Insertion sort in a doubly linked list - c++

Question

I'm making a list in C++ from scratch, not using the STL list funciton. My program is designed for the user to insert a new item (string) to the list, one at a time. The items are supposed to be sorted as they are inserted, and it partially works. The user is supposed to be able to print them forwards and backwards.
Examples:
I insert: 1, 3, 2 - result forward: 1, 2, 3 - backwards: 3, 2, 1
I insert 1, 3, 2, 4 - result forward: 1, 4, 2, 3 - backwards: 3, 2, 4, 1
I insert 4, 2, 3 - result forward: 2, 3, 4 - backwards: 4, 3, 2
I insert 4, 2, 3, 1 - result forward: 1, 4 - backwards: 1, 3 repetedly until program crashes
These are a few examples, I could include more. I can not for the life of me see what's wrong wth my code. I have gotten help from people I know who are great coders, but they were unable to see the problem, after looking at it for a couple hours. I've been stuck at this for days now, and I just don't see it.
Here is my code (comments are in Norwegian, sorry!):
main.cpp
#include <iostream>
#include <cstdlib>
#include <string>
using namespace std;
//definerer noden
struct node
{
string data; //det noden inneholder
node* next; //nestepeker
node* prev; //forrigepeker
};
//definerer funksjonene
bool isEmpty(node* first);
char menu();
void insertAsFirstElement(node* &first, node* &end, string data);
void insert(node* &first, node* &end, string data);
void remove(node* &first, node* &end, string data);
void showList(node* first);
void printBackwards(node* end);
//er noden tom?
bool isEmpty(node* first)
{
if(first == NULL)
return true;
else
return false;
}
//sender valgskjermen til konsollen
char menu()
{
char choice;
cout << "Welcome to LISTOMANIA 3000x! \n" << endl;
cout << "Press <1> to add an item" << endl;
cout << "Press <2> to remove item on top" << endl;
cout << "Press <3> to display the list forward" << endl;
cout << "Press <4> to display the list backwards" << endl;
cout << "Press <5> to exit" << endl;
cout << "\nInput: ";
cin >> choice;
return choice;
}
//hvis lista er tom, sett inn node som første element:
void insertAsFirstElement(node* &first, node* &end, string data)
{
cout << "temp is first\n";
node* temp = new node;
temp->data = data;
temp->next = NULL;
temp->prev = NULL;
first = temp;
end = temp;
}
//hvis lista ikke er tom, sett inn noden sortert:
void insert(node* &first, node* &end, string data)
{
if(isEmpty(first))
{
insertAsFirstElement(first, end, data);
}
else
{
node* temp = new node;
temp->data = data;
node* n = first;
while(n)
{
if(n->data > temp->data)
{
cout << "temp BEFORE n" << endl;
temp->prev = n->prev;
temp->next = n;
n->prev = temp;
if(temp->prev)
{
temp->prev->next = temp;
}
else
{
first = temp;
}
}
else if(n->data <= temp->data)
{
cout << "temp AFTER n" << endl;
//temp = new node;
//temp->data = data;
temp->prev = n;
temp->next = n->next;
n->next = temp;
if(temp->next)
{
temp->next->prev = temp;
}
else
{
end = temp;
}
break;
}
n = n->next;
}
}
}
//sletter valgt node
void remove(node* &first, node* &end, string data)
{
string delItem;
node* temp;
if(isEmpty(first))
cout << "\nNothing to delete, the list is empty!\n------------------------------------------------\n------------------------------------------------\n";
else if(first == end)
{
cout << "\nYou removed <" << first->data << ">!" << endl;
delete first;
first = NULL;
end = NULL;
cout <<"------------------------------------------------\n------------------------------------------------\n";
}
else
{
node* temp = first;
cout << "You removed <" << temp->data << ">!" << endl;
first = first->next;
delete temp;
cout <<"------------------------------------------------\n------------------------------------------------\n";
}
}
//skriver ut listen alfabetisk
void showList(node* first)
{
node * temp = first;
if(isEmpty(first))
{
cout << "\nThe list is empty!\n";
}
else
{
cout << "\nThe list contains: \n\n";
while(temp != NULL)
{
cout << temp->data << endl;
temp = temp->next;
}
}
cout << "------------------------------------------------\n";
cout << "------------------------------------------------\n";
}
//skriver ut listen omvendt alfabetisk
void printBackwards(node* end)
{
node * temp = end;
if(isEmpty(end))
{
cout << "\nThe list is empty!\n";
}
else
{
cout << "\nThe list contains: \n\n";
while(temp != NULL)
{
cout << temp->data << endl;
temp = temp->prev;
}
}
cout << "------------------------------------------------\n";
cout << "------------------------------------------------\n";
}
//mainfunksjon
int main()
{
node* first = NULL;
node* end = NULL;
char choice;
string data;
do
{
choice = menu();
switch(choice)
{
case '1': cout <<"\nPlease add something to the list: ";
cin >> data;
insert(first, end, data);
cout << "------------------------------------------------\n------------------------------------------------\n";
break;
case '2': remove(first, end, data);
break;
case '3': showList(first);
break;
case '4': printBackwards(end);
break;
case '5': cout << "\nSystem exit...\n";
break;
default: cout << "\nInvalid input!\n------------------------------------------------\n------------------------------------------------\n";
}
}while(choice != '5');
return 0;
}
Just ignore the lines of hyphens, they are just there to make the program look prettier when it is run.
Please note that I am a beginner! There might be lots of other mistakes too, other than my problem. Also ignore the remove function, as I haven't gotten to that problem yet...

