Problem with Linked list ()

Hi,
i'm new in c, I have a problem with a Linked list maybe you can help me :)
I made a linked list, and it's working fine, but i have a problem trying to access to an item using an index
This is my structure and functions (i'll put a red note where i think the error is):

Code:

---

typedef struct node{
        int a;
        int b;
        struct nodo *next;
}List;

//insert function:
List *insert(List **lst,    int a,    int b){
        assert(lst!=NULL);
        List *node=malloc(sizeof(List));
        node->next=*lst;
        *lst=node;
        node->a=a;
        node->b=b;
        return *lst;
}

List **get(List **lst, int index){
        int i=0;
        while ( (*lst != NULL) && (i< index) ){
                lst=&(*lst)->next;
                i++;
        }
        if (i==index){
            return lst;
        }else{
            return NULL;
        }
}

//im calling the get from other function using this:
int valuea=get(&alist,10)->a;

---

the error i'm receiving is:
error: request for member 'a' in something not a structure or union

I'll appreciate your help!
Thank you!

The problem is that get returns a pointer to a pointer to a List, but you are treating it as if it returned a pointer to a List. Since you do not need get to return a pointer to a pointer to List, you could just change it to return a pointer to a List. However, since you designed it to return a null pointer if the index does not exist, you may want to check that a null pointer was not returned.

how can i post a Tc like that an error always occur

Please use code tags: insert

Code:

---

before your source code and

---

after it in order to post.

I also need help for my program

Hi LaserLight!, thank you for your response!
I tried what you said, but now i'm receiving a warning (warning: return from incompatible pointer type) :(

Code:

---


List *get(List **lst, int index){
        int i=0;
        while ( (*lst != NULL) && (i< index) ){
                lst=&(*lst)->next;
                i++;
        }
        if (i==index){
            return lst;
        }else{
            return NULL;
        }
}

---

I know it's a newbie question, but i'm new with c and it's making me crazy.
Thank you again! :)

Quote:

---

Originally Posted by laserlight

The problem is that get returns a pointer to a pointer to a List, but you are treating it as if it returned a pointer to a List. Since you do not need get to return a pointer to a pointer to List, you could just change it to return a pointer to a List. However, since you designed it to return a null pointer if the index does not exist, you may want to check that a null pointer was not returned.

---

What I mean is something like this:

Code:

---

List *get(List *list, int index) {
    int i = 0;
    while (list != NULL && i < index) {
        list = list->next;
        ++i;
    }
    return list;
}

---

Your use would then be along the lines of:

Code:

---

/* Assume that alist is a pointer to List */
List *found = get(alist, 10);
int valuea = found ? found->a : 0;

---

Thank you thank you thank you, you save my day!!! :)

The idea of writing a linked-list function that obtains an item given its index is a fundamentally wrong thing to ever do.
The moment you introduce an O(n) operation to a container, and that is obviously the kind of thing people will commonly want to use within a loop, you're virtually guaranteed to have a program full of hideously inefficient O(n*n) behaviour (and needlessly so).

That is the reason why you will never find std::list or any other serious list implementation in any programming language exposing such a feature, unless the underlying structure is an array rather than a linked-list (which has a similar yet different problem if it includes erase or insert methods).

Hi iMalc, thank you for your suggestion.
I know i'm doing a bad use of this std creating high order functions, I tried to make a Vector structure (with getat, push, etc.) functions but i lost all the day and i couldn't, I'm experimented with Java, C# and other programming languages but not with c (is my first week programming in this language). If you can recommend me an apropiated std implementation to use i'll try to use it.
Thank you! :)

Quote:

---

Originally Posted by iMalc

The idea of writing a linked-list function that obtains an item given its index is a fundamentally wrong thing to ever do.
The moment you introduce an O(n) operation to a container, and that is obviously the kind of thing people will commonly want to use within a loop, you're virtually guaranteed to have a program full of hideously inefficient O(n*n) behaviour (and needlessly so).

That is the reason why you will never find std::list or any other serious list implementation in any programming language exposing such a feature, unless the underlying structure is an array rather than a linked-list (which has a similar yet different problem if it includes erase or insert methods).

---