Pointers Linked Lists C - c

Question

Did I do this right? I'm getting a strange error here...
linked.c: In function ‘push’:
linked.c:50:19: warning: assignment from incompatible pointer type [enabled by default]
linked.c: In function ‘main’:
linked.c:146:9: error: incompatible type for argument 1 of ‘push’
linked.c:45:6: note: expected ‘struct node **’ but argument is of type ‘struct node’
~/swen250/CLinkedList$ gcc -o linked linked.c
linked.c: In function ‘push’:
linked.c:50:19: warning: assignment from incompatible pointer type [enabled by default]
~/swen250/CLinkedList$ gcc -o linked linked.c
linked.c: In function ‘pop’:
linked.c:63:19: error: request for member ‘data’ in something not a structure or union
linked.c:64:20: error: request for member ‘next’ in something not a structure or union
linked.c: In function ‘copyList’:
linked.c:106:9: warning: passing argument 1 of ‘appendNode’ from incompatible pointer type [enabled by default]
linked.c:75:6: note: expected ‘struct node **’ but argument is of type ‘struct node *’
#include <stdio.h>
#include <stdlib.h>
struct node {
int data;
struct node* next;
};
static int length(struct node** head);
static void push(struct node** head, int data);
static int pop(struct node** head);
static void appendNode(struct node** head, int data);
static struct node *copyList(struct node** head);
static void printList(struct node** head);
/************************************************************
length - return length of a list
************************************************************/
int length(struct node** head) {
int count = 0;
struct node* current = NULL;
current = *head;
while (current != NULL) {
current = current->next;
++count;
}
return count;
}
/************************************************************
push - add new node at beginning of list
************************************************************/
void push(struct node** head, int data) {
struct node* new_ptr = NULL;
new_ptr = (struct node*)malloc(sizeof(struct node));
new_ptr->data = data;
new_ptr->next = *head;
*head = new_ptr;
}
/************************************************************
pop - delete node at beginning of non-empty list and return its data
************************************************************/
int pop(struct node** head) {
int val = 0;
struct node* temp = NULL;
if (*head != NULL) {
val = head->data;
temp = head->next;
free(head);
*head = temp;
}
return(val);
}
/************************************************************
appendNode - add new node at end of list
************************************************************/
void appendNode(struct node** head, int data) {
struct node* current = NULL;
struct node* previous = NULL;
struct node* new_ptr = NULL;
current = *head;
previous = current;
while (current != NULL) {
previous = current;
current = current->next;
}
new_ptr = (struct node*)malloc(sizeof(struct node));
new_ptr->data = data;
new_ptr->next = NULL;
previous = new_ptr;
}
/************************************************************
copyList - return new copy of list
************************************************************/
struct node* copyList(struct node** head) {
struct node* copy = NULL;
struct node* current = NULL;
struct node* new_ptr = NULL;
/* Copy current head to copy */
current = *head;
while (current != NULL) {
appendNode(copy, current->data);
current = current->next;
}
return copy;
}
/************************************************************
printList - print linked list as "List: < 2, 5, 6 >" (example)
************************************************************/
void printList(struct node** head) {
struct node* current = NULL;
printf("List: < ");
current = *head;
if (current == NULL)
printf("none ");
while (current != NULL) {
printf("%d", current->data);
current = current->next;
if (current != NULL)
printf(", ");
}
printf(" >\n");
}
void main() {
int i; // index used for loops
struct node *list_a; // a new list
struct node *list_a_copy; // copy of list
list_a = NULL; // initialize empty list
list_a_copy = NULL; // initialize empy list
// test push
for (i = 0; i < 4; ++i)
push(&list_a, i);
// test length
printf("Length of list = %d\n", length(&list_a));
// test print head list
printf("head:\n");
printList(&list_a);
// test append node
for (i = 4; i < 8; ++i)
appendNode(&list_a, i);
// test print head list
printf("head(append):\n");
printList(&list_a);
// make a copy of list
list_a_copy = copyList(&list_a);
// test pop head list
for (i = 0; i < 4; ++i)
printf("%d popped\n", pop(&list_a));
// test print copy list
printf("head copy:\n");
printList(&list_a_copy);
// test pop copy list
for (i = 0; i < 4; ++i)
printf("%d popped\n", pop(&list_a_copy));
}

