Question

update: added LinkedList.h and SymbolTable.h

So I’m having difficulty with a programming assignment from school. This is my first C++ course, the previous ones were taught in Java.

I created a linked list of SymbolTable objects and I need to retrieve the id (std::string) and value (int) from each when traversing the list, but any values I try to get just return null.

I think I’m missing something fundamental here, but not sure what.

this is the output I get, notice the blank line is from calling
cout << temp->data->getID() << endl;

I’m really at a loss of how to fix/get around this.

output:

C:\Users\spud\Documents\Visual Studio 2010\Projects\EECS1580_assign_01\Debug>EEC
S1580_assign_01.exe
Reading file...
Parsing...id_1 = 40
id_1
40

Parsing...var_2 = 50
var_2
50

Parsing...id_1 += 30
Parsing...var_2-=20
Parsing...id_1*=5
Parsing...k_27=80
k_27
80

Parsing...k_27 /= 20
done reading..
done

Here’s the code

LinkedList.cpp

#include "StdAfx.h"
#include "LinkedList.h"
#include <iostream> 
#include <string>
#include "SymbolTable.h"

using namespace std;


// define node for LinkedList
typedef struct node 
{ 
    public:
        SymbolTable *data;       // data in node 
        node *next;             // link to next node 
        node() 
        {
            next = NULL;
            data = new SymbolTable();
        }
}; 


node *head = NULL;               // empty linkedList
node *tail = NULL;
node *curr = NULL;


LinkedList::LinkedList() 
{
    head = new node();           
} 

LinkedList::~LinkedList() 
{ 
   node *q; 
   if( head == NULL ) 
        return; 

   while( head != NULL ) 
   { 
        q = head->next; 
      delete head; 
      head = q; 
   }                     
} 


// insert node at front of LinkedList
void LinkedList::insert(string id, int value) 

{ 
    SymbolTable *s = new SymbolTable(id, value);

    node *temp;                   //create a temporary node 
    temp = new node;              //allocate space for node 

    temp->data = s;               // store data(first field)
    temp->next = head;            // store the address of the pointer head(second field)
    head = temp;                  // transfer the address of 'temp' to 'head'
    cout << temp->data->getID() << endl;
} 

void LinkedList::display() 
{ 
     node *q; 
     cout << endl; 

     for(q = head; q != NULL; q = q->next) 
           cout << q->data->toString() << endl;
} 


void LinkedList::traverse() 
{ 

    node *curr;
    curr = head;
    SymbolTable *temp;

    if(head == NULL)
    cout << "End of list.\n";
    else    
    {
        while(curr != NULL) {
        //cout << temp->data.toString() << " ";
            temp->setID(curr->data->getID());
            temp->setValue(curr->data->getValue());
            cout << temp->getID() << endl;
            curr = curr->next;
        }
    }
} 

/*void find(string tofind, char op, int value)
{
     node *temp1;                               // create tmp node 
     temp1 = head;                              // transfer address of p to temp1 
     //string t = "";
     while(temp1 != NULL) 
     { 
        // t = temp1->data.getID();
        // if(tofind.compare(t) == 0)
         {
             cout << "match!" << endl;
        //   temp1->data.performOp(op, value);  // perform op
         }
        // temp1 = temp1->next;                   // transfer address of temp->next to temp             
     }     
     cout << "no match!" << endl;
}*/

void LinkedList::sort() 
{ 
    node *temp, *temp1, *temp2;
    //temp = new node;
    //temp1 = new node;
    //temp2 = new node;

    for(temp1 = head; temp1 != NULL; temp1 = temp1->next)
    for(temp2 = head; temp2 != NULL; temp2 = temp2->next)
    {
        if(temp1->data->toString() < temp2->data->toString())
        {
            temp->data = temp2->data;
            temp2->data = temp1->data;
            temp1->data = temp->data;
        }
    }
} 

SymbolTable.cpp

#include "StdAfx.h"
#include "SymbolTable.h"
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <cstdlib> 

using namespace std;

    string id;
    int value;
    int final;


    void SymbolTable::setID(string s)
    {
        id = s;
    }

    void SymbolTable::setValue(int a) 
    {
        value = a;
    }

    // Will perform the requested operation on the node
    int SymbolTable::performOp(char op, int val)
{
    // final value to return
    int final = 0;
    // determine which op to use, then exec it
        switch (op) {
            case '+':
                value += val; break;
            case '-':
                value -= val; break;
            case '*':
                value *= val; break;
            case '/':
                value /= val; break;
            case '%':
                value %= val; break;
            default:
                // no operation performed
                cout << "noOp" << endl;
                break;
        }

        cout << id << " New Value: " << value << endl;

        // return the new value
        return value;
}

    // returns a string with info about the node
    string SymbolTable::toString(void)
    {
        // create new string to return
        string mString;
        // append id and = to the string
        mString.append(id).append("=");
        // append in to string
        ostringstream oss;
        oss << value;
        mString += oss.str();
        cout << mString << endl;
        // return the final string
        return mString;
    }

    // return Id 
    const string SymbolTable::getID(void)
    {
        return id;
    }
    const int SymbolTable::getValue(void)
    {
        return value;
    }

    // return the link that this obj points to
    //SymbolTable* SymbolTable::getLink()
    //{
    //  return link;
    //}

    // set link
    //void SymbolTable::setLink(SymbolTable *s)
    //{
    //  link = s;
    //}

