How to delete node from linked list? - c++

Question

Adding integers to list works fine, but there's something wrong with deleting and printing.
I'm not friendly with debugger yet, but I found out that there is error from node pointer 'first'. Its value is -17891602. I don't know what happened...
#include <iostream>
using namespace std;
class nodeList;
class node {
friend class nodeList;
private:
int data;
node* link;
public:
node() { //constructor
data = 0;
link = NULL;
}
node(int d) { //constructor
data = d;
link = NULL;
}
node(int d, node* l){ //constructor
data = d;
link = l;
}
};
class nodeList {
private:
node* first;
int num = 0;
node* nt;
public:
nodeList() {
first = new node();
}
node* end(node* t){ //return pointer of last element
t = first;
for (int i = 0; i < num; i++){
t = t->link;
}
return t;
}
void add(int a){ //add 'a' at the end of the list
node* x = new node(a);
node* y = this->end(nt);
y->link = x;
num++;
}
void del(int n){ //n : data of element that you want to delete from list
node* temp = first;
node* pretemp = NULL;
node* x;
int i;
for (i = 0; i <= this->num; i++){ //loop to find 'n'
pretemp = temp;
temp = temp->link;
if (n == temp->data){
break;
}
}
temp = first;
for (int j = 0; j<i; j++){ //i : where 'n' is,
temp = temp->link;
}
x = temp->link;
pretemp->link = x;
delete temp;
num--;
}
void printList(){
node* temp = first;
temp = temp->link;
for (int i = 0; i<this->num; i++){
cout << temp->data << endl;
temp = temp->link;
}
}
};
int main(){
nodeList *l = new nodeList();
int a;
int select;
while (1){
cout << "1. ADD 2. DELETE 3. PRINT" << endl;
cin >> select;
if (select == 1){
cout << "Enter an integer: ";
cin >> a;
if (cin.fail()) {
cout << "Wrong input" << endl;
break;
}
l->add(a);
l->printList();
}
if (select == 2){
cout << "Enter the data of the element you want to delete: ";
cin >> a;
if (cin.fail()) {
cout << "Wrong input" << endl;
break;
}
l->del(a);
l->printList();
}
if (select == 3){
l->printList();
}
}
}

Answer 1

Your del function deletes pretemp node (node that was before the one that you need to delete).
Here's possible fix:
//n : data of element that you want to delete from list
void del(int n)
{
//loop to find 'n'
for (node *tmp = first; tmp->link; tmp = tmp->link)
{
if (n == tmp->link->data)
{
node *x = tmp->link;
tmp->link = tmp->link->link;
delete x;
num--;
break;
}
}
}
Also, was your del supposed to delete all nodes with data == n?

Answer 2

These functions are a bit complicated. Here is a simpler idea:
void del(int n){
node* pretemp = first, *temp = first->link;
if(pretemp->data == n) { //handle the deleting of the first node
first = first->link;
delete pretemp;
return;
}
while(temp != NULL && temp->data != n) { //find the node with the data member "n"
pretemp = temp;
temp = temp->link;
}
if(temp != NULL) { //if you found the node, delete it
pretemp->link = temp->link;
delete temp;
}
--num;
}

Answer 3

Your code is a bit over-complicated for what it needs.
Try something more like this instead:
#include <iostream>
#include <limits>
using namespace std;
class nodeList;
class node
{
friend class nodeList;
private:
int data;
node* link;
public:
node(int d = 0) //constructor
: data(d), link(NULL)
{
}
};
class nodeList
{
private:
node* first;
int num;
public:
nodeList()
: first(NULL), num(0)
{
}
~nodeList()
{
node *n = first, *t;
while (n)
{
t = n->link;
delete n;
n = t;
}
}
void add(int data) //add 'data' at the end of the list
{
node* n = new node(data);
if (!first)
first = n;
else
{
node *t = first;
while (t->link)
t = t->link;
t->link = n;
}
++num;
}
void del(int data) //data : data of element that you want to delete from list
{
node* n = first;
node* prev = NULL;
while (n) //loop to find 'data'
{
if (data == n->data)
{
if (prev)
prev->link = n->link;
if (n == first)
first = n->link;
delete n;
--num;
return;
}
prev = n;
n = n->link;
}
}
void printList()
{
for(node* n = first; n != NULL; n = n->link)
cout << n->data << endl;
}
};
int main()
{
nodeList l;
int input;
bool keepGoing = true;
do
{
cout << "1. ADD 2. DELETE 3. PRINT 4. EXIT" << endl;
if (!(cin >> input))
{
cout << "Wrong input" << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize_t>::max(), '\n');
}
else
{
switch (input)
{
case 1:
cout << "Enter an integer: ";
if (!(cin >> input))
{
cout << "Wrong input" << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize_t>::max(), '\n');
}
else
{
l.add(input);
l.printList();
}
break;
case 2:
cout << "Enter the data of the element you want to delete: ";
if(!(cin >> input))
{
cout << "Wrong input" << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize_t>::max(), '\n');
}
else
{
l.del(input);
l.printList();
}
break;
case 3:
l.printList();
break;
case 4:
keepGoing = false;
break;
default:
cout << "Wrong input" << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize_t>::max(), '\n');
break;
}
}
}
while (keepGoing);
return 0;
}
If you add an extra node* member to your nodeList class, you can simplify and speed up your add() method:
class nodeList
{
private:
node* first;
node* last;
int num;
public:
nodeList()
: first(NULL), last(NULL), num(0)
{
}
~nodeList()
{
node *n = first, *t;
while (n)
{
t = n->link;
delete n;
n = t;
}
}
void add(int data) //add 'data' at the end of the list
{
node* n = new node(data);
if (!first)
first = n;
if (last)
last->link = n;
last = n;
++num;
}
void del(int data) //data : data of element that you want to delete from list
{
node* n = first;
node* prev = NULL;
while (n) //loop to find 'data'
{
if (data == n->data)
{
if (prev)
prev->link = n->link;
if (n == first)
first = n->link;
if (n == last)
last = prev;
delete n;
--num;
return;
}
prev = n;
n = n->link;
}
}
void printList()
{
for(node* n = first; n != NULL; n = n->link)
cout << n->data << endl;
}
};
And if you can change your list to be a double-linked list instead of a single-linked list, you can simplify your del() method as well:
#include <iostream>
#include <limits>
using namespace std;
class nodeList;
class node
{
friend class nodeList;
private:
int data;
node* prev;
node* next;
public:
node(int d = 0) //constructor
: data(d), prev(NULL), next(NULL)
{
}
};
class nodeList
{
private:
node* first;
node* last;
int num;
public:
nodeList()
: first(NULL), last(NULL), num(0)
{
}
~nodeList()
{
node *n = first, *t;
while (n)
{
t = n->next;
delete n;
n = t;
}
}
void add(int data) //add 'data' at the end of the list
{
node* n = new node(data);
if (!first)
first = n;
if (last)
last->next = n;
n->prev = last;
last = n;
++num;
}
void del(int data) //data : data of element that you want to delete from list
{
for(node* n = first; n != NULL; n = n->next) //loop to find 'data'
{
if (data == n->data)
{
if (n->prev)
n->prev->next = n->next;
if (n->next)
n->next->prev = n->prev;
if (n == first)
first = n->next;
if (n == last)
last = n->prev;
delete n;
--num;
return;
}
}
}
void printList()
{
for(node* n = first; n != NULL; n = n->next)
cout << n->data << endl;
}
};