Singly Linked List inserting and deleting value in order - c++

Question

I was tasked with creating functions to add and delete nodes in a linked list given input data as an int and the char for with function to call. I'm not sure what I'm doing wrong. The only error I was given was: Exited with return code -11 (SIGSEGV). And a compiler method: main.cpp: In function ‘void listInsertValue(ListNode*&, ListNode*&, int)’:
main.cpp:111:23: warning: ‘toGoAfter’ may be used uninitialized in this function [-Wmaybe-uninitialized]
111 | toGoAfter->next = head;
Any help is appreciated. Thanks!
#include <iostream>
using namespace std;
struct ListNode
{
int data;
ListNode* next;
};
void listRemoveAfter(ListNode*&, ListNode*&, ListNode*);
void listPrepend(ListNode*&, ListNode*&, ListNode*&);
void listDeleteValue(ListNode*&, ListNode*&, int);
void listInsertValue(ListNode*&, ListNode*&, int);
void listInsertAfter(ListNode*&, ListNode*&, ListNode*, ListNode*);
int main()
{
ListNode *head = nullptr, *tail = nullptr;
ListNode *temp;
char choice;
int val;
//Write a main like you did in the previous lab
char command;
int number;
cin >> command;
while(command != 'Q')
{
if(command == 'I')
{
cin >> number;
listInsertValue(head,tail,number);
}
else
{
cin >> number;
listDeleteValue(head,tail,number);
}
cin >> command;
}
ListNode* current;
current = head;
while (current != nullptr)
{
cout << current->data << " ";
current = current->next;
}
cout << endl;
return 0;
}
//From previous lab - already complete
void listPrepend(ListNode*& h, ListNode*& t, ListNode*& n)
{
if (h == nullptr)
{
h = n;
t = n;
}
else
{
n->next = h;
h = n;
}
}
//From book, write yourself using the book code in 17.6 as a starting point
void listInsertAfter(ListNode*&head, ListNode*&tail, ListNode* curNode, ListNode* newNode)
{
if (head->next == nullptr)
{
head= newNode;
tail = newNode;
}
else if (curNode->next == tail)
{
tail->next = newNode;
tail = newNode;
}
else
{
newNode->next = curNode;
curNode->next = newNode;
}
}
//This function is mostly written, but you will need to add some code near the TODOs to complete the algorithm from the slides
void listInsertValue(ListNode*& head, ListNode*& tail, int val)
{
ListNode* toGoAfter, *newNode;
//TODO - create a new ListNode (newNode) with a data value of val (3 lines of code)
newNode = new ListNode;
newNode->data = val;
newNode->next = nullptr;
//TODO - check whether the list is empty in the if condition
if (head == nullptr)
{
listInsertAfter(head, tail, nullptr, newNode);
}
//TODO - use the else if to check whether the the value passed in is smaller than the value in the head
else if (head->data > val) //need to add to beginning of the list
{
listPrepend(head, tail, newNode);
}
else //need to add somewhere else in the list
{
//TODO - set toGoAfter to point to the head
toGoAfter->next = head;
//loop to find the location to insert the value
while (toGoAfter->next != nullptr && toGoAfter->next->data < val)
{
//TODO - set toGoAfter to point to the node after toGoAfter, like is done in traversals
toGoAfter = toGoAfter->next;
}
//We have found the location, so we can insert
listInsertAfter(head, tail, toGoAfter, newNode);
}
}
//modify
void listDeleteValue(ListNode* &head, ListNode*& tail, int val)
{
ListNode *temp;
//TODO - check if list is not empty in if condition
if (head->next == nullptr)
{
// TODO - check if value of head matches val passed in
if (head->data == val)
listRemoveAfter(head, tail, nullptr);
}
else
{
//loop searches for value to delete in node following temp
//TODO - set temp to point to the head
temp->next = head;
while (temp->next != nullptr && temp->next->data != val)
{
//TODO - set temp to point to the node after temp, like is done in traversals
temp = temp->next;
}
//TODO - make sure a node exists after temp, meaning the value to delete was found
if (temp->next != nullptr)
listRemoveAfter(head, tail, temp);
}
}
//From book, write yourself using the book code in 17.7 as a starting point
//Also add to the book's code, the code to delete nodes from memory
void listRemoveAfter(ListNode* & head, ListNode*& tail, ListNode* curNode)
{
ListNode *sucNode, *toDelete;
if (curNode->next == nullptr && head->next != nullptr)
{
sucNode = head->next;
head->next = sucNode;
if (sucNode->next == nullptr)
{ // Removed last item
tail->next = nullptr;
toDelete = head;
}
}
else if (curNode->next != nullptr)
{
sucNode = curNode->next->next;
curNode->next = sucNode;
if (sucNode-> next == nullptr)
{ // Removed tail
tail->next = curNode;
toDelete = curNode->next;
}
}
delete toDelete; //needed after the if/else if to remove the deleted node from memory
}

Answer 1

For most part you are not handling the case when there ia no element in the list. While inserting handle 4 use cases
Head==nullptr => head =newNode;
head->data > val
Tail->data < val
else case : insert in middle
Generic mistake: accessing ptr-> next, when ptr is nullptr
In general you want to use a debugger and any access to a memory 0x0 (nullptr) will start resolving your issues. ie head is 0x0 and you are doing operationa like head->data ==val