Printing Doubly Linked Lists - c++

I'm currently learning how to work with linked lists, specifically, doubly linked lists, and I have come across a problem with my program when I attempt to print it backwards.
Here is the portion of the code that I need help with:
#include <iostream>
using namespace std;
struct node
{
int data; //int to store data in the list
node *next; //pointer to next value in list
node *prev; //pointer to previous value in list
};
node *appendList(node *current, int newData) //Function to create new nodes in the list
{
node *newNode; //create a new node
newNode = new node;
newNode->data = newData; //Assign data to it
newNode->next = NULL; //At end of list so it points to NULL
newNode->prev = current; //Link new node to the previous value
current->next = newNode; //Link current to the new node
return newNode; //return the new node
}
node *createList(int maxLoop, node *begin, node *current, node *end) //Function to create list
{
//Allocate the starting node
current = new node;
current -> data = 1; //First data value is 1
current -> next = NULL; //next value is NULL
current -> prev = NULL; //previous value is NULL
begin = current; //This is the beginning of the list
for (int count = 2; count <= maxLoop; count++) //Loop to fill the list
{
current = appendList(current, count*count); //Create new nodes and fill with square numbers
}
end = current; //Now we are at the end of the list
return begin; //Return begin, this is the problem; I can't return end as well
}
void printForward (node *p) //Function to print the list forwards
{
node *curr = p; //current is the beginning value of the list
while (curr != NULL) //Continue while current is not at the end of the list
{
cout << curr->data << " "; //Print out the data of current
curr = curr->next; //Move one value along in the list
}
}
void printBackward (node *p) //Function to print the list backwards
{
node *curr = p; //current is the end value of the list
while (curr != NULL) //Continue while current is not at the beginning of the list
{
cout << curr->data << " "; //Print out the data of current
curr = curr->prev; //Move one value back in the list
}
}
int main()
{
//Initialize current, begin, and end
node *current = NULL;
node *begin = NULL;
node *end = NULL;
int maxLoop = 10; //The number of items in the list
cout << "The list has now been created." << endl;
begin = createList(maxLoop, begin, current, end); //function to create the list
cout << "Printed forwards, this list is: ";
printForward(begin); //Function to print the list forwards
cout << endl;
cout << "Printed backwards, this list is: ";
printBackward(end); //Function to print the list backwards
cout << endl;
return 0;
}
The purpose of this program is to create a list, print it forwards, backwards, insert an element, erase an element, and then destroy the list. I have chopped it down to just the create, print forward, and print backward functions.
The issue I have is that in the createList function I am modifying both begin and end but I can only return one or the other. This means that whichever I don't return is still NULL in the main function and therefore does not point to anything. I've tried setting the begin/current/end to not equal NULL but createList won't work if I do that.
Does anyone have any ideas for how I could modify both? Just to be clear, the list HAS TO be created in a function, it would be very easy to just initialize it in the main.
Thanks,
Tristan

Your problem is you're copying the pointers, when you should be passing them by reference, i.e., using a pointer-to-pointer or reference-to-pointer rather than just copying the value the pointer in main is originally pointing to. With what you're doing, you're unable to modify the original pointer variable that was declared in main ... passing-by-reference will allow you to-do that while also keeping all list setup code within your functions.
So for instance, change
node* createList(int maxLoop, node *begin, node *current, node *end)
to
void createList(int maxLoop, node** begin, node** current, node** end)
and then make sure to take the extra dereference into account in the body of your function
Finally, you would call it like:
createList(maxLoop, &begin, &current, &end);
And do the final assign to begin inside the body of the function of createList rather than in main.

