C++ Linked list

Question

I'm trying to make linked list similar too the one here:

linked list in C

That is to have the "head", I called it first, inside another struct. However I found doing that change. Makes it hard to add values to the list_item struct. I have tried some few things to see if it works. It compiles, however when I run the code it will crash. Any help would be helpful here. I know the cause of the crash is when I want to point the new_node to the linked_list.

#include <iostream>

using namespace std;

struct list_item
{
    int key;
    int value;
    list_item *next;
};

struct list
{
    struct list_item *first;
};

int main()
{
    list *head;
    list *new_node;

    head = NULL;
    head->first = NULL;

    for(int i = 0; i < 10; i++)
    {
        //allocate memory for new_node
        new_node = (list*)malloc(sizeof(list));
        new_node->first = (list_item*)malloc(sizeof(list_item));
        //adding the values
        new_node->first->key = i;
        new_node->first->value = 10 + i;

        //point new_node to first;
        new_node->first->next = head->first;

        //point first to new_node;
        head->first = new_node->first;

    }

    //print
     list *travel;
     travel->first = head->first;

     int i = 0;
     while(travel != NULL)
     {
         cout << travel->first->value << endl;
         travel->first = travel->first->next;
     }

    return 0;
}

Answer 1

You are creating 10 lists, I think you might try to do something like this:

#include <iostream>

using namespace std;

struct list_item
{
    int key;
    int value;
    list_item *next;
};

struct list
{
    struct list_item *first;
};

int main()
{
    //Just one head is needed, you can also create this
    // on the stack just write:
    //list head;
    //head.first = NULL;
    list *head = (list*)malloc(sizeof(list));
    list_item *new_node = NULL;

    head->first = NULL;

    for(int i = 0; i < 10; i++)
    {
        //allocate memory for new_node
        new_node = (list_item*)malloc(sizeof(list_item));
        //adding the values
        new_node->key = i;
        new_node->value = 10 + i;

        //if the list is empty, the element you are inserting
        //doesn't have a next element

        new_node->next = head->first;

        //point first to new_node. This will result in a LIFO
        //(Last in First out) behaviour. You can see that when you 
        //compile
        head->first = new_node;

    }

     //print the list 
     list_item *travel;
     travel = head->first;

     while(travel != NULL)
     {
         cout << travel->value << endl;
         travel = travel->next;
     }

    //here it doesn't matter, but in general you should also make
    //sure to free the elements
    return 0;
}

This is what is going on. At first you only have one head and no elements.

head
  |
  |
  V
 NULL

Then you add your first element. Make sure that the "new_node->next==NULL":

head
  |
  |
  V
node:   ------------------> NULL
key = 0
value = 10

Then you add another node in front but append your first node to its next node. you move the pointer from the head to the new node

head:
first
  |
  |
  V
node:   ---------> node:  -------------> NULL
key: 1             key: 0   
value: 11          value: 10  

etc.

Since you are using c++, you might consider using "new" and "delete". Just replace

new_node = (list_item*)malloc(sizeof(list_item));

with

list *head = new list

Answer 2

The next line only allocates memory for your list struct. The list contains only a pointer, you must also allocate memory for new_node->first before assigning to any of its members.

//allocate memory for new_node
new_node = (list*)malloc(sizeof(list));

Answer 3

I think you want something more like this:

#include <iostream>
#include <cstdlib>

using namespace std;

typedef struct tag_list_item
{
    int key;
    int value;
    struct tag_list_item *next;
} list_item;

typedef struct
{
    list_item *head;
} list;

int main()
{
    list my_list;
    list_item *new_node;
    list_item *previous_node = NULL;

    my_list.head = NULL;

    for(int i = 0; i < 10; i++)
    {
        //allocate memory for new_node
        new_node = (list_item*)malloc(sizeof(list_item));

        //adding the values
        new_node->key = i;
        new_node->value = 10 + i;

        if(previous_node == NULL)
        {
            my_list.head = new_node;
        }
        else
        {
            previous_node->next = new_node;
        }
        previous_node = new_node;    
    }

    //print
     list_item *iter = my_list.head;

     while(iter != NULL)
     {
         cout << iter->value << endl;
         iter = iter->next;
     }

    return 0;
}

Changes of note:

For malloc, I added:

#include <cstdlib>

I changed your list structures to typedefs, had to declare "next" using the tag since the typedef isn't complete at that point

typedef struct tag_list_item
{
    int key;
    int value;
    struct tag_list_item *next;
} list_item;

I changed your list name to "my_list" and declared it directly (without the pointer). In this case you can just have the compiler allocate it automatically on the stack.

list my_list;

I keep a pointer for "previous_node" so that you can assign the "next" pointer much more easily. Notice that the first node allocated is pointed to by the "head" pointer in the list structure. I believe that is the traditional name for the pointer to the first element in a list.

if(previous_node == NULL)
{
    my_list.head = new_node;
}
else
{
    previous_node->next = new_node;
}
previous_node = new_node;

Answer 4

head = NULL;
head->first = NULL;

There's the issue. You can't follow a pointer and set it to NULL if you've set the pointer itself to NULL.

That should be

head = malloc(sizeof(list));
head->first = NULL;

That should fix your code.

Hope that helps, Billy3

EDIT: There's also an issue with your FOR loop. When you allocate the list, you should only allocate the list itself once. When you insert an item, you only allocate a list_item. You're assigning a list pointer to a member which accepts only a list_item pointer ;)

See Gabe's post for a demonstration of correct behavior :)

Answer 5

Look at your struct declaration

struct list_item
{
    int key;
    int value;
    list_item *next;
};

That should be

struct list_item
{
    int key;
    int value;
    struct list_item *next;
};

Hope this helps, Best regards, Tom