Stuck on a linked list multiple class implementation - c++

Question

I have been working on this project for several days. This project contains 3 classes. The first is a DNA class that stores a DNA object. The second is a database class that reads a file and parses commands and data and deals with it accordingly. The last is a DNA list class that is a linked list of nodes that have pointers to DNA objects.
I've completed my linked list building method. It is required to be a push_back method that adds nodes at the end of the list. My problem arises when I try to search the list for a certain node. This has to be a method that returns a DNA* if a DNA object with id exists in the list; otherwise it returns NULL.
My plan is to use this method to print and also delete the node. I can't seem to get this method to work. Obviously I'm a little shaky with pointers. It took me several hours to implement my push_back method. Here is my code. Any guidance or help is appreciated.
DNA.h
#ifndef DNA_H
#define DNA_H
#include <string>
class DNA{
public:
// overloaded constructor for DNA class
DNA(std::string, int, std::string, int, int);
// print function
void print();
int getID();
private:
std::string m_label; // variable to hold label
int m_id; // variable to hold id
std::string m_sequence; // variable to hold sequence
int m_length; // variable to hold length
int m_index; // variable to hold index
};
#endif
DNA implementation
#include "DNA.h"
#include <iostream>
#include <string>
using namespace std;
DNA::DNA(string label, int id, string sequence, int length, int index){
m_label = label;
m_id = id;
m_sequence = sequence;
m_length = length;
m_index = index;
}
void DNA::print(){
cout << "DNA:" << '\t' << "label: " << m_label << '\t' << "ID: " << m_id << '\t' << "Sequence: " << m_sequence << endl << "Length: " << m_length << '\t' << "cDNAStartIndex: " << m_index << endl << endl;
}
int DNA::getID(){
return m_id;
}
Database class
#ifndef SEQUENCEDATABASE_H
#define SEQUENCEDATABASE_H
#include <string>
#include <fstream>
#include "DNA.h"
#include "DNAList.h"
class SequenceDatabase {
public:
SequenceDatabase();
// function to import the data file, parse the data, and perform the required output
void importEntries(std::string);
private:
DNAList list;
};
#endif
Database implemenation
#include "SequenceDatabase.h"
#include "DNA.h"
#include "DNAList.h"
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
SequenceDatabase::SequenceDatabase(){
DNAList list;
}
// function reads in the filename creates a data stream and performs the requested actions
void SequenceDatabase::importEntries(string inputFile){
ifstream dnaFile(inputFile);
char command;
string label, sequence;
int id, length, index;
while(dnaFile >> command){
DNA* p;
if(command == 'D'){
dnaFile >> label >> id >> sequence >> length >> index;
DNA data(label, id, sequence, length, index);
p = new DNA(label, id, sequence, length, index);
list.push_back(p);
}
if(command == 'P'){
dnaFile >> id;
cout << "Printing " << id << " ..." << endl << endl;
p = list.findId(id);
if(p == nullptr)
cout << "Can not find item " << "(" << id << ")!" << endl << endl;
else
p-> print();
}
}
dnaFile.close();
}
Finally my list class
#ifndef DNALIST_H
#define DNALIST_H
#include "DNA.h"
#include "sequenceDatabase.h"
struct DNANode{
DNA* data;
DNANode* next;
DNANode* prev;
};
class DNAList{
public:
DNAList();
DNAList(DNA* newDNA);
void push_back(DNA* newDNA);
DNA* findId(int);
void obliterate(int id);
int size();
private:
DNANode* head;
int list_size;
};
#endif
List implementation
#include "DNA.h"
#include "sequenceDatabase.h"
#include "DNAList.h"
#include <iostream>
using namespace std;
DNAList::DNAList(){
head = new DNANode;
head->next = nullptr;
list_size = 0;
}
DNA* DNAList::findId(int id){ // this function is my problem
DNANode* current;
current = head;
while(current->next != nullptr){
if(current->data->getID() == id){
return current->data;
}
current = current->next;
}
return nullptr;
}
int DNAList::size(){
return list_size;
}
void DNAList::push_back(DNA* newDNA){
DNANode* current;
DNANode* last;
DNANode* p;
p = new DNANode;
p->data = newDNA;
last = nullptr;
current = head;
cout << "Adding " << newDNA->getID() << " ..." << endl << endl;
while(current != nullptr){
last = current;
current = current->next;
}
if(current == head->next){
p->next = nullptr;
p->prev = head;
head->next = p;
}
else{
p->next = current;
p->prev = last;
last->next = p;
}
list_size++;
}
I wasn't sure if I should post the whole code, but i felt it was needed to understand the problem. My problem arises when i try to call the find function to print the data in the node.

Answer 1

Your head member variable of DNAList is initialized with new DNANode. Since DNANode doesn't have an explicit constructor, its compiler generated one will not initialize the pointers data, next and prev. next is initialized on the next line, but data is left as a garbage value.
Inside findId, this line is executed:
if (current->data->getID() == id){
However the first time around the loop, current is pointing head. This means you are trying to look at the garbage value, which will possibly crash.
One solution is to change the findId function to start at head->next, another is to initialise the data pointer in head to nullptr and check that data is not nullptr before you access it.
A better solution might be to just have head as nullptr to start with, rather than having a dummy DNANode at the top. This would involve changing some of the code in push_back, but might make it easier to understand.

Answer 2

Aha. I think what is causing the problem is that towards the end of your SequenceDatabase::importEntries() method you are setting if(p=nullptr) instead of making the comparison if(p == nullptr). This is no doubt causing the error you see. This is a common mistake.