Related

trying to make a simpler insertion at end of linked list program.Help needed with minor issues

I have managed to create a simple insertion at beginnning of linked list program but now i am struggling with insertion at end of linked list.
The program seems to be able to take values from user but the output list is not coming correct.Could you help me out?
If possible keep along the lines of my program as i am beginner and won't be able to understand a completely different method.
Logic i used-
If list is empty then insert value at beginning else if list is not empty then travel along the list till the next value being pointed at is NULL and then enter the new value in place of NULL.
#include<iostream>
using namespace std;
struct node
{
int data;
node *next;
};
node *start=NULL;
void insertend(int x)
{
node* temp=new node;
if(start==NULL)
temp->data=x;
else
{
while(temp!=NULL)
{
temp=temp->next;
}
temp->next=x;
}
}
void display()
{
node* disp=new node;
while(disp!=NULL)
{
cout<<disp->data<<endl;
disp=disp->next;
}
}
int main()
{
int x;
char ch;
do
{
cout<<"Enter data";cin>>x;
cout<<endl;
insertend(x);
cout<<"Do you want to continue?(y/n)";cin>>ch;
cout<<endl;
}while(ch=='y');
cout<<"Your list:"<<endl;
display();
}
The entry point to your list is the variable start. But you never set it. Take for example the first item a user inputs. You will call insertend(), it will check that start == NULL, but then it never sets start. You must set start = temp or something similar. You have the same problem in the else section -- you loop through the nodes starting with temp, but you should be starting with start. And again in the function display(), you create a pointer to a node and start looping from it, but it will have no data -- you should use start as the starting point of your loop.
struct node{
int data;
node* next;
};
node *first = NULL, *last = NULL;
void insert(int x){
if(first == NULL){
first = new node;
first->data = x;
first->next = NULL;
}else if(last == NULL){
last = new node;
last->data = x;
first->next = last;
last->next = NULL;
}else{
node *n = new node;
n->data = x;
n->next = NULL;
last->next = n;
last = n;
}
}
As you can see I am keeping track of first and last node in the list. Insert function checks if there is anything in the list with if(first == NULL)part. If there isn't it creates the first node. Similar thing happens with the else if. Finally in the else block we create a new node with data x. Then point the node stored in variable last to our new node and set last to be that node.
Here is the display function:
void display()
{
node *disp =first;
while(disp->next != NULL){
cout << disp->data << " ";
disp = disp->next;
}
cout << disp->data;
}
I also recommend that you do a cleanup after your program is finished running since you are creating new nodes.
void cleanup(node* n)
{
if(n->next == NULL)return delete n;
cleanup(n->next);
delete n;
}
and then at the end of main call cleanup(first)
Hope this makes sense :) Have a nice day!

Doubly linked list Core dump

Thanks in advance.
I am making a doubly linked list.
Everything was working fine, but I realized that when I added a new class node somewhere in the middle, the left pointer would still be pointing at the node previously before it (now two spaces away).
So I added a new node pointer on line 46.
Then on line 51 I told that node to now point to the new node.
So :
First I had new node temp off in space
Then I make pointer temp2 loop through the list
Lastly I tell temp3 to point to the node after temp2's node
After the function runs, the order should be temp2->temp->temp3
My main point: After I added line 51, my program core dumps(segmentation fault) and closes out.
How can I fix this? It only happens when I add something that isn't taking place of the head pointer.
void add(node *&head, node *&tail, node *&current)
{
node *temp = new node; //creates a pointer pointing to a new class node
cin >> temp->letter; // user input
current = head; // creates a pointer to point at the first node
while (current != NULL) // while list isn't empty
{
if (current->letter == temp->letter)
{ // letter already exists
cout << "DUPLICATE: " << temp->letter << endl << endl;
return;
}
else
{ // loop through list moving tail pointer to the end while checking for duplicates
tail = current;
current = current->right_link;
}
}
current = temp; // current = new added node
if (isEmpty(head))
{ // if first node
temp->left_link = NULL;
temp->right_link = NULL;
head = temp; // head and
tail = temp; // tail both point to first and only node.
}
else
{ // if new letter value is less than head value
if(temp->letter < head->letter)
{
temp->right_link = head; // node points (right) to head
temp->left_link = NULL; // left most node point to nothing.
head->left_link = temp; // head (currently the second node) points (left) to first node
head = temp; // head pointer moves to the first position
}
else
{ // if new node goes anywhere other than head
node *temp2 = head; // new node to cycle through list
while(temp2->right_link != NULL && temp2->right_link->letter < temp->letter)
{ // if temp2 points to a node and that node's value is less than temp node value
temp2 = temp2->right_link;
}
node *temp3 = temp2->right_link;
temp->right_link = temp2->right_link; // when temp2 stops looping, temp will point to
// the same node as temp2.
temp2->right_link = temp; // temp2's current node will point to temp, causing temp
// to be added into the list (after temp2)
temp3->left_link = temp; // point the node (after the newly inserted node) left to new node
temp->left_link = temp2; // connects the left pointer between temp and temp2
if(temp->right_link == NULL)
tail = temp;
}
}
cout << "ADDED : " << temp->letter << endl << endl;
}
if temp2->right_link == NULL
46 node *temp3 = temp2->right_link;
is a NULL pointer, so you can't
51 temp3->left_link = temp;
which should have been obvious if you used a debugger.

