C++ Linked List deleting the wrong item [closed] - c++

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I'm trying to review data structures and implement a basic linked list. When I run this code, I get the following output:
3 4 6
1 was found and deleted 4 was found and deleted
3
4 should be deleted, but obviously 1 should not, and I'm wondering where the error in my code/logic is.
Thanks in advance for any help.
#include <iostream>
using namespace std;
class List {
private:
struct node {
int data;
node * next;
};
node * head;
node * curr;
node * temp;
public:
List();
void addNode(int newData);
void deleteNode(int delData);
void printList();
};
int main() {
List test;
test.addNode(3);
test.addNode(4);
test.addNode(6);
test.printList();
cout << endl << endl;
test.deleteNode(1);
test.deleteNode(4);
cout << endl << endl;
test.printList();
}
List::List(){
head = NULL;
curr = NULL;
temp = NULL;
}
void List::addNode(int newData){
node * n = new node;
n->next = NULL;
n->data = newData;
if (head != NULL) { // List is intact
curr = head; // if List is not empty, make curr equal to the head, and start at the beginning of the list.
while(curr->next != NULL) { // Get to last item on the list
curr = curr->next;
}
curr->next = n; // Use the last item, and point to the new node.
}
else { // empty list
head = n; // new node is the head of the list.
}
}
void List::deleteNode(int delData){
node * n = new node;
temp = head;
curr = head;
if (head != NULL) {
while (curr->next != NULL && curr->data != delData) {
temp = curr;
curr = curr->next;
}
if (curr == NULL) {
cout << delData << " was not found in the list\n";
delete n;
}
else {
n = curr;
curr = curr->next;
temp->next = curr;
delete n;
cout << delData << " was found and deleted\n";
}
}
}
void List::printList(){
curr = head;
while (curr != NULL) {
cout << curr->data << endl;
curr = curr->next;
}
}

Answer 1

The following line allocates a new node.
node * n = new node;
As already pointed out in the comments, it is not clear why the deleteNode() is doing that. The subsequent lines of delete n is actually deleting this new node, not one of the nodes in the list.
I would try writing deleteNode() something like this:
void List::deleteNode(int delData) {
// Empty list
if (!head) return;
// The first node is to be deleted
if (head->data == delData) {
node * old_head = head;
head = head->next;
delete old_head;
return;
}
// A non-first node is to be deleted
for (node * cur = head; cur; cur = cur->next) {
if (cur->next && cur->next->data == delData) {
node * del_node = cur->next;
cur->next = cur->next->next;
delete del_node;
break;
}
}
}

Answer 2

Actually the problem in your code was that the condition :
curr-> next !=NULL had to be replaced by curr!= NULL
since the code stops one step before it requires to stop.
Here is your working code:
#include <iostream>
using namespace std;
class List {
private:
struct node {
int data;
node * next;
};
node * head;
node * curr;
node * temp;
public:
List();
void addNode(int newData);
void deleteNode(int delData);
void printList();
};
int main() {
List test;
test.addNode(3);
test.addNode(4);
test.addNode(6);
test.printList();
cout << endl << endl;
test.deleteNode(1);
test.deleteNode(4);
cout << endl << endl;
test.printList();
}
List::List(){
head = NULL;
curr = NULL;
temp = NULL;
}
void List::addNode(int newData){
node * n = new node;
n->next = NULL;
n->data = newData;
if (head != NULL) { // List is intact
curr = head; // if List is not empty, make curr equal to the head, and start at the beginning of the list.
while(curr->next != NULL) { // Get to last item on the list
curr = curr->next;
}
curr->next = n; // Use the last item, and point to the new node.
}
else { // empty list
head = n; // new node is the head of the list.
}
}
void List::deleteNode(int delData){
node * n = new node;
temp = head;
curr = head;
if (head != NULL) {
while (curr!= NULL && curr->data != delData) {
temp = curr;
curr = curr->next;
}
cout<<temp->data<<" ";
if (temp->next == NULL) {
cout << delData << " was not found in the list\n";
delete n;
}
else {
n = curr;
curr = curr->next;
temp->next = curr;
delete n;
cout << delData << " was found and deleted\n";
}
}
}
void List::printList(){
curr = head;
while (curr != NULL) {
cout << curr->data << endl;
curr = curr->next;
}
}

Answer 3

As #drescherjm said, that extra allocation could be skipped.
The final deletenode function is:
void List::deleteNode(int delData){
node * n ;
temp = head;
curr = head;
if (head != NULL) {
while (curr!= NULL && curr->data != delData) {
temp = curr;
curr = curr->next;
}
cout<<temp->data<<" ";
if (temp->next == NULL) {
cout << delData << " was not found in the list\n";
// delete n;
}
else {
n = curr;
curr = curr->next;
temp->next = curr;
delete n;
cout << delData << " was found and deleted\n";
}
}
}