So, I am relatively new to C and trying to implement a Queue using Linked Lists. Here is some code I wrote with help from the internet.
#include <stdio.h>
#include <stdlib.h>
#define pscan(prompt, x) printf(prompt); scanf("%d", &x)
#define nl() printf("\n");
typedef struct Node {
int data;
struct Node* next;
} Node;
typedef struct LinkedList {
Node* head;
Node* tail;
int size;
int (*add) (struct LinkedList*, int, int);
int (*append) (struct LinkedList*, int);
int (*get) (struct LinkedList*, int);
int (*remove) (struct LinkedList*, int);
void (*display_list) (struct LinkedList*);
Node* (*createNode) (int);
} LinkedList;
int add (LinkedList* self, int data, int position);
int append (LinkedList* self, int data);
int get (LinkedList* self, int position);
int rmv (LinkedList* self, int position);
void display_list (LinkedList* self);
LinkedList createLinkedList ();
Node* createNode (int data);
int add(LinkedList* self, int data, int position)
{
if (position > self->size || position < 0)
{
printf("Index out of bounds\n");
return 0;
}
Node* newNode = self->createNode(data);
Node* head = self->head;
Node* tail = self->tail;
if (position == 0)
{
if (head == NULL) self->head = newNode;
else
{
if (tail == NULL) tail = head;
newNode->next = head;
self->head = newNode;
}
self->size ;
}
else if (position == self->size)
{
if (head == NULL) self->head = newNode;
else
{
if (tail == NULL) tail = head;
tail->next = newNode;
self->tail = newNode;
}
self->size ;
}
else
{
Node* prev = head;
for(int i = 0; i < position-1; i )
{
prev = prev->next;
}
Node* node = prev->next;
prev->next = newNode;
newNode->next = node;
self->size ;
}
return 0;
}
int append(LinkedList* self, int data)
{
return self->add(self, data, self->size);
}
int get(LinkedList* self, int position)
{
if (self->size == 0)
{
printf("The list is empty.");
return 0;
}
else if (position >= self->size || position < 0)
{
printf("Index out of bound.");
return 0;
}
if (position == 0) return self->head->data;
else if (position 1 == self->size) return self->tail->data;
else
{
Node* node = self->head;
for(int i = 0; i < position; i ) node = node->next;
return node->data;
}
}
int rmv (LinkedList* self, int position)
{
int dt;
if (self->size == 0)
{
printf("The list is empty.");
return 0;
}
else if (position >= self->size || position < 0)
{
printf("Index out of bound");
return 0;
}
if (position == 0)
{
Node* head = self->head;
Node* next = head->next;
self->head = next;
dt = head->data;
free(head);
self->size--;
}
else if (position 1 == self->size)
{
Node* node = self->head;
Node* tail = self->tail;
for(int i = 0; i < self->size-2; i ) node = node->next;
node->next = NULL;
self->tail = node;
dt = tail->data;
free(tail);
self->size--;
}
else
{
Node* prev = self->head;
Node* next;
Node* node;
for(int i = 0; i < position-1; i ) prev = prev->next;
node = prev->next;
next = node->next;
prev->next = next;
dt = node->data;
free(node);
self->size--;
}
return dt;
}
void display_list(LinkedList* self)
{
if (self->size == 0) printf("This list is empty.\n\n");
else
{
Node* node = self->head;
printf("[");
for (int i = 0; i < self->size; i )
{
if (i > 0) printf(", ");
printf("%d", node->data);
node = node->next;
}
printf("]\n\n");
}
}
Node* createNode (int data)
{
Node* node = (Node*) malloc(sizeof(Node));
node->data = data;
node->next = NULL;
return node;
}
LinkedList createLinkedList ()
{
LinkedList l;
l.head = NULL;
l.tail = NULL;
l.add = &add;
l.append = &append;
l.get = &get;
l.remove = &rmv;
l.display_list = &display_list;
l.createNode = &createNode;
l.size = 0;
return l;
}
typedef struct queue
{
LinkedList items;
int *size;
int (*enqueue) (struct queue*, int);
int (*dequeue) (struct queue*);
int (*peek) (struct queue*, int);
void (*display) (struct queue*);
} Queue;
Queue CreateQueue();
int enqueue(Queue* self, int item);
int dequeue(Queue* self);
int peek(Queue* self, int pos);
void display(Queue* self);
Queue CreateQueue()
{
Queue q;
q.items = createLinkedList();
q.size = &(q.items.size);
q.enqueue = &enqueue;
q.dequeue = &dequeue;
q.peek = &peek;
q.display = &display;
return q;
}
int enqueue(Queue* self, int item)
{
self->items.append(&(self->items), item);
return 1;
}
int dequeue(Queue* self)
{
return self->items.remove(&(self->items), 0);
}
int peek(Queue* self, int pos)
{
return self->items.get(&(self->items), pos);
}
void display(Queue* self)
{
printf("%d items in queue.\n", *(self->size));
self->items.display_list(&(self->items));
}
void main()
{
Queue q = CreateQueue();
q.enqueue(&q, 3);
q.enqueue(&q, 7);
q.enqueue(&q, 4);
q.display(&q);
int item = q.dequeue(&q);
printf("Dequeued: %d\n", item);
q.display(&q);
q.enqueue(&q, 14);
q.display(&q);
}
The part I'm having an issue with is making the Queue's size
pointer point to the LinkedList's size
integer and then accessing that value.
On compiling and running, I get this: Output from the above code
Thanks in advance.
CodePudding user response:
The problem is in createQueue
:
Queue CreateQueue()
{
Queue q;
q.items = createLinkedList();
q.size = &(q.items.size);
q.enqueue = &enqueue;
q.dequeue = &dequeue;
q.peek = &peek;
q.display = &display;
return q;
}
You set q.size
to point to q.items.size
. This is a pointer to a local variable. You then return a copy of q
, but the size
member now points to a local that doesn't exist. Dereferencing a pointer to a variable whose lifetime has ended triggers undefined behavior.
There's no need for the size
element in Queue
. Just access the size
element of the items
member directly.