deleting a linked list node, C++ function not working - c++

Question

#include <iostream>
using namespace std;
class List {
public:
struct node {
int data;
node *next;
};
node* head = NULL;
node* tail = NULL;
node* temp = NULL;
node* prev = NULL;
public:
void addNum(int num) {
temp = new node;
temp->data = num;
temp->next = NULL;
if (head == NULL) {
head = temp;
tail = temp;
}
else {
tail->next = temp;
tail = temp;
}
}
void PrintList() {
temp = head;
while (temp != NULL) {
cout << temp->data << endl;
temp = temp->next;
}
}
void DelNum(int num) {
temp = head;
while (temp != NULL) {
if (temp->data == num) {
prev->next = temp->next;
free(temp);
}
temp = prev;
temp = temp->next;
}
}
};
int main() {
List list;
list.addNum(1);
list.addNum(2);
list.addNum(3);
list.addNum(4);
list.addNum(5);
list.addNum(6);
list.DelNum(3);
list.PrintList();
return 0;
}
What is wrong with my DelNum function? When I run the program nothing pops up. Doesn't matter what number I put in.

Answer 1

As mss pointed out the problem is in your DelNum() function where you assign temp = prev;. In your initialization you defined that node* prev = NULL; So, prev = NULL at the point when you assigned it to temp which caused segmentation fault when you try to use it like temp = temp->next;.

Answer 2

Two main problems are there in DelNum function:
first, when you are in while loop
, you should assign
prev = temp;
second, when you have found your target element, after deleting it you have to break out of the loop, which isn't done in your code
below is your corrected code( also correction of some other corner case in DelNum function ):
#include <iostream>
using namespace std;
class List {
public:
struct node {
int data;
node *next;
};
node* head = NULL;
node* tail = NULL;
node* temp = NULL;
node* prev = NULL;
public:
void addNum(int num) {
temp = new node;
temp->data = num;
temp->next = NULL;
if (head == NULL) {
head = temp;
tail = temp;
}
else {
tail->next = temp;
tail = temp;
}
cout<<num<<" is added \n";
}
void PrintList() {
temp = head;
while (temp != NULL) {
cout << temp->data << endl;
temp = temp->next;
}
}
void DelNum(int num) {
if(head==NULL)//empty
{
cout<<"empty linked list, can't be deleted\n";
return;
}
if(head->next==NULL)//means only one element is left
{
if(head->data==num)
{
node * fordelete=head;
head=NULL;
cout<<num<<"is deleted\n";
delete(fordelete);
}
else
{
cout<<"not found , can't be deleted\n";
}
return;
}
temp = head; // when more than one element are there
prev = temp;
while (temp != NULL) {
if (temp->data == num) {
prev->next = temp->next;
free(temp);
cout<<num<<" is deleted\n";
break;
}
prev= temp;
temp = temp->next;
}
if(temp==NULL)
{
cout<<"not found, can't be deleted\n";
}
}
};
int main() {
List list;
list.addNum(1);
list.addNum(2);
list.addNum(3);
list.addNum(4);
list.addNum(5);
list.addNum(6);
list.PrintList();
list.DelNum(3);
list.DelNum(7);
list.PrintList();
return 0;
}
I hope it will help you.