Answer 1

The issue is with the way how double pointers are used.
Here is the full working code:
I have made some changes on the way how double pointers were used.
You can see the changes in pop function and copyList function.
#include <stdio.h>
#include <stdlib.h>
struct node {
int data;
struct node* next;
};
static int length(struct node** head);
static void push(struct node** head, int data);
static int pop(struct node** head);
static void appendNode(struct node** head, int data);
static struct node *copyList(struct node** head);
static void printList(struct node** head);
/************************************************************
length - return length of a list
************************************************************/
int length(struct node** head) {
int count = 0;
struct node* current = NULL;
current = *head;
while (current != NULL) {
current = current->next;
++count;
}
return count;
}
/************************************************************
push - add new node at beginning of list
************************************************************/
void push(struct node** head, int data) {
struct node* new_ptr = NULL;
new_ptr = (struct node*)malloc(sizeof(struct node));
new_ptr->data = data;
new_ptr->next = *head;
*head = new_ptr;
}
/************************************************************
pop - delete node at beginning of non-empty list and return its data
************************************************************/
int pop(struct node** head) {
int val = 0;
struct node* temp = NULL;
if (*head != NULL) {
val = (*head)->data;
temp = (*head)->next;
free(*head);
*head = temp;
}
return(val);
}
/************************************************************
appendNode - add new node at end of list
************************************************************/
void appendNode(struct node** head, int data) {
struct node* current = NULL;
struct node* previous = NULL;
struct node* new_ptr = NULL;
current = *head;
previous = current;
while (current != NULL) {
previous = current;
current = current->next;
}
new_ptr = (struct node*)malloc(sizeof(struct node));
new_ptr->data = data;
new_ptr->next = NULL;
previous = new_ptr;
}
/************************************************************
copyList - return new copy of list
************************************************************/
struct node* copyList(struct node** head) {
struct node* copy = NULL;
struct node* current = NULL;
struct node* new_ptr = NULL;
/* Copy current head to copy */
current = *head;
while (current != NULL) {
appendNode(&copy, current->data);
current = current->next;
}
return copy;
}
/************************************************************
printList - print linked list as "List: < 2, 5, 6 >" (example)
************************************************************/
void printList(struct node** head) {
struct node* current = NULL;
printf("List: < ");
current = *head;
if (current == NULL)
printf("none ");
while (current != NULL) {
printf("%d", current->data);
current = current->next;
if (current != NULL)
printf(", ");
}
printf(" >\n");
}
void main() {
int i; // index used for loops
struct node *list_a; // a new list
struct node *list_a_copy; // copy of list
list_a = NULL; // initialize empty list
list_a_copy = NULL; // initialize empy list
// test push
for (i = 0; i < 4; ++i)
push(&list_a, i);
// test length
printf("Length of list = %d\n", length(&list_a));
// test print head list
printf("head:\n");
printList(&list_a);
// test append node
for (i = 4; i < 8; ++i)
appendNode(&list_a, i);
// test print head list
printf("head(append):\n");
printList(&list_a);
// make a copy of list
list_a_copy = copyList(&list_a);
// test pop head list
for (i = 0; i < 4; ++i)
printf("%d popped\n", pop(&list_a));
// test print copy list
printf("head copy:\n");
printList(&list_a_copy);
// test pop copy list
for (i = 0; i < 4; ++i)
printf("%d popped\n", pop(&list_a_copy));
}

Answer 2

From the error log, I notice that you have incompatible types multiples time. It would help if you post your main.cpp as well.
p.s Like Mike G said, double pointer is really excessive. You can implement a linked list using a single pointer.

Answer 3

Problem is in your pop() function...take a look.
you are assigning
val=head->data;
temp = head->next;
also you are freeing the head and then reassigning.
you should do like this
val=(*head)->data;
temp=(*head)->next;
and while freeing memory...free the temp and reassign head to its next.