C++, Linked list with user inputs and have it add to one head?

Question

So I need help with having the user input into just one head of the linked list and once the user types '+' into the program, the linked list will insert a new node to the list and start again and once the user types the '=', the two heads add with each other.

Thank you for the help in advance.

I'll provide my current code, output and desired output:

Code:

#include<iostream>
#include<string>
using namespace std;

struct Node {
    string data;
    Node* next;
};

void addToList(Node* head);

int main() {
    bool quit = false;
    int choice;
    Node* head = new Node;
    head->next = NULL;

    while (!quit) {
        cout << "1. add to list" << endl
            << "2. quit" << endl;
        cin >> choice;
        switch (choice) {
        case 1: addToList(head);
            break;
        case 2: quit = true;
            break;
        default:
            cout << "That is not a valid input, quitting program";
            quit = true;
        }
    }
}

void addToList(Node* head) {
    bool quit = false;
    string temp;
    Node* current;

    while (!quit) {

        cout << "Enter a word(quit to stop)";
        cin >> temp;

        if (temp == "quit") {
            quit = true;
        }
        else {
            // Allocate the new node here:
            current = new Node;
            current->data = temp;

            // the new node is inserted after the empty head
            // because head is an empty node in your implementation:
            current->next = head->next;
            head->next = current;

            // set current to head->next, because the head is empty in your implementation:
            current = head->next;
            while (current)
            {
                cout << current->data << endl;
                current = current->next;
            }
        }
    }
    return;
}

Current Output:

Enter a number(quit to stop)3
3
Enter a number(quit to stop)4
4
3
Enter a number(quit to stop)5
5
4
3

Desired Output:

Enter a number(quit to stop)3   
3    
Enter a number(quit to stop)4
34
Enter a number(quit to stop)5
345
Enter a number(quit to stop)+
Enter a number(quit to stop)1
1
Enter a number(quit to stop)2
12
Enter a number(quit to stop)=
357

Have you tried running your code line by line in a debugger while monitoring the values of all variables, in order to determine at which point your program stops behaving as intended? If you did not try this, then you may want to read this: [What is a debugger and how can it help me diagnose problems?](https://stackoverflow.com/q/25385173/12149471) You may also want to read this: [How to debug small programs?](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) — Andreas Wenzel, Apr 29 '22 at 15:24
So, have you tried making it do this? What happened when you tried? — user253751, Apr 29 '22 at 15:39

rturrado · Answer 1 · 2022-04-29T16:18:20.860

If you want to keep a pile of nodes (a list where you always insert at the end), it will be useful to have a tail pointer apart from the head pointer.
head and tail can be null pointers in a beginning; you don't need to have an empty node.
You'll need to pass them as references to addToList, since they can be modified (made to point to a new node).
If you use raw pointers, remember to delete the nodes you allocate via new.

[Demo]

#include <iostream>  // cin, cout
#include <string>

struct Node {
    std::string data{};
    Node* next{nullptr};
};

void addToList(Node*& head, Node*& tail) {
    bool quit{false};
    std::string temp{};
    Node* current{nullptr};

    while (!quit) {
        std::cout << "\tEnter a word (quit to stop):\n";
        std::cin >> temp;

        if (temp == "quit") {
            std::cout << "\tQuitting submenu.\n";
            quit = true;
        }
        else {
            // Allocate the new node here
            current = new Node;
            current->data = temp;

            if (tail) {
                tail->next = current;
                tail = current;
            }
            else {
                head = current;
                tail = current;
            }

            std::cout << "\t[";
            for (current = head; current; current = current->next) {
                std::cout << current->data << (current->next ? ", " : "");
            }
            std::cout << "]\n";
        }
    }
}

int main() {
    bool quit = false;
    int choice = 0;
    Node* head = nullptr;
    Node* tail = nullptr;

    while (!quit) {
        std::cout << "1. add to list, 2. quit:\n";
        std::cin >> choice;        
        switch (choice) {
            case 1:
                addToList(head, tail);
                break;
            case 2:
                std::cout << "Quitting main menu.\n";
                quit = true;
                break;
            default:
                std::cout << "That is not a valid input, quitting program.\n";
                quit = true;
        }
    }
    for (auto tmp{head}; tmp;) {
        tmp = tmp->next;
        delete head;
        head = tmp;
    }
}

// Input:
//
//   1
//   hello
//   quit
//   1
//   joe
//   quit
//   1
//   bye
//   quit
//   2
//
// Output:
//
//   1. add to list, 2. quit:
//      Enter a word (quit to stop):
//      [hello]
//      Enter a word (quit to stop):
//      Quitting submenu.
//   1. add to list, 2. quit:
//      Enter a word (quit to stop):
//      [hello, joe]
//      Enter a word (quit to stop):
//      Quitting submenu.
//   1. add to list, 2. quit:
//      Enter a word (quit to stop):
//      [hello, joe, bye]
//      Enter a word (quit to stop):
//      Quitting submenu.
//   1. add to list, 2. quit:
//   Quitting main menu.

Should you be able to use a std::vector instead of a list of Nodes, things would simplify a lot:

Adding a new element to the end would just require a push_back.
Printing the whole vector would also be a one-liner.
And, most importantly, you would forget about allocating and deallocating memory.

[Demo]

#include <fmt/ranges.h>
#include <iostream>  // cin, cout
#include <string>
#include <vector>

void addToList(std::vector<std::string>& list) {
    bool quit{false};
    std::string temp{};

    while (!quit) {
        std::cout << "\tEnter a word (quit to stop):\n";
        std::cin >> temp;

        if (temp == "quit") {
            std::cout << "\tQuitting submenu.\n";
            quit = true;
        }
        else {
            list.push_back(temp);
            fmt::print("\t{}\n", list);
        }
    }
}

int main() {
    bool quit = false;
    int choice = 0;
    std::vector<std::string> list{};

    while (!quit) {
        std::cout << "1. add to list, 2. quit:\n";
        std::cin >> choice;        
        switch (choice) {
            case 1:
                addToList(list);
                break;
            case 2:
                std::cout << "Quitting main menu.\n";
                quit = true;
                break;
            default:
                std::cout << "That is not a valid input, quitting program.\n";
                quit = true;
        }
    }
}

C++, Linked list with user inputs and have it add to one head?

1 Answers1