linked list not printing output - c++

Question

I have to dynamically allocate a list of robots for a school project. In an actual program, there will be other member functions that will require the list of names in order to perform certain functions.
As of right now, I just learned about this concept, and have tried really hard to put together some things I have seen online. The issue at the moment is that I can not tell if my list is properly being stored -- I am also getting wonky output when I try to call my display of list function.
Please help if you can. Also, I am happy to hear any tips for literally anything, as I am fairly new to programming.
class Node{
public:
std::string name_;
Node* next;
};
class linkedBotList{
public:
linkedBotList() {head = nullptr;} //constructor
~linkedBotList(){}; // destructure
void addNode();
void display();
private:
Node* head;
};
int main(int argc, const char * argv[]) {
linkedBotList* list = new linkedBotList();
int siz;
std::cout << "How many Robots?" << std::endl;
std::cout << "What are the names?" << std::endl;
std::cin >> siz;
for(int i = 0; i < siz; i++){
list->addNode();
}
delete list;
return 0;
}
void linkedBotList::addNode(){
std::string botName;
Node* newNode = new Node();
newNode->name_ = botName;
newNode->next = nullptr;
std::cin >> botName;
if(head == nullptr){
head = newNode;
}
else {
Node* temp = head; // head is not null
while(temp->next != nullptr){ // go until at the end of the list
temp = temp->next;
}
temp->next = new Node; // linking to new node
}
}
void linkedBotList::display() {
if (head == NULL) {
std::cout << "List is empty!" << std::endl;
}
else {
Node* temp = head;
while (temp != NULL) {
std::cout << "Made it to display funct.\n";
std::cout << temp->name_ << " ";
temp = temp->next;
}
std::cout << std::endl;
}
}
I did try a few things, like switching around my temp variable, and a few other re-assignments. Maybe someone can quickly spot the issue and help?

Answer 1

Your display function is fine.
The problem is that you have 2 logic flaws in addNode():
you are not storing strings in your list correctly. You are assigning botName to newNode->name_ before botName has been assigned a value. So all of your nodes have empty strings. Assigning botName afterwards will not update newNode->name_. 1
if the list is not empty, you iterate to the end of the list correctly 2, but then you assign a new blank node to temp->next instead of assigning your newNode that you already populated. And your Node constructor is not initializing the next member to nullptr, so you are creating a corrupted list, which will cause subsequent loops through the list to invoke undefined behavior.
Try this instead:
void linkedBotList::addNode(){
std::string botName;
std::cin >> botName; // <-- move up here
Node* newNode = new Node{botName, nullptr};
if (!head){
head = newNode;
}
else {
Node* temp = head;
while (temp->next){
temp = temp->next;
}
temp->next = newNode; // <-- linking to new node
}
}
Alternatively, you can eliminate the if like this:
void linkedBotList::addNode(){
std::string botName;
std::cin >> botName;
Node** temp = &head;
while (*temp){
temp = &((*temp)->next);
}
*temp = new Node{botName, nullptr};
}
1: A better design would be to have addNode() take in a string as an input parameter, and then move the cin call into your loop in main().
2: consider adding a tail member to your list to avoid having to loop on each addition.
Try this alternate design:
class Node{
public:
std::string name;
Node* next = nullptr;
};
class linkedBotList{
public:
linkedBotList() = default;
~linkedBotList();
void addNode(std::string name);
void display() const;
private:
Node* head = nullptr;
Node* tail = nullptr;
};
int main() {
linkedBotList list;
int siz;
std::string botName;
std::cout << "How many Robots?" << std::endl;
std::cin >> siz;
std::cout << "What are the names?" << std::endl;
for(int i = 0; i < siz; i++){
std::cin >> botName;
list.addNode(botName);
}
list.display();
return 0;
}
linkedBotList::~linkedBotList(){
Node *temp = head, *next;
while (temp) {
next = temp->next;
delete temp;
temp = next;
}
}
void linkedBotList::addNode(std::string name){
Node* newNode = new Node{name};
if (tail)
tail->next = newNode;
else
head = newNode;
tail = newNode;
}
void linkedBotList::display() const {
if (!head) {
std::cout << "List is empty!" << std::endl;
}
else {
Node* temp = head;
do {
std::cout << temp->name << " ";
temp = temp->next;
}
while (temp);
std::cout << std::endl;
}
}