Question

Question about queue's below:

Answer 1

I am trying to insert at the head and tail of a queue and im having some trouble with the tail portion. Heres what i have:

My structure:
```
typedef struct queue
{
int key;
struct queue *prev;
struct queue *after;
}DEQUEUE;
```
I am passing in my head and tail from main like this, and my create function works just fine as well
```
int main(void)
{
DEQUEUE *head,*tail;
head = NULL;
tail = NULL;
int x;

createQueue(&head,&tail);
```
i enter 1,2,3,4 and get the output
```
TAIL -> 4 <--> 3 <--> 2 <--> 1 <--> NULL
```

but when i try to enter a key to the end of the list using this function
```
void insertAtTail(DEQUEUE **T,int x)
{
DEQUEUE *new,*curr;
curr = *T;

new = malloc(sizeof(DEQUEUE));

if(!new)
{
printf("Malloc failure, out of memory.\n");
exit(1);
}

new->key = x;

if(!curr)
{
printf("You must create a list first, it is currently empty.\n\n");
}

else
{
curr->prev = new;
new->after = curr;
curr = new;
}
}
```
i get the output when i attempt to add a new number to the queue at the end of the list above as follows. Say i tried to add 55
```
TAIL -> 4 <--> 55 <--> NULL
```
im trying to teach myself this stuff and im obviously missing something so any help would be awesome!

Answer 2

and im wanting the output
```
TAIL -> 55 <--> 4 <--> 3 <--> 2 <--> 1 <--> NULL
```

Answer 3

```
#include <stdlib.h>
#include <stdio.h>
#include "dequeue.h"


int main(void)
{
DEQUEUE *head, *tail;
head = NULL;
tail = NULL;
int x;

createQueue(&head,&tail);

printf("Head = %d\n",head->key);
printf("Tail = %d\n",tail->key);

/*
printf("What number do you want to insert at the head: ");
scanf("%d",&x);
printf("\n");
insertAtHead(&head,x);
*/



printf("What number do you want to insert at the tail: ");
scanf("%d",&x);
printf("\n");
insertAtTail(&tail,x);



printQueue(head,tail);

return 0;
}

void createQueue(DEQUEUE **H, DEQUEUE **T)
{
{
DEQUEUE *new,*curr;

do{

new = malloc(sizeof(DEQUEUE));

if(new == NULL) // check to see if malloc worked
{
printf("Malloc failure, memory not allocated!!\n");
exit(1);
}


printf("Enter a number (-1 to quit): "); // lets user enter desired number or -1 to quit
scanf("%d",&new->key);
new->prev = NULL;
new->after = NULL;
printf("\n");

if(new->key == -1)
{
printf("List created\n\n"); // tell user list was created
}

else
{
if(*H == NULL && *T == NULL) // if its a new list, make the new value entered the head node
{
*H = new;
*T = new;
curr = new;
}

else
{
(*T)->after = new;
new->prev = (*T);
(*T) = new;

}
}

}while(new->key >0); // while the number entered wasnt negative
}
}

void printQueue(DEQUEUE *H,DEQUEUE *T)
{
DEQUEUE *curr,*temp;
curr = H;
temp = T;

if(!curr)
{
printf("Empty List\n");
exit(0);
}

else
{
printf("BACK <--> ");
while(temp!=NULL)
{
printf("%d <--> ",temp->key);
temp = temp->prev;
}

printf("FRONT\n\n");
}
}

void insertAtHead(DEQUEUE **H,int x)
{
DEQUEUE *new,*curr;
curr = *H;

new = malloc(sizeof(DEQUEUE));

if(!new)
{
printf("Malloc failure, out of memory.\n");
exit(1);
}


new->key = x;
new->prev = NULL;
new->after = NULL;

if(!curr)
{
printf("You must have an existing list first, create one.\n\n");
}

else
{
new->after = curr;
curr->prev = new;
curr = new;
}
}

void insertAtTail(DEQUEUE **T,int x)
{
DEQUEUE *new,*curr;
curr = *T;

new = malloc(sizeof(DEQUEUE));

if(!new)
{
printf("Malloc failure, out of memory.\n");
exit(1);
}

new->key = x;

if(!curr)
{
printf("You must create a list first, it is currently empty.\n\n");
}

else
{
curr->prev = new;
new->after = curr;
curr = new;
}
}
```
whole program if that helps any.

Answer 4

@e.mccormick