Question

I have a problem either adding to or traversing a linked list. The main item class is used by another class but I can add the correct number of these but it looks like when I add more data to the list the app no longer works.

I am not sure exactly where the error is. I know that when I try to traverse the list the application crashes. Any ideas or any improvements would be appreciated.

I can make the crash not happen by changing the AddOccurence method to not do the while loop.

Do

void Item::AddOccurence(int Item,int placeInLine){
    ItemOccurence* ocr=myHead;
    if(ocr)
    {

    }

instead of

void Item::AddOccurence(int Item,int placeInLine){
    ItemOccurence* ocr=myHead;
    while(ocr)
    {

    }

Basically hitting the first node but no more.

I have an object that contains a list.
Here is the .h file
#include
using namespace std;

class ItemOccurence{
public:
    ItemOccurence(int line,int placeInLine,ItemOccurence* link=NULL) :myLine(line),myPlaceInLine(placeInLine),myLink(link){}

    int myLine;
    int myPlaceInLine;
    ItemOccurence* myLink;
};

class Item {
public:
    Item();
    Item(string Item,int line,int placeInLine);
    virtual ~Item();
    void deallocate(ItemOccurence* p);
    void AddOccurence(int Item,int placeInLine);
    string myItem;
    ItemOccurence* myHead;
private:
    bool isEmpty();
};

And the .cpp file

#include "Item.h"
#include <string>
#include<iostream>
using namespace std;
Item::Item(string Item,int line,int placeInLine):myHead(NULL){
    myItem=Item; 

    myHead= new ItemOccurence(line,placeInLine,NULL);
}

Item::Item():myHead(NULL){
myHead=0;
}

Item::~Item() {
    deallocate(myHead);
    myHead=0;
}

void Item::deallocate(ItemOccurence* p){
    ItemOccurence* tmp;
    while(p){
        tmp=p;
        p=p->myLink;
        delete tmp;
    }
}

void Item::AddOccurence(int Item,int placeInLine){
    ItemOccurence* ocr=myHead;
    while(ocr)
    {
          cout<<"orrucence head while adding " << myHead->myLine << " " << myHead->myPlaceInLine <<"\n";
        ocr=ocr->myLink;
    }

    myHead = new ItemOccurence(Item,placeInLine,myHead);

    return;
}

bool Item::isEmpty(){
    if(myHead)
        return false;
    else
        return true;
}

EDIT:
I updated AddOccurence to be.

void Item::AddOccurence(int line,int placeInLine){
    ItemOccurence* prev = myHead;
    ItemOccurence* curr = myHead->myLink;

    while(curr){
        prev=curr;
        curr=curr->myLink;
        }

    // insert new ItemOccurence
    cout<<"adding " <<line<< " and " << placeInLine <<"\n";
    prev->myLink = new ItemOccurence(line,placeInLine); 

    return;
}

But I am still crashing. I am trying to debug but not sure what to look for.

Answer 1

It’s very hard to tell what your code is trying to do. Unfortunately, the hard truth is, it’s pretty far away from “working”.

Here’s a few hints:

• Reconsider your classes. What is Item and ItemOccurrence? A linked list is a list. It has items. You should probably name it List and Item. If you want, you can do this in a single class representing Item (the List is only a special case of the front Item).
• Each Item needs a string (the node data) and a next (the pointer to the next node).
• If you use List, it will need to have a pointer to head (the first Item).
• You don’t need to store placeInLine. placeInLine should only be used when searching for the place to insert a new Item.
• It’s unclear what ItemOccurrence::myLine is supposed to represent.
• When you initialize myHead(NULL), you don’t need to set it to 0.
• isEmpty() is not usually a private method.
• The algorithm for adding a node is:
  • Loop until you find the place before it needs to be inserted.
    • It looks like you need to check for two things: “end of list” and “placeInLine”
  • Set new_node->next = current->next->next node.
  • Set current->next = new_node node.
  • Be aware of the special case where current->next is NULL.
• Instead of if (x) return true; else return false;, it’s common to return x;