"lvalue required as left operand of assignment" error writing a linked list

I am currently learning some C++ for a course I am taking in school. I have basic understanding of lvalues and rvalues, but I am unable to determine why I am receiving a compiler error.
I am creating a singly linked list and need to be able to reverse it. As per my assignment I have two classes. The first is the node and just holds an int as well as a pointer.
class Node {
int data;
Node *next;
public:
//Constructor
Node(int d) {
data = d;
next = NULL;}
//Set to next Node
void SetNext(Node *nextOne) {
next = nextOne;}
//Returns data value
int Data(){return data;}
//Returns next Node
Node *Next() {return next;}
};
Then I have a linked list class that has a header pointer and then a number of functions for adding, printing etc. the list.
class LinkedList {
Node *head;
public:
//Constructor
LinkedList(){head = NULL;}
void AddNode(int d) {
//Create a new Node
Node *newNode = new Node(d);
//Create a temporary pointer
Node *temp = head;
//If there are already nodes in the list
if(temp != NULL) {
//Parse through to the end of the list
while(temp->Next() != NULL) {
temp = temp->Next();}
//Point the last Node in the list to the new Node
temp->SetNext(newNode);
}
//If adding as the first Node
else{
head = newNode;}
}
void PrintList() {
//Temporary pointer
Node *temp = head;
//If there are no nodes in the list
if(temp == NULL) {
std::cout << "The list is empty" << std::endl;}
//If there is only one node in the list
if(temp->Next() == NULL) {
std::cout << temp->Data() << std::endl;}
//Parse through the list and print
else {
do {
std::cout << temp->Data();
temp = temp->Next();
}
while(temp != NULL);
}
}
//Returns the number of nodes in the list
int CountList() {
//Temporary pointer
Node *temp = head;
//Counter variable
int counter = 0;
//If the list is empty
if(temp == NULL) {
return counter;}
//Parse through Nodes counting them
else {
do {counter++;
temp = temp->Next();
}
while(temp != NULL);
}
return counter;
}
//Reverses the list
Node *ReverseList() {
//Initially set to NULL then tracks the new head
Node *marker = NULL;
//Tracks the next one in the list
Node *nextOne;
//Sets the first Node to NULL and then sets the last Node to point to
//the first one and rotates through the list pointing the last to the
//first
while(head != NULL) {
nextOne = head->Next();
head->Next() = marker;
marker = head;
head = nextOne;
}
//Setting the head back to the start again
head = marker;
}
};
One of those functions is supposed to reverse the list. The line "head->Next() = marker;" in the ReverseList function is causing a "lvalue required as left operand of assignment" error when compiling.
Any insight as to why this is occurring and how I can correct the problem?
Thank you in advance!
The return from the call to Next() is an rvalue. As you are in a class function, you don't need to call the Next function to get at the private next pointer, you can just use it directly.
head->next = marker;
Your Next() function returns a pointer, and you then do this:
head->Next() = marker;
You're changing the pointer to marker and not what it's pointing at. To solve this you need to dereference that pointer:
*head->Next() = marker;
your signature for next is:
Node *Next() {return next;}
This makes a copy of next pointer at return and hence it is treated as r-value and not l-value.
One way of overcoming this would be to use a pointer-to-pointer:.
Node **Next() {return &next;}
And then use it as:
int main()
{
Node* marker=new Node(89);
Node* nod=new Node(9);
*(nod->Next())= marker;
cout<<(nod->next)->data<<endl;
cout << "Hello World" << endl;
return 0;
}
This makes it more complicated to use.

