C++入门教程：运算符、表达式与控制流程

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#include <iostream>
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using namespace std;
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int main() {
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    int a = 17, b = 5;
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    cout << "17 / 5 = " << a / b << endl;      // 3，整数除法截断
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    cout << "17 % 5 = " << a % b << endl;      // 2，取余
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    cout << "-17 % 5 = " << (-a) % b << endl;  // -2，符号跟随被除数
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    // 自增自减
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    int x = 10;
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    cout << "x++ 先取值后自增: " << x++ << endl;   // 10
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    cout << "此时 x = " << x << endl;               // 11
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    cout << "++x 先自增后取值: " << ++x << endl;   // 12
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    int age = 25;
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    bool hasTicket = true;
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    // 关系运算
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    cout << "age > 18: " << (age > 18) << endl;       // 1
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    cout << "age == 25: " << (age == 25) << endl;     // 1
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    // 逻辑运算：&&（与）、||（或）、!（非）
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    bool canEnter = (age >= 18) && hasTicket;
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    cout << "可以入场: " << canEnter << endl;          // 1
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    bool student = false;
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    cout << "是学生或已成年: " << (student || age >= 18) << endl;  // 1
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    cout << "不是学生: " << !student << endl;           // 1
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    int divisor = 0;
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    // 利用短路防止除零：divisor != 0 为假时，右侧 10/divisor 不会执行
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    if (divisor != 0 && 10 / divisor > 2) {
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        cout << "条件成立" << endl;
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    } else {
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        cout << "短路保护生效，未发生除零" << endl;
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    }
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    int a = 5;   // 0101
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    int b = 3;   // 0011
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    cout << "a & b  = " << (a & b) << endl;   // 1（0001）
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    cout << "a | b  = " << (a | b) << endl;   // 7（0111）
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    cout << "a ^ b  = " << (a ^ b) << endl;   // 6（0110）
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    cout << "~a     = " << (~a) << endl;      // -6
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    cout << "a << 1 = " << (a << 1) << endl;   // 10（左秽一位 = 乘2）
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    cout << "a >> 1 = " << (a >> 1) << endl;  // 2（��终位 = 隌）
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    int a = 2 + 3 * 4;      // 14（* 优先于 +）
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    int b = (2 + 3) * 4;    // 20（括号改变优先级）
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    bool c = 3 > 2 && 4 < 5; // true（关系运算优先于逻辑）
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    cout << a << endl;
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    cout << b << endl;
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    cout << c << endl;
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    // 建议：复杂表达式加括号，提高可读性
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    int score = 85;
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    char grade = score >= 90 ? 'A' : (score >= 80 ? 'B' : 'C');
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    cout << "等级: " << grade << endl;  // B
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    int score;
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    cout << "请输入成绩: ";
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    cin >> score;
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    if (score >= 90) {
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        cout << "优秀" << endl;
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    } else if (score >= 80) {
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        cout << "良好" << endl;
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    } else if (score >= 60) {
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        cout << "及格" << endl;
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    } else {
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        cout << "不及格" << endl;
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    }
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    int day;
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    cout << "输入星期几（1-7）: ";
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    cin >> day;
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    switch (day) {
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        case 1: cout << "周一" << endl; break;
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        case 2: cout << "周二" << endl; break;
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        case 3: cout << "周三" << endl; break;
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        case 4: cout << "周四" << endl; break;
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        case 5: cout << "周五" << endl; break;
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        case 6:
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        case 7: cout << "周末" << endl; break;   // 多个 case 共享
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        default: cout << "无效输入" << endl;     // 缺省分支
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    }
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    // 计算 1 到 100 的和
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    int sum = 0, i = 1;
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    while (i <= 100) {
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        sum += i;
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        i++;
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    }
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    cout << "1+2+...+100 = " << sum << endl;  // 5050
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    int n;
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    // do-while 至少执行一次，适合「先输入后验证」
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    do {
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        cout << "请输入一个正数: ";
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        cin >> n;
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    } while (n <= 0);
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    cout << "你输入了: " << n << endl;
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    // 打印 9x9 乘法表
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    for (int i = 1; i <= 9; i++) {
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        for (int j = 1; j <= i; j++) {
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            cout << j << "x" << i << "=" << (i * j) << "\t";
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        }
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        cout << endl;
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    }
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    // break: 立即终止整个循环
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    for (int i = 1; i <= 10; i++) {
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        if (i == 5) break;
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        cout << i << " ";
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    }
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    cout << endl;  // 输出: 1 2 3 4
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    // continue: 跳过本次循环剩余语句，进入下一次
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    for (int i = 1; i <= 10; i++) {
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        if (i % 2 == 0) continue;   // 跳过偶数
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        cout << i << " ";
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    }
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    cout << endl;  // 输出: 1 3 5 7 9
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    return 0;
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}

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#include <iostream>
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using namespace std;
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int main() {
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    // 找出第一个满足 i*j == 24 的组合
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    for (int i = 1; i <= 10; i++) {
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        for (int j = 1; j <= 10; j++) {
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            if (i * j == 24) {
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                cout << "找到: " << i << " x " << j << " = 24" << endl;
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                goto found;  // 跳出多重循环
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            }
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        }
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    }
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    cout << "未找到" << endl;
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found:
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    cout << "程序结束" << endl;
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    return 0;
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}

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#include <iostream>
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#include <cstdlib>
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#include <ctime>
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using namespace std;
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int main() {
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    // 用当前时间做随机种子
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    srand(time(nullptr));
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    int secret = rand() % 100 + 1;  // 1~100 随机数
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    int guess, attempts = 0;
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    cout << "===== 猜数字游戏 =====" << endl;
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    cout << "我已想好一个 1~100 之间的数字，你来猜！" << endl;
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    while (true) {
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        cout << "请输入你的猜测: ";
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        cin >> guess;
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        attempts++;
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        if (guess > secret) {
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            cout << "太大了！" << endl;
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        } else if (guess < secret) {
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            cout << "太小了！" << endl;
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        } else {
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            cout << "恭喜！你猜对了！" << endl;
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            cout << "数字是 " << secret << "，你共猜了 " << attempts << " 次。" << endl;
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            break;
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        }
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        if (attempts >= 10) {
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            cout << "次数用尽，答案是 " << secret << "，下次加油！" << endl;
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            break;
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        }
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    }
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    return 0;
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}

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===== 猜数字游戏 =====
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我已想好一个 1~100 之间的数字，你来猜！
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请输入你的猜测: 50
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太大了！
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请输入你的猜测: 25
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太小了！
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请输入你的猜测: 37
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恭喜！你猜对了！
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数字是 37，你共猜了 3 次。