Doubly linked list

[Apprentice123]

I'm trying to make doubly linked list in c, only not quite understand the theory. The code is as follows:

typedef struct list
{
    int num;
    struct list *pre;
    struct list *next;
} LIST;

void insert_later(LIST *L, int n)
{
    LIST *aux;
    
    aux = (LIST *) malloc(sizeof(LIST));
    aux->num = n;
    aux->next = L->next;
    aux->pre= *L;
    L->next = *aux;
}

I know that is not correct, what are my mistakes in the theory of lists and in the language?

[julmuri]

This might be better way to do it.

struct List
{
    int      data;
    List*    head;
    List*    tail;

}; // struct List


int append( List* list, int data )
{
    //
    // is valid list
    if ( 0 == list )
        return -1;

    //
    // allocate new node
    List* tail = (List*)malloc( sizeof(List) );
    if ( 0 == tail )
        return -1;

    //
    // forward to last valid node
    for ( ; 0 != list->tail; list = list->tail )
    { /* ... */ }

    //
    // set tail to newly allocated node
    list->tail = tail;

    // set head to old list tail and
    // tail to 0 so we can tell it is last node
    tail->data = data;
    tail->head = list;
    tail->tail = 0;

    return 0;
}


int main( int argc, char* argv[] )
{
    List list = {0};

    for ( int it = 1; it < 5; ++it )
        append( &list, it );

    for ( List* it = &list; 0 != it->tail; it = it->tail )
        printf( "List::data=%d\n", it->data );

    //
    // clean up
    return 0;
}

Hope this helps.

[Apprentice123]

I would like to create a list structure.

The first function I tried was to insert value at the end of the list. How do I insert the end of the list?

[Apprentice123]

I find this code in internet:

#include <stdio.h>
#include <stdlib.h>

struct node
{
    int info;
    struct node *right;
    struct node *left;
};

typedef struct node *nodeptr;

nodeptr getnode()
{
    nodeptr p;
    p = (struct node*) malloc(sizeof(struct node));
    return p;
}

nodeptr initialize(nodeptr S)
{
    S = NULL;
    return S;
}

int empty(nodeptr S)
{
    if(S==NULL)
        return 1;
    else
        return 0;
}

void insertright(nodeptr p, int num)
{
    nodeptr q;
    nodeptr temp;
    q = getnode();
    temp = getnode();
    if(p==NULL)
    {
        q->info = num;
        q = p->right;
        q->left = p;
    }
    else 
    {
        temp = p->right;
        p->right = q;
        q->left = p;
        q->right = temp;
        temp->left = q;
    }
}

void insertleft(nodeptr p, int num)
{
    nodeptr q;
    nodeptr temp;
    q = getnode();
    temp = getnode();
    if(p==NULL)
    {
        p->info = num;
        q->left = p;
        p->right = q;
    }
    else 
    {
        p->info = num;
        temp = q->right;
        q->right = p;
        q = p->left;
        p->right = temp;
        temp->left = p;
    }
}

void removenode(nodeptr p)
{
    nodeptr q;
    q = p;
    p = p->right;
    p->left = NULL;
    free(q);
}

void display(nodeptr S)
{
    nodeptr p1;
    p1 = S;
    printf("\n LINKED LIST \n");
    while(p1!=NULL)
    {
        printf("&#37;d \n",p1->info);
        p1 = p1->right;
    }
}

int main()
{
    nodeptr mainnode;
    initialize(mainnode);
    insertright(mainnode,10);
    insertright(mainnode,20);
    insertright(mainnode,30);
    insertleft(mainnode,40);
    insertleft(mainnode,50);
    display(mainnode);
    return 0;
}

But to execute the program show the message of the windows that the memory can not be read