Program crashes while adding a new node in linked list in c++

Question

I am trying to implement an insert function of the linked but as soon as I add the third element the program just crashes and execution is stopped, even though the same code worked on hackerrank's compiler.

Here is my code.

#include<bits/stdc++.h>
using namespace std;

class Node{
    public:
        int data;
        Node * next;
        Node(int data){
            this -> data = data;
            this -> next = nullptr;
        }
};

Node * insert_tail(Node * head, int data){
    Node * node = new Node(data);
    if(head == nullptr) return node;
    Node * current = head;
    while(head -> next != nullptr) current = current -> next;
    current -> next = node;
    return head;
}

void print_linkedlist(Node * head){
    while(head -> next != nullptr){
        cout << head -> data << " -> ";
        head = head -> next;
    }
    cout << head -> data << " -> nullptr";
}

int main(){
    Node *  head = nullptr;
    head = insert_tail(head, 1);
    head = insert_tail(head, 5);
    head = insert_tail(head, 3);
    head = insert_tail(head, 5);
    head = insert_tail(head, 8);
    head = insert_tail(head, 17);

    print_linkedlist(head);
    return 0;
}

You should learn to use a debugger and step through the code to see what's happening. — JHBonarius
– JHBonarius, Commented Oct 28, 2020 at 19:36
Sorry about that I added the definition, but as @MikeCAT mentioned it was a logical error. — Shaikh Abuzar
– Shaikh Abuzar, Commented Oct 28, 2020 at 19:36
I am new to C++ can you send me some tutorial on how to use debugger. — Shaikh Abuzar
– Shaikh Abuzar, Commented Oct 28, 2020 at 19:37
Tactical note: Loops like while(head -> next != nullptr) are risky and require additional tests to ensure that head is valid before the first head -> next. I usually find it better to write while(head != nullptr) and build the initial test into the loop. — user4581301
– user4581301, Commented Oct 28, 2020 at 19:58

MikeCAT · Accepted Answer · 2020-10-28 19:29:24Z

1

The line

    while(head -> next != nullptr) current = current -> next;

in the function insert_tail is wrong. It will run endlessly when head->next is not nullptr.

It should be

    while(current -> next != nullptr) current = current -> next;

answered Oct 28, 2020 at 19:29

MikeCAT

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Comments

Vlad from Moscow · Accepted Answer · 2020-10-28 19:38:42Z

1

Here is a typo

while(head -> next != nullptr) current = current -> next;
      ^^^^^^^^^^^^

Write

while(current -> next != nullptr) current = current -> next;
      ^^^^^^^^^^^^

An alternative definition of the function can look the following way,

void insert_tail( Node * &head, int data )
{
    Node **current = &head;

    while ( *current ) current = &( *current )->next;

    *current = new Node( data );
}

And the function can be called simply like

insert_tail(head, 1);

Also the function print_linkedlist can be written like

std::ostream & print_linkedlist( const Node * head, std::ostream &os = std::cout )
{
    for ( ; head; head = head->next )
    {
        os << head -> data << " -> ";
    }

    return os << "nullptr";
}

and can be called like

print_linkedlist(head) << '\n';

edited Oct 28, 2020 at 19:38

answered Oct 28, 2020 at 19:30

Vlad from Moscow

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