Answer 1

check two lines that I added with comments //1.this line and //2.this line the fist one is called when temp<n so you have to compare temp with previous node of n, and the second one is for when temp>=n so you have to do is to compare temp with next node of n. with what I said so far it is clear that you have to remove the line that I commented with //4.you should remove this line : n = temp->next; this is what cause your last problem. Also you have another mistake, you have to remove the break; it leaves your listed in the middle of the sort so your sort never completed and that is what cause the other problem.
while(n)
{
if(n->data > temp->data)
{
cout << "temp BEFORE n" << endl;
temp->prev = n->prev;
temp->next = n;
n->prev = temp;
if(temp->prev)
{
temp->prev->next = temp;
}
else
{
first = temp;
}
n = temp->prev; //1.This line
}
else if(n->data <= temp->data)
{
cout << "temp AFTER n" << endl;
//temp = new node;
//temp->data = data;
temp->prev = n;
temp->next = n->next;
n->next = temp;
if(temp->next)
{
temp->next->prev = temp;
}
else
{
end = temp;
}
n = temp->next; //2.This line
//3.you should comment this line : break;
}
//4.you should remove this line : n = temp->next;
}

Answer 2

I got some help from more people, and we found a solution. Here is my insert fucntion:
//hvis lista ikke er tom, sett inn noden sortert:
void insert(node* &first, node* &end, string data)
{
if(isEmpty(first))
{
insertAsFirstElement(first, end, data);
}
else
{
node* temp = new node;
temp->data = data;
temp->next = first;
temp->prev = NULL;
first->prev = temp;
first = temp;
}
//sortering
if(first != NULL)
{
node* current = first;
node* prev = NULL;
node* tempNode = NULL;
while(current->next != NULL)
{
tempNode = current->next;
//cout << "current: " << current->data << " tempNode: " << tempNode->data << endl;
if(current->data > tempNode->data)
{
//cout << "swapped " << "current: " << current->data << " tempNode: " << tempNode->data << endl;
swap(current->data, tempNode->data);
}
else
{
prev = current;
current = current->next;
}
}
}
}
This sorts the data if it is a string, with a little bug on numbers, interpreted as a string, which can be solved with converting all numbers to an int.
EDIT:
That bug with the nubmers isn't really a bug at all. The list thinks 11 is smaller than 5, but that is because every input is interprated as a string. So then if you think that 1 = a and 2 = b, then it makes perfect sense that 11 < 5. So if you want them to be sorted how it would logically be done, having 11 > 5, you'd have to convert all numbers to an int.