Linking a LinkedList as an element to a different LinkedList

Question

I'm working on a C++ assignment where I'll create a search engine on a linked list of linked lists. As per the requirements, I can't use other libraries nor STL.

Basically it will be like this (I removed the variables from small list since they are irrelevant):

My structs are these:

struct small
{
    int data;
    struct small *next;
};


struct big
{
    int playerID;
    string playerName;
    string playerTeam;
    struct small *goals;
    struct big *next;
};

Here's the relevant code snippet, I think the problem is at addGoals(...) where I'm failing to assign the small element to the temp->goals.

class biglist
{
private:
    big *head, *tail;

public:
    biglist()
    {
        head = NULL;
        tail = NULL;
    }

. . .

void createbig(int ID, string name, string team)
    {
        big *temp = new big;
        temp->playerID = ID;
        temp->playerName = name;
        temp->playerTeam = team;
        temp->goals = NULL;
        temp->next = NULL;

        if (head == NULL)
        {
            head = temp;
            tail = temp;
            temp = NULL;
        }
        else
        {
            tail->next = temp;
            tail = temp;
        }
    }

void addGoals(int id, small *s)
    {
        big *temp = head;
        while (temp != NULL)
        {
            if (temp->playerID == id)
            {
                temp->goals = s;
                break;
            }
            temp = temp->next;
        }
    }

    void test()
{
    big *temp = head;
    while (temp != NULL)
    {
        if (temp->playerID == 1)
        {
            if (temp->goals !=NULL)
            {
                cout << temp->goals->data << endl;
            }
            else
            {
                cout << "goals null" << endl;
            }
        }
        temp = temp->next;
    }
}
}
. . .

class smalllist
{
private:
    small *head, *tail;

public:
    smalllist()
    {
        head = NULL;
        tail = NULL;
    }

    void createsmall(int ID, biglist b)
    {
        small *temp = new small;
        temp->data = ID;
        temp->next = NULL;

        if (head == NULL)
        {
            head = temp;
            tail = temp;
            temp = NULL;
        }
        else
        {
            tail->next = temp;
            tail = temp;
        }


        b.addGoals(1, temp);
    }
};

Finally, my main code:

int main()
{
    biglist obj;
    obj.createbig(1, "Player1", "Team1");
    obj.createbig(2, "Player2", "Team2");
    obj.displaybig();

    smalllist sml;
    sml.createsmall(9, obj);
    sml.displaysmall();

    obj.displaybig();
    obj.test();
}

Debugging throws an exception at:

cout << temp->goals->data << endl;

saying that

Exception thrown: read access violation. temp->goals was nullptr.

I'm 90% sure I messed up something with pointers; but other stuff I've tried gave errors before compiling. I checked out some books / tutorials but couldn't figure it out.

Also if you have a better approach or saw one of the horrible mistakes that I'm making, please don't hold back :)

Thanks.

EDIT I changed my createbig() like this. Currently it works with following codes:

void createbig(int ID, string name, string team, small *s)
    {
        big *temp = new big;
        temp->playerID = ID;
        temp->playerName = name;
        temp->playerTeam = team;
        temp->goals = s;
        temp->next = NULL;

        if (head == NULL)
        {
            head = temp;
            tail = temp;
            temp = NULL;
        }
        else
        {
            tail->next = temp;
            tail = temp;
        }
    }

and added this to small

small getsmall(int i)
    {
        small *temp = head;
        while (temp != NULL)
        {
            if (temp->data == i)
            {
                return *temp;
            }
        }
    }

My final main function is

int main()
{
    smalllist sml;
    sml.createsmall(9);
    sml.displaysmall();

    biglist obj;
    small s = sml.getsmall(9);
    obj.createbig(1, "Player1", "Team1", &s);
    //obj.createbig(2, "Player2", "Team2");
    obj.displaybig();

    obj.test();

}

While it ends successfully now, it gives the address of goals and I get this in debug section:

JaMiT · Accepted Answer

Let's look at what your code does, going through the main function. (Being able to walk through code like this is a useful skill. You can also use a debugger to help out, stepping through your function line-by-line.)

biglist obj;

Default construct a biglist. The head and tail are null. (By the way, nullptr is C++'s replacement for C's NULL.)

obj.createbig(1, "Player1", "Team1");
obj.createbig(2, "Player2", "Team2");

Add entries in obj for players with IDs 1 and 2. Their goals are null.

obj.displaybig();

Presumably an output of obj?

smalllist sml;
sml.createsmall(9);
sml.displaysmall();

These lines do something with a smalllist, but do not reference obj, so they are not relevant to this issue.

obj.displaybig();

Presumably an output of obj? Kind of redundant since nothing affected obj since the last display.

obj.test();

Call the test code, which finds the element for player ID 1 and outputs the data of that player's first goal. However, if you look up where that player was added, the goal is null, so you get a crash.

Separate from the above, there is probably some confusion in createsmall. Inside that function, a new biglist is created (not obj), and that list is told to add a goal to the player with ID 1. However, this has no effect the biglist in the main function.

Linking a LinkedList as an element to a different LinkedList

Answers (2)

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