c++ - 在 C++ 中显示列表节点期间的额外输出

Question

出于某种奇怪的原因，当我尝试显示我的列表范围时，它给出了额外的输出。该程序要求用户输入要显示的开始和结束节点整数，然后由于某种原因它将显示一个空白节点，然后显示第一个节点。大约 10 秒后，它将显示正确的节点范围。

例如，我会输入

ball,   4,   9.99
doll, 2, 10.00
lava lamp, 5, 24.99

但是当输入 2 到 3 的范围时它会输出

2. (blank), (blank), (blank)
3. ball, 4, 9.99
(pause for 10 seconds that is not called for)
2. doll, 2, 10.00
3. lava lamp, 5, 24.99

谁知道这是为什么？ (有问题的函数是void displayRange)

#include <iostream>
#define nullptr 0
#include <cstdlib>
#include <algorithm>
#include <string>
#include <conio.h>

using namespace std;
int menu();

class ItemList {
    private:
        struct ListNode{
            string IName;
            string QQuantity;
            string PPrice;
            double value;
            struct ListNode * next;
                };
ListNode *head;
    public:
        ItemList()
            {
                head = new ListNode;
                head->next=nullptr;
            }
            ~ItemList();

            void insertNode(string Item, string Quantity, string Price)
            {
                ListNode *newNode;
                ListNode *nodePtr;
                ListNode *previousNode=nullptr;

                newNode=new ListNode;
                newNode->IName=Item;
                newNode->QQuantity=Quantity;
                newNode->PPrice=Price;

                if(!head)
                {
                    head=newNode;
                    newNode->next=nullptr;
                }
                else
                {
                    nodePtr=head;
                    previousNode=nullptr;

                    while(nodePtr != nullptr && nodePtr->IName < Item)
                    {
                        previousNode=nodePtr;
                        nodePtr=nodePtr->next;
                    }
                    if(previousNode==nullptr)
                    {
                        head=newNode;
                        newNode->next=nodePtr;
                    }
                    else
                    {
                        previousNode->next=newNode;
                        newNode->next=nodePtr;
                    }
                }
            }
            void displayNode()
            {
                ListNode *nodePtr;
                nodePtr=head->next;
                int i=0;
                while(nodePtr)
                {
                    i++;

                    cout << i << ". " << nodePtr->IName << ",  ";
                    cout << nodePtr->QQuantity << "  ";
                    cout << "$" << nodePtr->PPrice << "\n" << endl;
                    nodePtr=nodePtr->next;
                }
                if(!head)
                {
                    cout << "The store is empty." << endl;
                }
            }

            void modifyNode(string Item)
            {
            ListNode *nodePtr;
            ListNode *nodePrev;
            string newName, newQuantity, newPrice;
            int modify;
            if (!head)
            {
                return;
                cout << "Store is empty." << endl;
            }
            else
            {
                nodePtr = head;
                if (head->IName==Item)
                    nodePtr = head->next;
                else
                {
                    while (nodePtr != nullptr && nodePtr->IName != Item)
                    {
                        nodePrev = nodePtr;
                        nodePtr = nodePtr->next;
                    }
                }
                if (nodePtr)
                {
                    cout << nodePtr->IName << "\t" << nodePtr->QQuantity << "\t" << nodePtr->PPrice << endl;
                    cout << "What would you like to change?\n";
                    cout << "1. Item" << endl;
                    cout << "2. Quantity" << endl;
                    cout << "3. Price" << endl;
                    cout << "4. Whole Entry" << endl;
                    cin >> modify;
                    switch (modify)
                    {
                        case 1:
                            cout << "Change to what?\n";
                            cin.sync();
                            getline(cin,newName);
                            transform(newName.begin(), newName.end(), newName.begin(), ::toupper);
                            nodePtr->IName = newName;
                            break;
                        case 2:
                            cout << "Change to what?\n";
                            cin >> newQuantity;
                            transform(newQuantity.begin(), newQuantity.end(), newQuantity.begin(), ::toupper);
                            nodePtr->QQuantity = newQuantity;
                            break;
                        case 3:
                            cout << "Change to what?\n";
                            cin >> newPrice;
                            transform(newPrice.begin(), newPrice.end(), newPrice.begin(), ::toupper);
                            nodePtr->PPrice = newPrice;
                            break;
                        case 4:
                            cout << "What is the product called?\n";
                            cin.sync();
                            getline(cin,newName);
                            transform(newName.begin(), newName.end(), newName.begin(), ::toupper);
                            nodePtr->IName = newName;
                            cout << "How many are there really?\n";
                            cin >> newQuantity;
                            transform(newQuantity.begin(), newQuantity.end(), newQuantity.begin(), ::toupper);
                            nodePtr->QQuantity = newQuantity;
                            cout << "What is the actual price?\n";
                            cin >> newPrice;
                            transform(newPrice.begin(), newPrice.end(), newPrice.begin(), ::toupper);
                            nodePtr->PPrice = newPrice;
                    }
                }
                else
                    cout << "Product not found\n";
            }
        }

            void deleteNode(string Item)
            {
                ListNode *nodePtr;
                ListNode *previousNode;

                if(!head)
                    return;
                if(head->IName==Item)
                {
                    nodePtr=head->next;
                    delete head;
                    head=nodePtr;
                }
                else
                {
                    nodePtr=head;
                    while(nodePtr!=nullptr && nodePtr->IName!=Item)
                    {
                        previousNode=nodePtr;
                        nodePtr=nodePtr->next;
                    }
                    if(nodePtr)
                    {
                        previousNode->next=nodePtr->next;
                        delete nodePtr;
                    }
                    else
                    {
                        cout << "Nothing to delete." << endl;
                    }
                }
            }

            void deleteRangeNode(int start, int stop)
            {
                ListNode *nodePtr;
                ListNode *newNode;
                nodePtr = head;
                int i=-1;
                cin.sync();
                while(nodePtr!=nullptr)
                {
                    i++;

                    if((i>=start)&&(i<=stop))
                    {
                        newNode->next = nodePtr -> next;
                        cout << "Deleted Product: " << nodePtr->IName << endl;
                        delete nodePtr;
                        nodePtr=newNode;
                    }
                    newNode=nodePtr;
                    nodePtr=nodePtr->next;