Search Function in Linked List - C++ [closed]

Closed. This question needs debugging details. It is not currently accepting answers.
Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 8 years ago.
Improve this question
My aim is to produce a function that searches for a number already in the list and print that it has been found.
My initial idea was to follow my remove function which searches through the list until it finds a number (to then delete).
This seemed the logical way to code the search function. If this isn't correct how would I modify it to search through my list and display that a number has been found?
I have nodes *head, *current and *temp as well as node pointer next and number as the data type in a class on a .h file.
Thank you.
NOTE - I used my remove() function under the search() function.
#include <iostream>
#include <string>
#include <fstream>
#include "LinkedList.h"
using namespace SDI;
int main()
{
LinkedList menu;
menu.insert(5);
menu.insert(4);
menu.insert(2);
menu.insert(3);
menu.insert(8);
menu.remove(4);
menu.reverse();
menu.display();
menu.search(2);
system("pause");
};
LinkedList::LinkedList()
{
head = NULL;
current = NULL;
temp = NULL;
};
LinkedList::~LinkedList()
{
};
void LinkedList::insert(int add) //insert function, data is stored in add from function body
{
Node* newnode = new Node; //definition of add node, make new node and make node* point to it
newnode->next = NULL; //point and set up to last node in the list (nothing)
newnode->number = add; //adds data to list
if (head != NULL) //if head is pointing to object then we have list
{
current = head; //make current pointer point to head
while (current->next != NULL) //check to see if end at list, is it the last node?
{
current = current->next; //advances current pointer to end of list
}
current->next = newnode; //adds new node next to value already stored
}
else
{
head = newnode; //if we don't have element in list
}
};
void LinkedList::remove(int remove) //remove function, data is stored in remove from function body
{
Node* remove1 = NULL; //searches through for same value in remove and deletes
temp = head;
current = head;
while (current != NULL && current->number != remove) //check if current node is one we want to delete...if not advance current pointer to next one
{
temp = current; //keep temp pointer one step behind
current = current->next; //advance to next node, traverse list till at the end
}
if (current == NULL) //pass through whole list and value not found
{
std::cout << "N/A\n";
delete remove1; //removes spare number floating around in memory
}
else
{
remove1 = current; //pointing to value we want to delete
current = current->next; //advances current pointer to next node
temp->next = current; //stops hole that occurs in list, patches this up
if (remove1 == head) //if pointer is pointing to front of list
{
head = head->next; //advance the head to next
temp = NULL;
}
delete remove1;
}
};
void LinkedList::search(int searchNum)
{
Node* searchnumber = nullptr;
temp = head;
current = head;
while (current != NULL && current->number != searchNum)
{
temp = current;
current = current->next;
}
if (current != NULL)
{
searchnumber = current;
current = current->next;
std::cout << "-" << searchnumber << " Found";
}
else
{
std::cout << "N/A";
}
};
void LinkedList::display()
{
current = head; //point to start of list
while (current != NULL) //while it points to something in list
{
std::cout << current->number; //display list starting from start
current = current->next; //advance to next pointer
}
};
void LinkedList::reverse()
{
Node *new_head = nullptr; //create new head as we want it to start from last element
for (current = head; current;) //same as display, ask it to go through list from head then outside loop assign to new head and switch sides
{
temp = current; //keep temp pointer one step behind
current = current->next; //goes through each element in the list
temp->next = new_head; //scrolls through backwards from new head
new_head = temp;
}
head = new_head; //assign head to new head
};
Your search algorithm seems to be wrong. Change it to :
if (current != NULL) // (current == NULL) is wrong because it means the value wasn't found
{
searchnumber = current;
current = current->next;
std::cout << "-" << searchnumber->number << " Found"; // here searchnumber is the node's address. You need to print its value, so use searchnumber->number
}
And you don't need to remove nodes till you find the desired value.
You can just use your search algorithm to find if a number already in the list. If that's what you want.
While a list is unordered a comparison of search algorithms doesn't have any sense. Simply iterate over all nodes one by one and apply match criteria.

