Thread: Something about typedef (data structures)(Linkedlist)

Hello,
There is a program which sorts the chars we input alphabetically by a linked list.At first we define structer.

Code:

struct linkedlist {
stuct linkedlist *nextptr;
char data ;
} ;

typedef linkedlist Linkedlist;
Linkedlist *Linkedlistptr;
/* I can not understand the definition with star. I figured out as it
  automatically defines struct variables as pointers. I dont know if I am true
*/

Then there is a inserter function to which we send the startptr which is defined buy *Linkedlistptr definer. Here how we call the inserter function:

>inserter(&startptr);

and we instruct the inserter :
>void inserter(Linkedlistptr *startptr);

What I couldnt understand here is , isnt startptr is already a pointer , why are we sending it's adress?We definitely dont use startptr's adress in inserter function. So if I narrow down my questions as two :

1- Linkedlist *linkedlistptr , provides linkedlistptr to automatically define struct pointers. Is it true ? ( Linkedlistptr startptr; So here startptr is a pointer right? )
2-why do we need to send startptr's adress?

Oh one more =>

3-typedef linkedlist Linkedlist;
Linkedlist *Linkedlistptr;

Why did the book define Linekdlistptr by two steps . Couldnt we do

>typedef linkedlist *Lişnkedlistptr ; ,

???
Thank you for all responses.

>I can not understand the definition with star.
"T *obj" means that obj is a pointer to T, where T is any reasonably simple type.

>1- Linkedlist *linkedlistptr , provides linkedlistptr to automatically define struct pointers. Is it true ?
Not with the code you gave. Presumably you meant this:

Code:

typedef struct linkedlist Linkedlist;
typedef Linkedlist *Linkedlistptr;

This defines two type aliases. The first is an alias for the structure, and it's primarily there to save you from typing the required struct keyword. The second is an alias for a pointer to the structure.

>2-why do we need to send startptr's adress?
The inserter function doesn't return anything, so how do you update the list if the start pointer changes? The two ways to do that are to return a pointer to the new list start:

Code:

struct linkedlist *insert ( struct linkedlist *start );

/* ... */

start = insert ( start );

And pass in a pointer to the start of the list:

Code:

void insert ( struct linkedlist **start );

/* ... */

insert ( &start );

Without the typedefs you can more easily see that we're working with a pointer to a pointer. Adding a level of indirection means that any changes to *start will be reflected back in the calling code. Pointers are passed by value, which is why you can't simply pass a pointer and expect any changes to the pointer itself to stick.

>Why did the book define Linekdlistptr by two steps
Probably in anticipation of using just a bare structure object instead of always using pointers.

Your question is a bit vague and there seem to be several copying mistakes.

Firstly, the last line is not a typedef but a declaration of a pointer to struct Linkedlist.

The typedef is used for convenience. Without the typedef you'd have to delcare the pointer the following way:

Code:

struct linkedlist {
struct linkedlist *nextptr;
char data ;
};

struct linkedlist *Linkedlistptr;

Since it might be inconvenient to add the struct keyword each time you want to delare a linkedlist pointer (instance), you can use the typedef to remove the need of it:

Code:

struct linkedlist {
struct linkedlist *nextptr;
char data ;
};

typedef struct linkedlist Linkedlist;
Linkedlist *Linkedlistptr;

What happens is that for the compiler Linkedlist translates to struct linkedlist, as in the previous snippet.

May-be more usual, but equivalent way to create this typedef is

Code:

typedef struct linkedlist {
struct linkedlist *nextptr;
char data ;
} Linkedlist;

Linkedlist *Linkedlistptr;

>Anon I did not copy them. So I did not make a mistake. My source was writing like that.
Then your book sucks, and from what I've heard of your book, it doesn't suck that much. Let's take a look:

>stuct linkedlist *nextptr;
It's struct, not stuct.

>typedef linkedlist Linkedlist;
Illegal C. To specify a structure type, you must include the struct keyword.

>void inserter(Linkedlistptr *startptr);
With the definition of Linkedlistptr as an object, you can't use it as a type. This is illegal as well.

The only possible corrections that make sense for your question are:

Code:

struct linkedlist {
struct linkedlist *nextptr;
char data ;
} ;

typedef struct linkedlist Linkedlist;
typedef Linkedlist *Linkedlistptr;

Originally Posted by ozumsafa

Well , to be honest , I did take it a little bit wrong. Right , but he wrote aggresively as you can feel. Anyway , the code works on a couple of compilers I have. Also that is the way deitel&detiel used. I still cant get your point.

The point is that if you are using typedef, you should use "typedef struct xx yy;", not "typedef xx yy;" - the latter is valid in C++, but not in C. If your compiler is C/C++ compiler it MAY accept the latter as an extension, but it's not good practice to use C++ things in C.

--
Mats

>Hey my friend , you must be upset of something.
I don't get upset from trivial things like this.

>That stuct , the forgeting of struct keyword , these were not that important anyway.
I think they're very important, because the inconsistency you created with these problems makes your questions ambiguous.

>I suggest you to think more detailed about that.
I suggest you think more carefully before incorrectly correcting someone. Here are quotes from your original post:

Code:

typedef linkedlist Linkedlist;
Linkedlist *Linkedlistptr;

and we instruct the inserter :
>void inserter(Linkedlistptr *startptr);

The function declaration is a syntax error because Linkedlistptr is not a type, it's an object. Feel free to compile it yourself:

Code:

struct linkedlist {
  struct linkedlist *nextptr;
  char data;
};

typedef struct linkedlist Linkedlist;
Linkedlist *Linkedlistptr;

void inserter(Linkedlistptr *startptr);

int main ( void )
{
  return 0;
}

If what you wrote isn't what you meant to write, you have no excuse when someone corrects you.

>Anyway , the code works on a couple of compilers I have.
Then you're probably compiling as C++, which has subtly different semantics than C. I highly recommend figuring out how to compile strictly as C with your compiler if you intend to learn C.

Originally Posted by ozumsafa

Well , to be honest , I did take it a little bit wrong. Right , but he wrote aggresively as you can feel. Anyway , the code works on a couple of compilers I have. Also that is the way deitel&detiel used. I still cant get your point.

Hint: don't assume all programmers are male.

>I was not talking about typedef struct matsp.
We help with the big picture, not just the exact questions that are asked. You can learn more that way, but if you're not interested in learning, we're not forcing you to hang around.