temp, char c) {
FullList remember;
FullList ptr;
while (temp.head->c != c) {
    remember.head = temp.head;
    temp.head = temp.head->next;
}
ptr.head = temp.head->next;
free(temp.head);
remember.head->next = ptr.head;
return temp;
}

int main(void) {
FullList List;
char c, s;
List.head = NULL;

while ((c=getchar()) != '.') {
    List = addToEnd(List, c);
}
scanf(" %c", &s);
List = DeleteNode(List, s);
while (List.head != NULL) {
    printf("%c", List.head->c);
    List.head = List.head->next;
}
return 0;
}

---

typedef struct List {
 char c;
 struct List *next;
}List;

typedef struct {
 List *head;
 List *tail;
}FullList;

 List *insertList(char c, List *t1) {
  List *t = (List*)calloc(1, sizeof(List));
  t->c = c ;
  t->next = t1;
 return t;
 }

FullList addToEnd(FullList c, char element) {
 if (c.head == NULL) { 
    c.head = c.tail = insertList(element, NULL);
 }else {
    c.tail->next = insertList(element, NULL);
    c.tail = c.tail->next;
 }
 return c;
} 

void DeleteNode(FullList temp, char c) {
 FullList remember;
 FullList ptr;
 while (temp.head->c != c) {
    remember.head = temp.head;
    temp.head = temp.head->next;
 } 
 ptr.head = temp.head->next;
 free(temp.head);
 remember.head->next = ptr.head;
} 


int main(void) {
 FullList List;
 char c, s;
 List.head = NULL;

 while ((c=getchar()) != '.') {
    List = addToEnd(List, c);
 }
 scanf(" %c", &s);
 DeleteNode(List, s);
 while (List.head != NULL) {
    printf("%c", List.head->c);
    List.head = List.head->next;
 }
 return 0;
}

Answer 1

The way it is now, inside DeleteNode , when you change the argument, it only changes the local variable, not the one outside the function.

You either have to pass the FullList to DeleteNode by pointer so that modifications done to it will be visible to the caller, or modify a local one and return it, and the caller must assign the returned FullList to its list.

Either way, the changes made by DeleteNode must become visible to the caller.

Answer 2

You can't do it without changing your existing code.

You're passing your FullList struct to your DeleteNode() function. This means that any changes to that struct are not visible back in main - the function is getting a copy of it.

You would need to change DeleteNode() to accept a pointer:

void DeleteNode(FullList *temp, char c)

Then when calling it main() you would do:

DeleteNode(&List, s);

By doing this, you can change the value of temp->head in your function and it will be visible back in main()

temp->head = temp->head->next;

Edit: The logic you'll need is:

• Check to see if temp->head->c == c
• If yes, replace temp->head with temp->head->next
• else assign temp->head to a temp pointer *previous . Assign temp->head->next to a pointer *current . Loop through the list, moving both pointers. When you find a match in current->c , assign current->next to previous->next and free() the current node.