VS Code 2022乘法游戏代码求助：缺11的乘法题及负数异常

我在VS Code Community 2022中开发了一款简易乘法游戏，功能是输入指定乘法表数字（比如5），随机生成该数字1到12倍的乘法题。但运行时遇到两个问题：

• 从未出现含11的乘法题
• 会随机出现负数题目

原代码如下：

#include<iostream>
#include<cstdlib>
#include<ctime>
#include<random>
using namespace std;
int main() {

int a, ans, points = 0;
int x[12] = {1,2,3,4,5,6,7,8,9,10,11,12};

cout << "Welcome to my multiplication game";

cout << "\nEnter the multiplication table you want to play with\n";
cin >> a;

random_shuffle(&x[0], &x[11]);

for(int i=1;i<=12;i++)
{
    int multiplication = x[i] * a;
    cout << "What is: " << x[i] << " x " << a << "\n";
    cin >> ans;
    if (ans == a * x[i])
    {
        points++;
    }
}

if (points >= 10)
{
    cout << "You have " << points << " points, Well Done!";
}

else if (points >= 5 && points <= 9)
{
    cout << "You have " << points << " points, Good try!";
}

else
{
    cout << "You have " << points << " points, Better luck next time!";
}
}

问题原因分析 #

1. 未出现11的乘法题 #

• random_shuffle的第二个参数要求是待打乱序列末尾的下一个位置，原代码中&x[11]指向数组第12个元素（值为12）的前一个位置，导致最后一个元素12没被纳入打乱范围；同时数组索引10对应的11虽在打乱范围内，但循环从i=1开始，跳过了索引0的元素，且当i=12时访问的是数组越界位置，根本没遍历到所有12个元素。
• 数组x的有效索引是0到11，共12个元素，循环需从i=0开始，到i<12结束，才能遍历全部元素。

2. 出现随机负数题目 #

当循环执行到i=12时，x[12]属于数组越界访问，该位置的内存是未初始化的垃圾数据，可能是负数，和输入的a相乘后就会出现负数题目。

修复后的代码 #

#include<iostream>
#include<cstdlib>
#include<ctime>
#include<random>
using namespace std;
int main() {

int a, ans, points = 0;
int x[12] = {1,2,3,4,5,6,7,8,9,10,11,12};

cout << "Welcome to my multiplication game";

cout << "\nEnter the multiplication table you want to play with\n";
cin >> a;

// 设置随机数种子，保证每次运行打乱结果不同
srand(time(0));
// 修正random_shuffle参数，覆盖全部12个元素
random_shuffle(x, x + 12);

// 循环从i=0开始，遍历全部12个元素
for(int i=0;i<12;i++)
{
    int multiplication = x[i] * a;
    cout << "What is: " << x[i] << " x " << a << "\n";
    cin >> ans;
    if (ans == multiplication)
    {
        points++;
    }
}

if (points >= 10)
{
    cout << "You have " << points << " points, Well Done!";
}
else if (points >= 5 && points <= 9)
{
    cout << "You have " << points << " points, Good try!";
}
else
{
    cout << "You have " << points << " points, Better luck next time!";
}
return 0;
}

内容的提问来源于stack exchange，提问作者MSHypezzz