Deleting a node in a doubly linked list - c++

Question

Im trying to delete a node from a doubly linked list with data that's been read in from a file and rewrite it to the file with the non-deleted nodes. But i'm doing this incorrectly.
Either it deletes only a part of the node or it doesn't delete anything (the starting node).
Here's my header
hw07.h
#ifndef HW07_H_
#define HW07_H_
#include <iostream>
#include <fstream>
using namespace std;
typedef struct slist slist;
struct stud
{
string term;
string title;
string description;
string tktNum;
string location;
string lecDay;
string instructor;
string labLoc;
string labDay;
string labInstruct;
string units;
string preReqs;
string grade;
};
struct sentry
{
slist *list;
sentry *next;
sentry *prev;
stud *data;
};
struct slist
{
int length;
sentry *first;
sentry *last;
};
void readFile(slist *&header);
void deleteDataTN(slist *&header);
#endif
my main
hw07.cpp
#include "hw07.h"
int main()
{
slist *header = NULL;
readFile(header);
deleteDataTN(header);
return 0;
}
my readFile function
readFile.cpp
#include "hw07.h"
void readFile(slist *&header)
{
ifstream fin;
sentry *node, *temp;
fin.open("sbclassdb.txt");
while(!fin.eof())
{
if(header == NULL)
{
header = new slist;
header-> length = 0;
header-> first = NULL;
header-> last = NULL;
node = new sentry;
header-> first = node;
header-> last = node;
node-> prev = NULL;
node-> next = NULL;
}else
{
node = new sentry;
node-> prev = header-> last;
node-> next = NULL;
temp = header -> last;
temp-> next = node;
header-> last = node;
}
node->data = new stud;
getline(fin, node->data->term);
getline(fin, node->data->title);
getline(fin, node->data->description);
getline(fin, node->data->tktNum);
getline(fin, node->data->location);
getline(fin, node->data->lecDay);
getline(fin, node->data->instructor);
getline(fin, node->data->labLoc);
getline(fin, node->data->labDay);
getline(fin, node->data->labInstruct);
getline(fin, node->data->units);
getline(fin, node->data->preReqs);
getline(fin, node->data->grade);
header->length++;
string blankLine;
if(!getline(fin, blankLine))
break;
}
fin.close();
}
my deleteData function
deleteDataTN.cpp
#include "hw07.h"
void deleteDataTN(slist *&header)
{
sentry *node, *temp;
string ticketNum;
ofstream fout;
int index;
bool found;
found = false;
index = 0;
cout << "Ticket Number: ";
getline(cin, ticketNum);
node = header->first;
while(index < header->length && !found)
{
if(node->data->tktNum == ticketNum)
{
found = true;
free(node->data);
}
else
{
++index;
temp = node;
node = temp->next;
}
fout.open("sbclassdb.txt");
node = header->first;
for(int i=0; i<header->length; ++i)
{
fout << node->data->term << endl;
fout << node->data->title << endl;
fout << node->data->description << endl;
fout << node->data->tktNum << endl;
fout << node->data->location << endl;
fout << node->data->lecDay << endl;
fout << node->data->instructor << endl;
fout << node->data->labLoc << endl;
fout << node->data->labDay << endl;
fout << node->data->labInstruct << endl;
fout << node->data->units << endl;
fout << node->data->preReqs << endl;
fout << node->data->grade << endl;
temp = node;
node = temp->next;
fout << "\n";
}
}
my text file used to read in file
sbclassdb.txt
Fall 2017
CS1A
Intro to Computer Science 1
11111
SM101
TTH 100PM
John Doe
SM102
TTH 300PM
John Doe
5
NA
A
Spring 2018
CS1B
Intro to Computer Science 2
22222
SM101
TTH 200PM
Jane Doe
SM102
TTH 300PM
Jane Doe
5
CS1A
NA
Spring 2018
CS4A
Into to Java for Computer Science
12345
BGS101
MWF 200PM
Bob Smith
BGS102
Bob Smith
4
CS1B
NA

Answer 1

new should be cleaned up with delete (not free which is for malloc)
struct sentry{stud *data;...} can be changed to struct sentry{stud data;...}. And there is no need for sentry::list member:
struct sentry
{
sentry *next;
sentry *prev;
stud data;
};
There is no gain in declaring sentry::data as a pointer. If you simply declare the data as stud data; then you don't have to allocate/free with new/delete. (you still need new/delete for the nodes)
void deleteDataTN(slist *&header) can be declared as void deleteDataTN(slist &header), the you simply call deleteDataTN(myheader);
Don't use while(!fin.eof()){...}. There are many SO topics on this issue. See for example how-does-ifstreams-eof-work. I wrote a code earlier which I believe was wrong, here is the corrected version:
while(fin.good())
{
sentry *node = new sentry;
if(header.first == nullptr)
{
node->prev = nullptr;
node->next = nullptr;
header.first = node;
header.last = node;
}
else
{
header.last->next = node;
node->prev = header.last;
node->next = nullptr;
header.last = node;
}
...
}
To remove a node, you can add a function to slist:
struct slist
{
...
void remove(sentry *node);
};
void slist::remove(sentry *node)
{
if(!node) return;
if(!first) return;//empty list, do nothing
if(first->next == nullptr)//deleting the only node
{
first = last = nullptr;
}
else if(node == last)//deleting the last node
{
last = node->prev;
node->prev->next = nullptr;
}
else if(node == first)//deleting the first node
{
first = node->next;
first->prev = nullptr;
}
else//deleting a node in the mid
{
node->prev->next = node->next;
node->next->prev = node->prev;
}
length--;
delete node;
}
Now you can remove any node
node = header.first;
while(node)
{
if(node->data.tktNum == "xyz")
{
header.remove(node);
break;
}
node = node->next;
}