Singly linked list , project stops working [closed] - c++

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I am new to c++ , trying to write singly linked list in c++ , and it's various operations like , print, insert at the end, insert the end, delete etc. I am doing step by step. I have written my first insert method. It is working fine no error , but the first time it goes to the if condition of insert i.e the list is empty and create the first node here the problem is coming is after entering the first node the output screen is closing saying "project has stopped working" . what I am trying to do is after entering one node it should ask "do u wish to add another node?" if users enter "y" it should continue otherwise should go back to main function : Here is my code :
#include <iostream>
using namespace std;
struct node
{
int num;
node *next;
};
class link_list
{
node *head;
node *list;
public:
link_list()
{
head = NULL;
list = NULL;
//cout << list;
}
// void display_options(link_list &b);
void print_list();
void insert();
//void insert_at_beg();
// void insert_at_middle();
};
void link_list::print_list()
{
if (list == NULL)
{
cout << "Empty list" << endl;
//return;
}
else
{
int count = 0;
while (list->next != NULL)
{
list = list->next;
count++;
cout << "Node " << "value " << endl;
cout << count << " " << list->num << endl;
}
}
}
void link_list::insert()
{
// head = new node;
//list = head;
int y;
char a;
do
{
if (head == NULL)
{
head = new node;
cout << "enter the first node" << endl;
cin >> y;
list->num = y;
list->next = NULL;
}
else
{
node * newNode;
list == head;
while (list->next != NULL)
{
list->next = list;
}
newNode = new node;
cin >> y;
newNode->num = y;
list->next = newNode;
newNode->next = NULL;
}
cout << "do u wish to add another node?" << endl;
cin >> a;
} while (a == 'y' || a == 'Y');
}
int main()
{
link_list ll;
char choice;
do{
cout << "select one" << endl;
cout << "press 1 for insert ." << endl;
cout << "press 2 for insert at beginning ." << endl;
cout << "press 3 for insert at the middle ." << endl;
cout << "press 4 delete ." << endl;
cout << "print 5 print the linked list :" << endl;
cout << "print 6 exit :" << endl;
int no;
cin >> no;
switch (no)
{
case 1:
ll.insert();
break;
case 5:
ll.print_list();
case 6:
return 0;
default:
cout << "oops wrong choice" << endl;
}
fflush(stdin);
cout << "Do u wanna make another choice?" << endl;
cin >> choice;
cout << choice << endl;
} while (choice == 'Y' || choice == 'y');
cout << "Thanks!" << endl;
system("pause");
return 0;
}

Answer 1

You need to keep a more careful eye on your compiler warnings, and you need to learn how to use a debugger. There are many problems with this code, but most of them are easy to find by debugging.
This line in link_list::insert is not an assignment, it is a comparison. This would likely be raised as a compiler warning in most modern compilers. You'll probably want to fix that.
line == head
This loop doesn't do anything useful:
while (list->next != NULL)
{
list->next = list;
}
If list starts off as non-null, all this code will do is loop forever, repeatedly setting list->next to list. You probably wanted the operation to be the other way around.
list = list->next;
which will get you the last element in the list.
Still in the same function, this is broken:
head = new node;
cout << "enter the first node" << endl;
cin >> y;
list->num = y;
list->next = NULL;
because you haven't initialised list. Using head instead of list is probably what you wanted.
This is silly:
newNode = new node;
cin >> y;
because you don't prompt for the node value.
Once these changes are made, the code appears to function, at least for insertion and printing, but I'm not going to investigate further.
Perhaps this might be better moved to https://codereview.stackexchange.com/

Answer 2

head = new node;
cout << "enter the first node" << endl;
cin >> y;
list->num=y; //<-- problem
You are using list without initializing it. You should write
list = head;
before the statement list->num=y;
This will fix the bug for first time insertion. but there are several other problems in you code while inserting subsequently, which is pointed out by other answers. Modified code will be
void link_list::insert()
{
int y;
char a;
do
{
if (head == NULL)
{
head = new node;
cout << "enter the first node" << endl;
cin >> y;
head->num = y;
head->next = NULL; // no need to list as a member at all
}
else
{
node* list = head;
while (list->next != NULL)
{
list= list->next;
}
list->next= new node;
cin >> y;
list->next->num = y;
list->next->next = NULL;
}
cout << "do u wish to add another node?" << endl;
cin >> a;
} while (a == 'y' || a == 'Y');
}
void link_list::print_list()
{
if (head == NULL)
{
cout << "Empty list" << endl;
//return;
}
else
{
int count = 0;
node* list=head;
while (list->next != NULL)
{
count++;
cout << "Node " << "value " << endl;
cout << count << " " << list->num << endl;
list = list->next;
}
}
}