Doubly Linked list instream - C++ - c++

Question

I am doing a project that reads from an input file and puts the digits in two separate doubly linked lists and does operations (addition, multiplication, ) between the two and outputs them to a third doubly linked list.
I am stuck in how to make the loop read character by character and create a new node for each digit.
The intention, in the end, is to change to different sizes of digits per node.
The file would look similar to this:
0*0
890+0
0*400
7650.4+100.26
160008800999008800+4
976976*863586589
Here is the code:
#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
using namespace std;
struct node// original node
{
int data;
node* next;
node* prev;
};
struct node *newNode(int data)// add new node with int data input
{
struct node *new_node = (struct node *) malloc(sizeof(struct node));
new_node->data = data;
new_node->next = NULL;
new_node->prev = NULL;
return new_node;
};
// This is supposed to add a new node to a doubly linked list at the end
void insert(node** head_ref,int n)// NOT WORKING PLEASE HELP
{
node* temp = newNode(n);
temp->data = n;
temp->next = NULL;
temp->prev = (*head_ref);
(*head_ref)->next = temp;
temp = (*head_ref);
/*last->next = temp;
last = temp;*/
}
void push(struct node** head_ref, int new_data)// insert at the beginning of the list
{
/* allocate node */
struct node* new_node = newNode(new_data);
/* link the old list off the new node */
new_node->next = (*head_ref);
/* move the head to point to the new node */
(*head_ref) = new_node;
}
/* Adds contents of two linked lists and return the head node of resultant list */
struct node* addTwoLists(struct node* first, struct node* second)
{
struct node* res = NULL; // res is head node of the resultant list
struct node *temp, *prev = NULL;
int carry = 0, sum;
while (first != NULL || second != NULL) //while both lists exist
{
// Calculate value of next digit in resultant list.
// The next digit is sum of following things
// (i) Carry
// (ii) Next digit of first list (if there is a next digit)
// (ii) Next digit of second list (if there is a next digit)
sum = carry + (first ? first->data : 0) + (second ? second->data : 0);
// update carry for next calulation
carry = (sum >= 10) ? 1 : 0;
// update sum if it is greater than 10
sum = sum % 10;
// Create a new node with sum as data
temp = newNode(sum);
// if this is the first node then set it as head of the resultant list
if (res == NULL)
res = temp;
else // If this is not the first node then connect it to the rest.
prev->next = temp;
// Set prev for next insertion
prev = temp;
// Move first and second pointers to next nodes
if (first) first = first->next;
if (second) second = second->next;
}
if (carry > 0)
temp->next = newNode(carry);
// return head of the resultant list
return res;
}
void printList(node* list)
{
node* temp = list;
while (temp != NULL)
{
cout << temp->data << "";
temp = temp->next;
}
}
int main(int argc, char* argv[])
{
struct node* res = NULL;
struct node* first = NULL;
struct node* second = NULL;
// create 2 lists for testing, not the final intention
push(&first, 6);
push(&first, 4);
push(&first, 9);
push(&first, 5);
push(&first, 7);
push(&second, 4);
push(&second, 8);
//Set input
string filename = argv[1];
filename.erase(0, 14);
std::string s = filename;
std::string delimiter = "=";
size_t pos = 0;// Get digits per node
std::string token;
while ((pos = s.find(delimiter)) != std::string::npos) {
token = s.substr(0, pos);
s.erase(0, pos + delimiter.length());
}
int digitsPerNode = std::stoi(s);//obtain DigitsPerNode
std::string f = filename;//get correct filename
std::string cutoff = ";";
std::string reducedname = f.substr(0, f.find(cutoff)); // token is "scott"
ifstream instream(reducedname);//Obtain Input Stream
string operands;
char currentchar;
string group;
//Char solution
bool isfirstoperation= true;
char operation = '+';
while (instream.get(currentchar))
{
for (int i = 0; i < digitsPerNode; i++)
{
if (currentchar == '\n')
{
cout << "piripitiflautica" << endl;
}
cout << currentchar << endl;
if (isdigit(currentchar))
{
}
if (currentchar == '+')
{
isfirstoperation = false;
operation = '+';
}
if (i != digitsPerNode)
{
instream.get(currentchar);
}
}
if (currentchar == '\n')
{
cout << "newline" << endl;
}
/*cout <<endl << "Final: "<< group;
group = "";
cout <<endl;*/
}
//Operate
printList(first);
cout << operation;// display operation
printList(second);
cout << "=" ;//Display answer
if (operation = '+')
{
res = addTwoLists(first, second);
//printf("Resultant list is ");
printList(res);
}
cout << endl;
return 0;
}

Answer 1

Two problems to get you started: In insert (which you are not calling), temp = (*head_ref); is backwards; it should be *head_ref = temp;.
And in push you're not updating the prev node in the existing list (watch out for an empty list when you handle this).