Question

I need some serious help with understanding Linked Lists in C++ I am suppose to take a program that I wrote a couple weeks ago using array structures and convert them into linked lists and add a couple new functions. My big concern is I do not feel confident in linked lists and have been spending time on here and other sites gaining knowledge on them. But I cannot find a source that helps me relate to the problem I am facing right now.

Here is my original code:

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

#define MAX 100

struct YouTubeVideo { 
char video_name[1024];        // YouTube video name
int video_ranking;            // Number of viewer hits
char video_url[1024];         // YouTube URL
};

struct YouTubeVideo Collection[MAX];

int tail = 0;

//-- Forward Declaration --// 
void printall();
void insertion();
void sort();
void branching(char option);        
void menu(); 
void load_file();
void save_file();

int main()
{
char ch; 

load_file();

printf("\n\nWelcome to CSE240: YouTube Classic Hits\n");

do {
     menu();
     fflush(stdin);                 // Flush the standard input buffer 
     ch = tolower(getchar());       // read a char, convert to lower case
     branching(ch);
} while (ch != 'q');

return 0; 
}

void menu()
{
 printf("\nMenu Options\n");
 printf("------------------------------------------------------\n");
 printf("i: Insert a new favorite\n");
 printf("p: Review your list\n"); 
 printf("q: Save and quit\n");
 printf("\n\nPlease enter a choice (i, p, or q) ---> "); 
}

void branching(char option)
{
switch(option)
{
    case 'i':
        insertion();
        sort();
    break;

    case 'p':
        printall();
    break;

    case 'q':
        save_file();
    break;

    default:
        printf("\nError: Invalid Input.  Please try again..."); 
    break;
}
}

void insertion()
{
if(tail < MAX)
{
    printf("\nWhat is the name of the video? (No spaces characters allowed)\n");
    scanf("%s", Collection[tail].video_name);

    printf("\nHow many viewer hits does this video have?\n");
    scanf("%d", &Collection[tail].video_ranking);

    printf("\nPlease enter the URL: ");
    scanf("%s", Collection[tail].video_url);

    tail++;
}
else
{
    printf("\nERROR: Your collection is full. Cannot add new entries.\n");
}
}

void sort()
{
int i = 0, j = 0; 
struct YouTubeVideo temp;

for(i = 0; i < tail; i++)
{
    for(j = i+1; j < tail; j++)
    {
        if(Collection[i].video_ranking < Collection[j].video_ranking)
        {
            temp = Collection[i];
            Collection[i] = Collection[j];
            Collection[j] = temp;
        }
    }
}

//RA: I think it's easier (and faster) to assume your current list is already
//    sorted and then insert your new element into the correct position. (You
//    can show this maintains a sorted list by induction.)

printf("\nSorting Complete...\n");
}

void printall()
{
int i; 

printf("\nCollections: \n"); 

for(i = 0; i < tail; i++)
{
    printf("\nVideo Name: %s", Collection[i].video_name);
    printf("\nRanking (Hits): %d", Collection[i].video_ranking);
    printf("\nURL: %s", Collection[i].video_url);
    printf("\n");
}
}

void save_file() { 
FILE *fileName;                                     // declare a pointer to File type 
char ch; 
int index = 0; 

fileName = fopen("ranking.dbm", "wb");              // "b" for binary mode 
                                                        // ìwî for write


if(fileName != NULL) 
{   
    fwrite(&tail, sizeof(int), 1, fileName);        // Write tail to the     file for later retrieval.

    for(index = 0; index < tail; index++)
    { 
        fwrite(&Collection[index].video_name, 1024, 1, fileName); 
        fwrite(&Collection[index].video_ranking, sizeof(int), 1, fileName);     
        fwrite(&Collection[index].video_url, 1024, 1, fileName);
    } 

    fclose(fileName);
} 
else 
    printf ("ERROR: Could not open file for saving data !\n"); 
}

void load_file() { 
FILE *fileName;                         // declare a pointer     to File type 
int index = 0; 

fileName = fopen("ranking.dbm", "rb");  // "b" for binary mode 
                                    // ìrî           for read

if(fileName != NULL) {   
    fread(&tail, sizeof(int), 1, fileName);

    for(index = 0; index < tail; index++) 
    {
        fread(Collection[index].video_name, 1024, 1, fileName);
        fread(&Collection[index].video_ranking, sizeof(int), 1, fileName);
        fread(Collection[index].video_url, 1024, 1, fileName);
    }

    fclose(fileName);
}
else 
    printf ("ERROR: Could not open file for loading data !\n"); 
}

These are the exact instructions for what I am suppose to do:

Convert the “YouTubeVideo” array structure (Collection) into a linked-list. The program must sort (by “video_name”) the entries as they are inserted into the linked-list. [30 points] (*Note: You will lose 10 points if the linked list is not sorted.)

