Linked List insertion issue - c++

Question

I was trying out Linked List and I have a bit of problem with my insertNode1 function. This function is supposed to insert a new node after the inputted node by the user. The result doesn't show what I wanted to show. I wanted the number 5 to be after 3, but it doesn't show how I wanted it.
#include <iostream>
using namespace std;
class List {
struct node {
float data;
node* next;
};
node* head;
public:
void appendNode(float);
void insertNode(float);
void insertNode1(float,float);
void deleteNode(float);
void displayList();
List(){
head = NULL;
}
~List(){
node *nodePtr, *nextNode;
nodePtr = head;
while (nodePtr != NULL){
nextNode = nodePtr->next;
delete nodePtr;
nodePtr = nextNode;
}
}
};
void List::appendNode(float d)
{
node *newNode, *nodePtr;
newNode = new node;
newNode->data = d;
newNode->next = NULL;
if (!head){
head = newNode;
}
else {
nodePtr = head;
while (nodePtr->next){
nodePtr = nodePtr->next;
}
nodePtr->next = newNode;
}
}
void List::insertNode(float d){
node *newNode, *nodePtr, *prevNode=NULL;
newNode = new node;
newNode->data = d;
if (head== NULL)
{
head = newNode;
newNode->next = NULL;
}
else {
nodePtr = head;
while (nodePtr != NULL && nodePtr->data < d){
prevNode = nodePtr;
nodePtr = nodePtr->next;
}
if (prevNode == NULL){
head = newNode;
newNode->next = nodePtr;
}
else {
prevNode->next = newNode;
newNode->next = nodePtr;
}
}
}
void List::insertNode1(float p, float d){
node *newNode, *nodePtr, *selectedNode=NULL;
newNode = new node;
newNode->data = d;
nodePtr = head;
selectedNode = nodePtr;
selectedNode->data = p;
if (head != NULL){
head = newNode;
newNode->next = NULL;
}
else {
while (nodePtr->next != NULL && nodePtr->data != p){
nodePtr = nodePtr->next;
}
if (selectedNode == NULL){
head = newNode;
selectedNode->next = NULL;
}
else {
selectedNode->next = newNode;
}
}
}
void List::deleteNode(float da){
node *nodePtr, *previousNode=NULL;
if (head == NULL){
return;
}
if (head ->data == da){
nodePtr = head->next;
delete head;
head = nodePtr;
}
else{
nodePtr = head;
while (nodePtr != NULL && nodePtr->data != da){
previousNode = nodePtr;
nodePtr = nodePtr->next;
}
previousNode->next = nodePtr->next;
delete nodePtr;
}
}
void List::displayList(){
node *nodePtr;
nodePtr = head;
while (nodePtr!= NULL){
cout << nodePtr->data << endl;
nodePtr = nodePtr->next;
}
}
int main(){
List lobj;
lobj.appendNode(3);
lobj.appendNode(6.4);
lobj.appendNode(4.5);
lobj.appendNode(7.8);
lobj.insertNode(1.5);
lobj.displayList();
cout << "after deleting and adding a new node" << endl;
lobj.deleteNode(6.4);
lobj.displayList();
lobj.insertNode1(3, 5);
lobj.displayList();
system("pause");
}

Answer 1

At the beginning of your void List::insertNode1(float p, float d) function:
nodePtr = head;
selectedNode = nodePtr;
selectedNode->data = p;
Here, selectedNode points at the head of the list and you seem to overwrite the data of that node with the value of p, which I cannot help but feel is not what you meant to do.

Answer 2

There are a few issues inside List::insertNode1.
nodePtr = head;
selectedNode = nodePtr;
selectedNode->data = p;
Here selectedNode points at head but then you set the first data value to p.
if (head != NULL){
head = newNode;
newNode->next = NULL;
}
This part only executes when head is not null. This means you get rid of the entire list every time this is run. This is why the last call to list::displayList only prints 5.
if (selectedNode == NULL){
head = newNode;
selectedNode->next = NULL;
}
Here selectedNode is NULL. first you set head to be a new node but this doesn't change selectedNode. If you ran this method on an empty list your program would segfault here. You should be setting the next of head or newNode to NULL here, not selectedNode.
else {
selectedNode->next = newNode;
}
Here selectedNode still points to head. You change nodePtr but this doesn't also change selectedNode.