    // Constructor
    SymbolTable::SymbolTable(string a, int v)
    {

        // initialize the id
        id = a;
        cout << getID() << endl;
        // initialize the value
        value = v;
        cout << getValue() << endl;
        final = 0;
    }
    SymbolTable::SymbolTable()
    {
        id = "";
        value = 0;
    }

    // deConstructor
    SymbolTable::~SymbolTable(void)
    {
        // clear the string
        id.clear();
        // set value to 0
        value=0;
    }

Main

#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <iostream> 
#include <algorithm>
#include "SymbolTable.h"
#include "LinkedList.h"

using namespace std;

//SymbolTable *root = NULL;

LinkedList mylist;

void parse(string line) 
{ 
     int value;
     string svalue;
     string id;
     char op;

     cout << "Parsing..." << line << endl;

     // remove all spaces                                             
     for(int i=0; i<line.length(); i++)
         if(line[i] == ' ')
             line.erase(i,1);

     int eqSpot = line.find('=');                             // find the = 
     svalue = line.substr(eqSpot+1, line.length()-1);         // set value 
     switch(line.at(eqSpot-1)){                               // set op 
                      case '+':  op = '+';  eqSpot--; break; 
                      case '-':  op = '-';  eqSpot--; break; 
                      case '*':  op = '*';  eqSpot--; break; 
                      case '/':  op = '/';  eqSpot--; break;
                      default:   op = '=';  break;          
     };

     id = line.substr(0, eqSpot);                           // set id

     stringstream ss(svalue);                               // convert std::string svalue to int value
     ss >> value;

     if(op == '=')                                          // if decleration.. 
     { 
         // create new node in the LinkedList      
                mylist.insert(id, value);                           // insert s into LL
     } 
     else                                                   // else is operation 
     { 
               // myList.find(id, op, value);
     }
} 

/*// sorts the nodes in the LL
void sort()
{
    SymbolTable *temp1;
    SymbolTable *temp2;
    SymbolTable *temp;

    cout << "Sorting Nodes..." << endl;
    for( temp1 = root ; temp1!=NULL ; temp1 = temp1->getLink() )
    {
          for( temp2 = temp1->getLink() ; temp2!=NULL ; temp2 = temp2->getLink() )
          {
                if( temp1->getID() > temp2->getID() )
                {
                      temp = temp1->getLink();
                      temp1->setLink(temp2->getLink());
                      temp2->setLink(temp);
                }
          }
    }
}
// should print out the toString for each node
void printNodes()
{
    SymbolTable *node = root;

    do
    {
        cout << node->toString() << endl;
    }while((node = node->getLink()) != NULL);
}
// Will traverse the string, comparing IDs and will return a pointer to that node or null if not found
SymbolTable* traverse(string id)
{
     SymbolTable *temp1;                          // create tmp node 
     temp1 = root;

     while(temp1 != NULL) 
     {  
        cout << temp1->toString() << endl;
        if(id.compare(temp1->getID()) == 0)
        {
            // Found!
            cout << "MATCH" << endl;
            return temp1;
        };
        cout << root->getLink() << endl;
        if(temp1 == temp1->getLink())
            temp1 = NULL;
        else
            temp1 = temp1->getLink();                // transfer address of temp->next to temp 
     };

     cout << "NO MATCH FOUND" << endl;
     return NULL;

}
*/

// some test values
int _tmain(int argc, _TCHAR* argv[])
{
    // Read input
    mylist = LinkedList();
    string line;
    ifstream myfile ("input.txt");              // input file
    if (myfile.is_open())
    {
        cout << "Reading file..." << endl;
        while ( myfile.good() )                 // read file line by line
        {
            getline(myfile,line);
            if(line.compare("</end/>") == 0)    // end of file
                break;
            else
                parse(line);                    // parse the line
        }
        myfile.close();
        cout << "done reading.." << endl;
    }
    else cout << "Unable to open file"; 

    //myList.sort();

    //myList.traverse();

    cout << "done";
    //return 0;

    return EXIT_SUCCESS; 
} 

LinkedList.h

#pragma once
#include "SymbolTable.h"
#include <string>

class LinkedList
{
private: 

public:
    LinkedList(void);
    ~LinkedList(void);
    void insert(std::string id, int value); 
    void display(); 
    void traverse();
    void sort();
    //void find(std::string, char, int);
};

SymbolTable.h

#pragma once
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <cstdlib> 

class SymbolTable
{
public:
    int performOp(char, int);
    std::string toString(void);
    const std::string getID(void);
    void setValue(int);
    void setID(std::string);
    const int getValue();
    SymbolTable(std::string, int);
    SymbolTable();
    ~SymbolTable(void);
};

Answer 1

One problem is removing spaces in parse function

 // remove all spaces                                             
 for(int i=0; i<line.length(); i++)
     if(line[i] == ' ')
         line.erase(i,1);

When space is removed index should not be incremented, so that you can remove the possible next space.

But you main problem is having global variables that should have been members of SymbolTable:

string id;
int value;
int final;

Instead of this above you should have the following (in SymbolTable.h):

class SymbolTable
{
    string id;
    int value;
    int final;

    // declaration of all the member functions
    ...
};

Also, in this code you don’t need destructor (the one you call deConstructor). All members will be automatically deleted when the object is deleted itself. You only have to worry about deleting objects of which you only have a pointer.

Global variables that pertain to LinkedList should also be members of that class, instead of being global:

class LinkedList
{
    node *head;
    node *tail;
    node *curr;

    // declaration of all the member functions
    ...
};

And their initialization should be done in constructor:

LinkedList::LinkedList()
{
    head = NULL;               // empty linkedList
    tail = NULL;
    curr = NULL;
}