A c++ program to implement singly linked list with operations like insertFront, insertLast, display, search, update, deleteLast, deleteFront, insertAfter

#include <iostream>
#include <cstdlib>
using namespace std;
struct Node {
   int data;
    Node *next;
};

struct Node *head = NULL, *p, *temp;

void insertfront(int new_data) {
   struct Node *new_node = new Node; //create empty node
   new_node->data = new_data;
   new_node->next = NULL;
   if (head == NULL){
        head = new_node;
   }
   else{
        new_node->next = head;
        head = new_node;
   }
}

void deleteFront(){
    if (head == NULL)  //empty list
        return;
    else{
        p = head;
        head = head->next;
        p->next = NULL; //unlink before deleting
        delete p;
    }
}

void deleteLast(){
    if (head == NULL) //empty list
        return;
    else if (head->next == NULL){ //if it contains one element only
        delete head;
        head = NULL;
    }
    else{ //more than one elements
        p = head;
        while (p->next->next != NULL){
            p = p->next;
        }
        delete p->next;
        p->next = NULL;
    }
}

void insertlast(int item){
    struct Node *new_node = new Node;
    new_node->data = item;
    new_node->next = NULL;
    if (head == NULL){ //if empty
        head = new_node;
    }
    else {
        p = head;
        while (p->next != NULL){
            p = p->next;
        }
        p->next = new_node;
    }
}

void insertAfter(int key, int item){
    if (head == NULL) return;
    else{
        bool found = false;
        p = head;
        while (p != NULL){
            if (p->data == key){
                found = true;
                break;
            }
            p = p->next;
        }
        if (found){
            struct Node *new_node = new Node;
            new_node->data = item;
            new_node->next = p->next;
            p->next = new_node;
        }
        else {
            cout << "key not found\n";
        }
    }
}

void display() {
    cout<<"The linked list is: ";
    p = head;
    if (head == NULL) cout << "empty\n";
    else {
       while (p != NULL) {
          cout<< p->data <<" ";
          p = p->next;
       }
       cout<<"\n";
    }
}

void update(int key, int item){
    p = head;
    bool found = false;
    while(p != NULL){
        if (p->data == key){
            found = true;
            break;
        }
        p = p->next;
    }
    if (found){
        p->data = item;
    }
    else {
        cout<<"key not found :(\n";
    }
}

void search(int key){
    p = head;
    int index = 0;
    bool found = false;
    while(p != NULL){ //to look for a key on the last node...
        if (p->data == key){
            found = true;
            break;
        }
        p = p->next;
        index++;
    }
    if (found){
        cout << "key " << key << " found at index " << index << "\n" ;
    }
    else {
        cout<<"key not found :(\n";
    }
}


int main() {


    cout<<"#########################################################\n";
    cout<<"###                                                   ###\n";
    cout<<"###    C++ program to implement singly linked list    ###\n";
    cout<<"###                                                   ###\n";
    cout<<"#########################################################\n";

    while(true){
        cout<<"   --  Enter your choice to continue  --   \n";
        cout<<"1. Insert Front\n2. Insert Last\n3. insert After\n4. delete Front\n"<<
        "5. delete Last\n6. update\n7. search\n8. display\n9. exit\n";
        int command, value, position;
        cin >> command;
        switch (command){
        case 1:
            cout<<"enter the value to be inserted : "; cin >> value;
            insertfront(value); break;
        case 2:
            cout<<"enter the value to be inserted : "; cin >> value;
            insertlast(value); break;
        case 3:
            cout<<"enter the value to be inserted : "; cin >> value;
            cout<<"Enter the position at which node to be inserted after : "; 
            cin >> position;
            insertAfter(position,value); break;
        case 4:
            deleteFront();
            break;
        case 5:
            deleteLast();
            break;
        case 6:
            cout<<"enter the value to be updated : "; cin >> position;
            cout<<"Enter the new value : "; cin >> value;
            update(position,value);
            break;
        case 7:
            cout<<"enter the value you are looking for : "; cin >> value;
            search(value);
            break;
        case 8:
            display();
            break;
        case 9:
            exit(1);
            break;

        default:
            cout << "wrong choice\n";
            break;
        }
    }
}