Now I have given it as good a go as I believe I can with my current understanding but I am running into a problem right now.

Here is the code with my attempt at a solution in it:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

using namespace std;

#define MAX 100

 struct YouTubeVideo
{
char name[1024];        // YouTube video name
int ranking;                // Number of viewer hits
char url[1024];             // YouTube URL
};

struct YouTubeVideo Collection[MAX];

int tail = 0;

//-- Forward Declaration --//
void printall();
void insertion();
void branching(char option);
void menu();


int main()
{
char ch;

// TODO: Add code to load save data from file

cout << "\n\nWelcome to CSE240: YouTube Classic Hits\n";

do {
    menu();
    cin >> ch; // read a char, convert to lower case
    cin.ignore();

    ch = tolower(ch);
    branching(ch);
} while (ch != 'q');

return 0;
}

void menu()
{
cout << "\nMenu Options\n";
cout << "------------------------------------------------------\n";
cout << "i: Insert a new favorite\n";
cout << "p: Review your list\n";
cout << "s: Search\n";
cout << "d: Delete an entry\n";
cout << "q: Save and quit\n";
cout << "\n\nPlease enter a choice (i, p, s, d, or q) ---> ";
}

void branching(char option)
{
switch(option)
{
    case 'i':
        insertion();
        break;

    case 'p':
        printall();
        break;

    case 's':
        // TODO: Add code to search for a particular node by name
        break;

    case 'd':
        // TODO: Add code to remove a node
        break;

    case 'q':
        // TODO: Add code to save data into a file
        break;

    default:
        cout << "\nError: Invalid Input.  Please try again...";
        break;
}
}

void insertion() { // insert a new entry
struct YouTubeVideo *p, *temp;
p = (struct YouTubeVideo *) malloc(sizeof(struct YouTubeVideo)); if (p == 0) {
    printf("out of memory\n"); return; }
printf("Enter Video name, Views, URL: \n"); scanf("%s", p->name); // p->name is array         scanf("%d", &p->phone);
scanf("%s", p->ranking);
temp = head;
if ((head == NULL)||(strcmp(p->name, temp->name) <=0)) {
    p->next = head;
    head = p;
}

else {
    while (temp->next != NULL) {
        if (stricmp(p->name, temp->next->name) <=0) { p->next = temp->next;
            temp->next = p;
            return;
        } else
            temp = temp->next; }
    p->next = NULL;
    temp->next = p;
} }

void printall()
{
int i;

cout << "\nCollections: \n";

for(i = 0; i < tail; i++)
{
    cout << "\nVideo Name: " << Collection[i].name << "\n";
    cout << "\nRanking (Hits): " << Collection[i].ranking << "\n";
    cout << "\nURL: " << Collection[i].url << "\n";
    cout << "\n";
}
}

The problem I am having is with my insertion I am getting error undeclared identifier head and no member named next in YouTubeVideo. I tried placing and declaring them in a bunch of places but cannot seem to fix these errors.

I would really appreciate some help and any possible knowledge you could bestow upon me. I really have given this a big go, but am just stuck fro the moment.

Answer 1

You need to actually implement a linked list. This looks like a homework assignment, so im wondering how far into c++ you are. If you arent really doing classes and Object oriented programming yet, the easiest way to fix this is going to be to add a next and previous object to your youtube video struct. like this:

struct YouTubeVideo {   
    char video_name[1024];        // YouTube video name  
    int video_ranking;            // Number of viewer hits 
    char video_url[1024];         // YouTube URL  
    YouTubeVideo* next;
    YouTubeVideo* previous;
};  

next, you will need to add a head declaration. this is going to be of type YouTubeVideo*. When you add the first video, set the head to point at thsi video. then, whenever you add a new video after that, set the next pointer of the head video to point to the new video, and the previous pointer on the new video should point to the head video. Thats the start of your linked list, but your solution is still very messy.

If I was you, I would look at how some Linked List classes are implemented. Here is the header file for the first Linked List class i wrote:

#ifndef LINKEDLIST_H 
#define LINKEDLIST_H

#include "List.h" 

class LinkedList : public List { 
 public: 
  LinkedList();
  virtual double get(int index) const; 
  virtual void add(double item); 
  virtual void insert(int index, double item); 
  virtual double delete_item(int index); 
  virtual int size() const;  
 private:
  class Node;
  Node* first;
  Node* last;
  int listsize;
}; 
class LinkedList::Node{
 public:
  double value;
  Node* next;
};

#endif

You can see that this list has a class Node in it, and this class contains a value double value. If you wanted to use this code, you would have to make your node class have a field that was YouTubeVideo* value.