C++ Singly Linked List Insertion Sort - c++

Question

So hey, I'm having a problem with this project I have. I'm supposed to read integers from a file and insert them into a list. There's a findSpot function that needs to be implemented that traverses the linked list and if the next node's value is larger than what is being checked, it returns the current "spot". And then we output the linked list to a separate file.
Here's the code.
#include <iostream>
#include <fstream>
using namespace std;
class listNode {
public:
int value;
listNode* next;
friend class linkedList;
listNode()
: value(0)
, next(NULL)
{
}
public:
~listNode(){
};
};
class linkedList {
listNode* listHead;
public:
linkedList()
: listHead(NULL)
{
}
bool isEmpty()
{
return (listHead == 0);
}
void listInsert(int data, listNode* spot)
{
listNode* newNode;
newNode->value = data;
newNode->next = NULL;
if (isEmpty()) {
listHead = newNode;
}
else {
newNode->next = spot->next;
spot->next = newNode;
cout << newNode;
}
}
/*void listDelete ()
{
}*/
listNode* findSpot(int data)
{
listNode* spot;
spot = listHead;
while (spot->next != 0 && spot->next->value < data) {
spot = spot->next;
}
return spot;
}
void printList(listNode* spot)
{
listNode* newNode = spot;
while (newNode != NULL) {
cout << "Inserting " << newNode->value << ": "
<< "listHead-->(" << newNode->value << "," << newNode->next->value << ")-->(";
newNode = newNode->next;
}
cout << endl;
}
/*~linkedList()
{
listNode* temp = spot->next;
spot->next = spot->next->next;
delete temp;
}*/
};
int main(int argc, char* argv[])
{
int data;
listNode* spot;
ifstream infile;
infile.open(argv[1]);
ofstream outfile(argv[2]);
cout << "Reading Data from the file" << endl;
while (infile >> data) {
cout << data << endl;
}
infile.close();
linkedList myList;
infile.open(argv[1]);
while (infile >> data) {
myList.findSpot(data);
myList.listInsert(data, spot);
myList.printList(spot);
}
cout << "Printing your linked list to the output file.";
/*while (outfile.is_open())
{
myList.printList();
}*/
infile.close();
outfile.close();
return 0;
}
I don't know if the problem lies mainly in the insertList function or if it's the findSpot function. The findSpot function seems correct to me but I may just be missing something.
As I run the code, the actual reading of the file the first time is fine. Actually inserting anything into the linked list causes the program to hang.

Answer 1

Ok, lets try this again. I'll actually include some code, but please try to use this as a learning point, and not something to just copy paste. I know you said you were copying your teachers algorithm, but what they gave you is probably just that, an algorithm. It is your job to actually implement that in working code, checking for error conditions, etc. Anyway, here we go:
For the function findSpot:
listNode* linkedList::findSpot(int data) {
listNode* spot = listHead; // Initialize spot to start of list
if ( isEmpty() ) // if list is empty, return NULL
return NULL;
// now we know listHead isn't null, so look through the list and
// find the entry that has a value greater than the one provided
// return the list item BEFORE the one with the greater value
while (spot->next != 0 && spot->next->value < data) {
spot = spot->next;
}
// return the one we found; This could be the same as listHead
// (start of list), something in the middle, or the last item on the
// list. If we return from here, it will not be NULL
return spot;
}
Now we can do the insert function:
void linkedList::listInsert(int data, listNode* spot) {
// We need a new item to put on the list, so create it
listNode* newNode = new listNode();
newNode->value = data;
newNode->next = NULL;
// If the list is empty, update the head to point at our new object
if ( isEmpty() ) {
listHead = newNode;
// otherwise point spot to new item, and new item to the one spot
// pointed to
} else {
newNode->next = spot->next;
spot->next = newNode;
}
}
Looking at your print function, that is going to have it's own issues. It looks like you want to print the whole list, but it seems that you are starting to print from "spot". It's all very confused. It also has an issue using newNode->next->value, without checking if newNode->next is NULL. Here's a short example of what I think you are trying to do... note that I don't even need to pass in spot, just the data point added:
void linkedList::printList(int data) {
// if some huckleberry called this before calling insert,
// list will be empty... always a good idea to check
if ( isEmpty())
return;
// first line of output... just print out the data point
// added and start of output text
cout << "Inserted " << data << ": " << "listHead-->(";
// start at start of list
listNode* newNode = listHead;
// loop through until we find the end
while (newNode != NULL) {
cout << newNode->value; // print the value
newNode = newNode->next; // move to the next item on the list
// We moved to the next node; It might be NULL and the loop will end
// if not, we want to print an open bracket since we know another one
// is going to be printed
if ( newNode != NULL )
cout << ")-->(";
}
// last item was just printed, so close off the last bracket
cout << ")" << endl;
}
Hope that is somewhat helpful

Answer 2

Since this looks like a homework assignment, I'm going to give you one fix:
change
myList.findSpot(data);
to
spot = myList.findSpot(data);
If you look closely, spot is used, but never assigned anything.

Answer 3

Well, there are several problems with your program (besides formatting). In the function findSpot(), you have:
listNode* spot;
spot = listHead;
while (spot->next != 0 && spot->next->value < data) {
spot = spot->next;
}
return spot;
The problem here is that the first time you call this, listHead is NULL, so the
while (spot->next
is going to fail, since spot is NULL.
I also notice that nowhere in your code do you call new(). In listInsert, you need to use new() to initialize your newNode variable.
Lastly, find spot has 2 conditions where it can return NULL. If the list is empty, it should return NULL, and you would want to insert at the start of the list. If the new value you are adding is greater than all the others, you will also return NULL and you would have to add to the end of the list.
Since this is a homework assignment, I don't want to write the code for you, but hopefully that helps.