Re: [Dev-C++] A double chain list problem about delete the special node

[Per W. <pw...@ia...>]

I haven't had the time to check all code, but there are a number of
errors here. For some of them, see below:

/Per W

On Thu, 3 Jun 2004, [gb2312] Bruce jjs wrote:

>
>     I have write a double chain list program that do only some simple ope=
ration.But the result of the function DeleteList() is not natural.It will d=
elete all the node before the ith node.I haven't found the reason of the pr=
oblem by far.Can someone can me why and how to improve it?
>
> ---Thanks,
>
>    Bruce.J
>
> My program is list belove:
>
> /* ************************************************************ */
>
> #include <stdio.h>
>
> enum bool {FALSE, TRUE};
> typedef enum bool BOOL;
> typedef int DATA;
> typedef struct DoubleLink {
>     DATA    data;
>     struct DoubleLink *prev;
>     struct DoubleLink *next;
> } DoubleLink;
>
> /* Input the value of n entries element to create a double chain list */
> DoubleLink *CreateList(DoubleLink *p, int n)
> {
>     DoubleLink *ptr;
>     int i;
>
------
What are you using the parameter p for, since you immediately
assigns your own value to it? Besides, when you call CreateList()
from main(), you send in an undefined pointer. Maybe you thought
of sending in a pointer to a pointer?
------
>     p =3D (DoubleLink *) malloc(sizeof(DoubleLink));
>
>     /* First create a empty double chain list with a head pointer */
>     p->next =3D NULL;
>     p->prev =3D NULL;
>     for (i =3D 0; i < n; ++i) {
>         ptr =3D (DoubleLink *) malloc(sizeof(DoubleLink));
>         scanf("%d", &ptr->data);
>         ptr->prev =3D p;
>         ptr->next =3D p->next;
>         p->next =3D ptr;
>     }
>     return p;
> }
>
> /* Get the length of the double chain list */
> int GetLength(DoubleLink *p)
> {
>     DoubleLink *p1;
>     int i =3D 0;
>
>     p1 =3D p->next;
>     while (p1) {
>         ++i;
>         p1 =3D p1->next;
>     }
>     return i;
> }
>
> /* Delete the ith element */
> /* It has a problem that it will delete the all element which before the =
ith element  when I run it,but I don't know why.*/
> BOOL DeleteList(DoubleLink *p, int i)
> {
>     if (i<1 || i>GetLength(p))
>         return FALSE;
>
>     DoubleLink *p1;
>     int j =3D 0;
>
>     p1 =3D p;
>     while(p1 && j<i) {
------
Depending on i, this might loop until p1 =3D=3D NULL
------
>         p1 =3D p1->next;
>         ++j;
>     }
------
And if p1 is NULL, you're not allowed to try p1->next
------
>     if (p1->next =3D=3D NULL)
------
Let's say that p1->next is NULL. Then you still have to
check if p1->prev is !=3D and in that case set p1->prev->next =3D NULL;
since p1->prev will loose it's last entry.
------
>         free(p1);  /* If it is a last node,direct to free the memory */
>     else {
>         p1->prev->next =3D p1->next;
>         p1->next->prev =3D p1->prev;
>         free(p1);
>     }
>     return TRUE;
> }
>
> /* Printf the double chain list*/
> void OutputList(DoubleLink *p)
> {
>     DoubleLink *p1;
>
>
>     p1 =3D p;
>     while (p1 =3D p1->next)
>         printf("%d,", p1->data);
>     printf("\n");
> }
>
> /* free the memory of double chain list If not to use it latter */
> void DestoryList(DoubleLink *p)
> {
>     while (p) {
------
You can't first delete p, and then get p->next. You have to
take a copy of p->next before calling free(p)
------
>         free(p);
>         p =3D p->next;
>     }
> }
>
> main()
> {
>     DoubleLink *ptr;
>     int n, i;
>
>     printf("Please input the number of data you want to create:\n");
>     scanf("%d", &n);
>     ptr =3D CreateList(ptr, n);
>     OutputList(ptr);
>
>     printf("Please input the ith where you want to delete:\n");
>     scanf("%d", &i);
>     if (DeleteList(ptr, i)) {
>         printf("Delete success!\n");
>         OutputList(ptr);
>     } else
>         printf("Delete failed!\n");
>     DestoryList(ptr);
>     system("PAUSE");
> }
>
>
>
>
> ---------------------------------
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