打印双向链表-无结果

Question

I am new to c++ and programming in general. I am trying to implement a doubly linked list. I think the list is created successfully, but I am having trouble printing the list out entirely. Can you please let me know what's wrong with my printListForward method below? My code is not complete yet. Would really appreciate any tips and suggestions as well.

#include "MagicSquare.hpp"
#include <iostream>

class MagicSquaresList{

private:

    struct MagicSquaresNode{

        int nodeIndex;
        MagicSquaresNode *pleft;
        MagicSquaresNode *pright;
        MagicSquaresNode *pup;
        MagicSquaresNode *pdown;
    };

    MagicSquaresNode *head;
    MagicSquaresNode *tail;

public:
    MagicSquaresList (){ 
        head = NULL;
        tail = NULL;
    }

    int getListLength(){
        int length = 1;
        MagicSquaresNode *temp = new MagicSquaresNode;
        temp = head;

        if(isEmpty()){
            return 0;
        }else{
            while(temp != tail){
                length++;
                temp = temp->pright;
            }
        }
        return length;
    }

    bool isEmpty(){
        return head == NULL;
    }

    void appendToEnd(int val){
        MagicSquaresNode *newNode = new MagicSquaresNode;
        newNode->nodeIndex = val;

        if(isEmpty()){
            tail = newNode;
        } else {
            tail->pright = newNode;
            newNode->pleft = tail;
        }

        tail = newNode;
    }

    void printListForward() {
        MagicSquaresNode *ptr = head;

        while(ptr != tail){
            std::cout << ptr->nodeIndex << " ";
            ptr = ptr->pright;
        }

        std::cout << std::endl;
    }

};


int main(){

    /*********** temporary *****************/
    int matrixSize, listSize;
    matrixSize = 3;
    listSize = matrixSize * matrixSize;
    /****************************************/

    MagicSquaresList list1;

    for (int i = 1; i <= listSize; i++){
        list1.appendToEnd(i);
    }

    list1.printListForward();
    std::cout << list1.getListLength() << std::endl;
    return 0;

}

Answer 1

You need to set the head.

  void appendToEnd(int val){
    MagicSquaresNode *newNode = new MagicSquaresNode;
    newNode->nodeIndex = val;

    if(isEmpty()){
        tail = newNode;
        head = newNode;
    } else {
        tail->pright = newNode;
        newNode->pleft = tail;
    }

    tail = newNode;
    }

Answer 2

Just a few comments. First you want to use proper indents. For beginner it is important to learn to write a simple Makefile. In your case, I wrote one for you.

Makefile:

  1 bin_PROGRAMS=doublelink
  2 GCCLIBDIR= /usr/local/lib64
  3 CXXFLAGS=-g -std=c++11
  4 CC=g++   
  5 LDFLAGS=-L$(GCCLIBDIR)
  6          
  7 all : $(bin_PROGRAMS)
  8 
  9 doublelink : doublelink.o
 10    $(CC) $(CXXFLAGS) -o $@ $^ $(LDFLAGS)

For your source code I did simple editing and named your file: doublelink.cpp:

//#include "MagicSquare.hpp"
#include <iostream>

using namespace std;

class MagicSquaresList{
   private:
      struct MagicSquaresNode {
         MagicSquaresNode(int ni) : nodeIndex(ni), pleft(0), pright(0), pup(0), pdown(0) { }
         int nodeIndex;
         MagicSquaresNode *pleft;
         MagicSquaresNode *pright;
         MagicSquaresNode *pup;
         MagicSquaresNode *pdown;
      };

      MagicSquaresNode *head;
      MagicSquaresNode *tail;

   public:
      MagicSquaresList () { 
         head = 0;
         tail = 0;
      }

      int getListLength(){
         MagicSquaresNode *temp = head;
         if (temp == 0) {
            return 0;
         }
         int length = 0;
         while (temp != 0) {
            ++length;
            temp = temp->pright;
         }
         return length;
      }

      bool isEmpty(){
         return head == 0;
      }

      void appendToEnd(int val){
         MagicSquaresNode *newNode = new MagicSquaresNode(val);
         if (tail == 0) {
            head = newNode;
         } 
         else {
            tail->pright = newNode;
            newNode->pleft = tail;
         }
         tail = newNode;
      }

      void printListForward() {
         MagicSquaresNode *ptr = head;
         while (ptr != 0) {
            //cout << ptr << endl;
            std::cout << ptr->nodeIndex << " ";
            ptr = ptr->pright;
         }
         std::cout << std::endl;
      }

};


int main(){
    /*********** temporary *****************/
    int matrixSize, listSize;
    matrixSize = 3;
    listSize = matrixSize * matrixSize;
    /****************************************/

    MagicSquaresList list1;

    for (int i = 1; i <= listSize; i++){
        list1.appendToEnd(i);
    }

    list1.printListForward();
    std::cout << list1.getListLength() << std::endl;
    return 0;
}

With both file in the directory, you type

make

a binary file doublelink will appear in your directory.
you run this program by typing its name:

$ doublelink 1 2 3 4 5 6 7 8 9 9

But with all these efforts. You should never need to implement double linked list. You should use the C++ standard library and customize the data type for your purpose. The std::list is implemented as double linked list. Please read the document at http://www.cplusplus.com/reference/list/list/ . You should create your structure of interest by

list<MagicSquare> myfancySquareList;

myfancySquareList.push_back(MagicSquare(somevalue));

Your double linked list is also missing the destructor, and you are having memory leak. There are many other missing things from your implementation which is usually covered by a text book of several hundred pages. Hope this get you started. When you have trouble, you can run your program in debug mode: gdb doublelink. You can step through it and figure out where is your problem. Your initial problem is a segmentation fault. Try to run your original program and see where it terminate.