Printing Linked List in C++

My following code print just only first element. In print_list() function, it stops after printing first element. It says after first element, head->next is 0. Shouldn't point towards second element?
I want to simply print whole list.
#include<iostream>
#include<cstdlib>
using namespace std;
struct node {
int x;
node *next;
};
node* add_element(node*);
bool is_empty(node*);
void print_list(node*);
node* search(node*);
int main()
{
node *head;
head=NULL;
node* current=head;
for(int i=0;i<5;i=i+1)
{
if (current==NULL)
{
current=add_element(current);
head=current;
}
else{
current=add_element(current);
}
}
cout<<head->next<<endl;
// DOUBT: head->next gives NULL value. It should give me pointer to 2nd node
print_list(head);
}
node* add_element(node* current)
{
node* temp;
temp=new node;
temp->next=NULL;
cout<<"enter element"<<endl;
cin>>temp->x;
current=temp;
return current;
}
bool is_empty(node* temp)
{
return temp==NULL;
}
void print_list(node* temp)
{
if (is_empty(temp)==false)
{
cout<<"here temp(head)"<<temp->next<<endl;
while(temp!=NULL)
{
cout<<temp->x<<endl;
temp = temp->next;
}
}
}
Print function print first element because you have just one node in the Linked List! Actually the mistake is present in add_element(node*) function, you overwrite address of head node with new node (so having memory leak) as I marked below:
node* add_element(node* current)
{
node* temp;
temp = new node; <---" You allocated memory"
temp->next = NULL; <---" Set next NULL"
cout<< "enter element" << endl;
cin>> temp->x; <---" Assign a value in new node"
// Replace below two line with suggested
current = temp; <---"MISTAKE: Overwrite first node"
"temp next is NULL so losing address of other nodes"
return current; <--- "return first node"
}
Next of new node (so first node) is NULL hence the print function prints only first node's value.
Suggestion:
You should Correct as follows to add new node as a first node in linked list:
temp -> next = current; // new nodes next if present first node
return temp; // new code becomes first node
Be careful current should be NULL initially.
With my suggestion in add_element() function also change the for loop code in main() as follows:
for(int i=0; i < 5; i = i + 1){
current = add_element(current);
}
head = current;
And check the working code at Codepade (instead of user input I added value using y = 100 variable).
Edit To append new node:
You need to check whether new node is first node of not (read comments).
// returns first node address in linked list = head
node* add_element(node* head){
node *temp, *new_nd;
// Create new node
new_nd = new node;
new_nd->next = NULL;
cout<<"enter element"<<endl;
cin>>new_nd->x;
// Is new node is the first node?
if(!head)
return new_nd;
// move to last
temp = head;
while(temp->next) temp = temp->next;
// add new node at last
temp->next = new_nd;
// return old head
return head;
}
Also simply main() as below:
int main(){
node *head = NULL;
for(int i = 0; i < 5; i = i + 1){
head = add_element(head);
}
print_list(head);
}
check this working code.
Your problem is here:
node* add_element(node* current)
{
node* temp; //You created a new node
temp=new node; //You allocated it here
temp->next=NULL; //You set its next property to null
cout<<"enter element"<<endl; //
cin>>temp->x;
current=temp; //This should be current->next = temp. You are overwriting it!
return current; //And now you are returning essentially the temp object that
//You created and you set its next property to NULL
}
You are assigning the node you created in temp = new node to the current node that was passed in. What you want to do is assign the node you just created to the current node's next property. It should be current->next = temp
head->next is NULL because you set it so in add_element(). To have a linked list, you should set current->next = temp.
As you're using C++, you might consider using std::list instead of implementing your own linked list.
if (current==NULL)
{ current=add_element(current);
head=current;
}
else
{ current->next=add_element(current);
current=current->next;
}
The correct code.
You have to make a small correction in the loop.
You have to add a new node and then make it point to the next of the current node.
so the simplified code is current->next=add_element(current)
and then make current point to the new current.