Why does my linked-list remove function not work? - c

Question

My C/C++ linked-list remove function does not remove an element from the list. Here follows some of my code;
struct listIntElement {
struct listIntElement *next;
int data;
};
typedef struct listIntElement ListIntElement;
ListIntElement *head = NULL;
/*
Inserts a new element infront of the list.
*/
bool insert(ListIntElement **head, int data) {
// Allocate memory for new element. The cast is needed here as we are using a C++ compiler.
ListIntElement *newElement = (ListIntElement *) malloc(sizeof(ListIntElement));
// Check if memory allocation was succesfull.
if (newElement == NULL)
return false;
// Set the data for the new element of the list.
newElement->data = data;
// Keep track of the new head of the list.
newElement->next = *head;
*head = newElement;
return true;
}
/*
Deleting an element in the list.
*/
bool remove(ListIntElement **head, ListIntElement *elementToDelete) {
ListIntElement *element = *head;
// Check for NULL pointers.
if (head == NULL || *head == NULL || elementToDelete == NULL)
return false;
// Special case for the head.
if (elementToDelete == *head) {
*head = element->next;
free(elementToDelete);
return true;
}
// Traversal of the list to find the element to remove.
while (element != NULL) {
if (element->next == elementToDelete) {
// Relink the list so that it does not include the element to be deleted.
element->next = elementToDelete->next;
free(elementToDelete);
return true;
}
element = element->next;
}
// elementToDelete was not found.
return false;
}
/*
Finding an element in the list.
*/
ListIntElement find(ListIntElement **head, int data) {
// Take care of the head as we don't want to use the head
// in the traversal operation.
ListIntElement *element = *head;
while (element != NULL && element->data != data) {
element = element->next;
}
return *element;
}
/*
Displaying the list.
*/
void displayList(ListIntElement **head) {
ListIntElement *element = *head;
// Check if list is empty.
if (head == NULL | *head == NULL) {
printf("List is empty\n");
}
while (element != NULL) {
printf("%d --> ", element->data);
element = element->next;
}
printf("NULL");
printf("\n");
}
Here is my test code;
/*
* Testing a linked list.
*/
ListIntElement found;
printf("Linked list test\n");
insert(&head,0);
insert(&head, 1);
insert(&head, 2);
insert(&head, 3);
insert(&head, 4);
insert(&head, 5);
displayList(&head);
printf("size is: %d\n", size(&head));
found = find(&head, 5);
printf("This was found: %d\n", found.data);
remove(&head,&found);
displayList(&head);
I have found this section to be the section where things go wrong in the remove function;
// Special case for the head.
if (elementToDelete == *head) {
*head = element->next;
free(elementToDelete);
return true;
}
Take notice that I am using MS Visual Studio 2015 to write C code and using a C++ compiler.

Answer 1

From find function, you were returning a copy, and not the address itself, so your changes were not reflected in the calling function. Fixed it.
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#define true 1
#define false 0
struct listIntElement {
struct listIntElement *next;
int data;
};
typedef struct listIntElement ListIntElement;
ListIntElement *head = NULL;
/*
Inserts a new element infront of the list.
*/
bool insert(ListIntElement **head, int data) {
// Allocate memory for new element. The cast is needed here as we are using a C++ compiler.
ListIntElement *newElement = (ListIntElement *) malloc(sizeof(ListIntElement));
// Check if memory allocation was succesfull.
if (newElement == NULL)
return false;
// Set the data for the new element of the list.
newElement->data = data;
// Keep track of the new head of the list.
newElement->next = *head;
*head = newElement;
return true;
}
/*
Deleting an element in the list.
*/
bool removeElement (ListIntElement **head, ListIntElement *elementToDelete) {
ListIntElement *element = *head;
// Check for NULL pointers.
if (head == NULL || *head == NULL || elementToDelete == NULL)
return false;
// Special case for the head.
if (elementToDelete == *head) {
*head = element->next;
free(elementToDelete);
return true;
}
// Traversal of the list to find the element to remove.
while (element != NULL) {
if (element->next == elementToDelete) {
// Relink the list so that it does not include the element to be deleted.
element->next = elementToDelete->next;
free(elementToDelete);
return true;
}
element = element->next;
}
// elementToDelete was not found.
return false;
}
/*
Finding an element in the list.
*/
ListIntElement *find(ListIntElement **head, int data) {
// Take care of the head as we don't want to use the head
// in the traversal operation.
ListIntElement *element = *head;
while (element != NULL && element->data != data) {
element = element->next;
}
return element;
}
/*
Displaying the list.
*/
void displayList(ListIntElement **head) {
ListIntElement *element = *head;
// Check if list is empty.
if (head == NULL | *head == NULL) {
printf("List is empty\n");
}
while (element != NULL) {
printf("%d --> ", element->data);
element = element->next;
}
printf("NULL");
printf("\n");
}
int main () {
ListIntElement *found;
printf("Linked list test\n");
insert(&head,0);
insert(&head, 1);
insert(&head, 2);
insert(&head, 3);
insert(&head, 4);
insert(&head, 5);
displayList(&head);
printf("size is: %d\n", sizeof(&head));
found = find(&head, 5);
printf("This was found: %d\n", found->data);
removeElement(&head,found);
displayList(&head);
}
Output:
Linked list test
5 --> 4 --> 3 --> 2 --> 1 --> 0 --> NULL
size is: 4
This was found: 5
4 --> 3 --> 2 --> 1 --> 0 --> NULL