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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));

}
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1
  • One of the issues that I see is, your using a double pointer in the context of a single pointer. Commented Nov 25, 2013 at 3:36

3 Answers 3

Reset to default
2

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));
}
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Comments

0

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.

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Comments

0

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.

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