                }

            }

            void displayRange(int start, int stop)
            {
                ListNode * nodePtr;
                nodePtr=head;
                int i=-1;
                bool found=false;
                cin.sync();



                while(nodePtr!=nullptr)
                {
                    i++;
                    if((i>=start && i<=stop))
                    {
                        cout << i << ". " << nodePtr->IName << ",  ";
                        cout << nodePtr->QQuantity << "  ";
                        cout << "$" << nodePtr->PPrice << "\n" << endl;
                        nodePtr=nodePtr->next;
                    }
                }
            }
        };


ItemList::~ItemList()
{
    ListNode *nodePtr;
    ListNode *nextNode;

    nodePtr=head;
    while(nodePtr!=nullptr)
    {
        nextNode=nodePtr->next;
        delete nodePtr;
        nodePtr=nextNode;
    }
}

int main()
{
    ItemList pro;
    int method;
    while(method!=0)
    {
    int method=menu();
    system("cls");
    string It, Q, P;
    int begin, end;
    switch(method)
    {
    case 1:
        int count;
        cout << "How many products would you like to put in?" << endl;
        cin >> count;
        system("cls");
        for(int i=0; i<count; i++)
        {
            cout << "Product #" << i + 1 << endl;
            cout << "Enter the item name: ";
            cin.sync();
            getline(cin,It);
            transform(It.begin(), It.end(), It.begin(), ::toupper);
            cout << "Enter the Quantity: ";
            cin >> Q;
            transform(Q.begin(), Q.end(), Q.begin(), ::toupper);
            cout << "Enter the Price: ";
            cin >> P;
            pro.insertNode(It, Q, P);
            cout << "\n";
        }
        break;

    case 2:
        int dis;
        cout << "How many products would you like to display?" << endl;
        cout << "1. Entire Store" << endl;
        cout << "2. Range of Products" << endl;
        cin >> dis;
        system("cls");
        switch(dis)
        {
            case 1:
                pro.displayNode();
                break;
            case 2:
                cout << "The display should START at: ";
                cin >> begin;
                cout << "The display should END at: ";
                cin >> end;
                pro.displayRange(begin,end);
                system("pause");
                break;
        }

        break;

    case 3:
        pro.displayNode();
        cout << "What product do you wish to modify? (by item name)" << endl;
        cin.sync();
        getline(cin, It);
        transform(It.begin(), It.end(), It.begin(), ::toupper);
        system("cls");
        pro.modifyNode(It);
        break;

    case 4:
        int del;
        cout << "Do you wish to delete one product or more?" << endl;
        cout << "1. One" << endl;
        cout << "2. Range of Products" << endl;
        cout << "3. Entire Store" << endl;
            cin >> del;
            system("cls");
            switch(del)
            {
                case 1:
                    cout << "What product do you wish to delete? (by item name)" << endl;
                    pro.displayNode();
                    cout << "\n";
                    cin.sync();
                    getline(cin,It);
                    transform(It.begin(), It.end(), It.begin(), ::toupper);
                    pro.deleteNode(It);
                    cout << "\n";
                    break;
                case 2:
                    pro.displayNode();
                    cout << "What range of items do you wish to delete?" << endl;
                    cout << "START: ";
                    cin >> begin;
                    cout << "STOP: ";
                    cin >> end;
                    pro.deleteRangeNode(begin, end);
                    break;
                case 3:
                    pro.~ItemList();
                    cout << "All items deleted." << endl;
                    break;
            }
        break;


    case 0:
        cout << "Exiting the program." << endl;
        return 0;
    }
    system("pause");
    system("cls");
    }
    return 0;
}

int menu()
{
    string space1= "                       ";
    string space2= "                                  ";
    int method;
    cout << space1 << "What would you like to do to the store?" << endl;
    cout << space2 << "1. Add Product" << endl;
    cout << space2 << "2. Display" << endl;
    cout << space2 << "3. Modify Product" << endl;
    cout << space2 << "4. Delete Product" << endl;
    cout << space2 << "0. Exit\n" << endl;
    cout << space2;
    cin >> method;
    return(method);
}

Answer 1

问题的根源在于您在列表中创建了一个哨兵节点，但随后没有在 displayRange 中忽略它。当您调用 displayNode 时，您将忽略它。您看到延迟的原因是 displayNode 中的循环依赖于有符号整数溢出(这是未定义的行为)来终止。将 nodePtr 的增量移到范围检查之外将解决此问题。

这段代码有几个问题。不实现自己的列表容器的原因有很多，但最重要的一个是因为除非您对这种语言有经验，否则很难在第一次就完全正确。我强烈建议您查看 std::vector .这是我找到的项目列表。

1. #define nullptr 0 不要这样做。该标准不保证这是定义明确的行为，并且两者不具有相同的类型。
2. 调用 cin.sync() 不能保证做任何事情(它是实现定义的)。
3. 在尝试对其调用 std::getline 之前，您需要清除输入流中的空白。这包含在 reference page 中。在“备注”下。
4. 插入新节点时，需要将newNode->next设置为null。
5. 打开编译器的警告。您有多种未初始化变量的用法，这可能会导致未定义的行为。