如何在选择选项 5 后阻止程序自行终止

Question

我正在做一个在链表的特定位置插入节点的功能。

除了选项 5) 在特定位置插入和 9) 显示之外，一切都运行良好。

如果我选择 5)，程序会要求输入一个数字。提供数字后，它会自行终止。

同样，当我选择9)时，程序会显示节点中的数据然后终止，但我已经放入了一个

while

循环来停止。

我的主文件：

#include <iostream>

using namespace std;
int value, count, value2, count2;

struct node
{
    int data;
    node * next;
};

node *list = nullptr;
node * p;
node * q;
node * r;

void insertFront()
{
    cout << "ENTER A VALUE=";
    cin >> value;

    if (list == nullptr)
    {
        p = new node();
        p->data = value;
        p->next = nullptr;
        list = p;
    }
    else
    {
        p = new node();
        p->data = value;
        p->next = list;
        list = p;
    }
}

void display()
{
    p = list;
    while (p != nullptr)
    {
        cout << p->data << " ";
        p = p->next;
    }

    cout << p->data << endl;
}

void insertSpec()
{
    cout << "Enter the number after which you want to enter a node=";
    cin >> value;

    if (list == nullptr && value > 1)
    {
        cout << "THERE IS NO OTHER NODE SO YOU CANNOT PUT A NODE AT " << value;
    }

    if (list != nullptr && value > 0)
    {
        cout << "ENTER THE NUMBER YOU WANT TO ENTER=";
        cin >> value2;
        count = 1;
        count2 = 1;
        while (count != (value))
        {
            p = p->next;
            count++;
        }

        while (count2 != (value + 1))
        {
            q = q->next;
            count2++;
        }

        r = new node();
        r->data = value2;
        p->next = r;
        r->next = q;
    }
}

int main()
{
    int choice;
    cout << "1) Insert at front" << endl;
    cout << "5) Insert at specified place" << endl;
    cout << "9) Display" << endl << endl;
    while (choice != 99)
    {
        cout << "Your choice:";
        cin >> choice;

        switch (choice)
        {
        case 1:
            {
                insertFront();
                break;
            }
        case 5:
            {
                insertSpec();
                break;
            }
        case 9:
            {
                display();
                break;
            }
        case 99:
            {
                cout << "PROGRAM TERMINATED :)";
                break;
            }
        }
    }

    return 0;
}

Answer 1

在

insertSpec()

中，如果用户指定的节点号超过列表末尾，则第一个循环具有未定义的行为，第二个循环具有未定义的行为，因为

q

从未被分配指向有效的

 node

.

display()

在循环结束后尝试访问 p->data 时有

undefined behavior

因为

p

到那时是

nullptr

.

尝试更像这样的东西：

#include <iostream>
using namespace std;

struct node
{
    int data;
    node *next = nullptr;
};

node *list = nullptr;

void insertFront()
{
    int value;

    cout << "ENTER A VALUE=";
    cin >> value;

    list = new node{value, list};
}

void display()
{
    for(node *p = list; p != nullptr; p = p->next)
    {
        cout << p->data << " ";
    }
    cout << endl;
}

void insertSpec()
{
    int number, value;

    cout << "Enter the node number after which you want to enter a new node=";
    cin >> number;
    
    if (number < 1)
    {
        cout << "INVALID NODE NUMBER!" << endl;
        return;
    }

    node **p = &list;
    int n = number;

    while (*p != nullptr && n > 0)
    {
        p = &((*p)->next);
        --n;
    }

    if (n > 0)
    {
        cout << "YOU CANNOT PUT A NODE AFTER " << number << endl;
        return;
    }

    cout << "ENTER THE VALUE YOU WANT TO ADD=";
    cin >> value;

    *p = new node{value, *p};
}

int main()
{
    int choice;

    cout << "1) Insert at front" << endl;
    cout << "5) Insert at specified place" << endl;
    cout << "9) Display" << endl;
    cout << "99) Exit" << endl << endl;

    do
    {
        cout << "Your choice:";
        cin >> choice;

        switch (choice)
        {
        case 1:
            insertFront();
            break;

        case 5:
            insertSpec();
            break;

        case 9:
            display();
            break;
        }
    }
    while (choice != 99);

    cout << "PROGRAM TERMINATED :)";

    return 0;
}

在线演示