C++ 入门教程（三）：流程控制

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

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#include <iostream>
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int main() {
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    char op;
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    double a, b;
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    std::cout << "输入: 数 运算符 数（如 3 + 4）: ";
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    std::cin >> a >> op >> b;
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    switch (op) {
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        case '+':
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            std::cout << a + b << "\n";
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            break;                    // ⚠ 不写 break 会"贯穿"到下一分支！
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        case '-':
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            std::cout << a - b << "\n";
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            break;
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        case '*':
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            std::cout << a * b << "\n";
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            break;
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        case '/':
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            if (b != 0) std::cout << a / b << "\n";
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            else        std::cout << "除数不能为0\n";
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            break;
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        default:                      // 兜底分支
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            std::cout << "不支持的运算符\n";
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    }
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    return 0;
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}

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// 打印 1~10 的平方
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for (int i = 1; i <= 10; ++i) {
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    std::cout << i << "²= " << i * i << "\n";
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}
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// 死循环的两种写法
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for (;;) { /* ... */ }
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while (true) { /* ... */ }
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// for 的三个部分都可空：分号不能省
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int i = 0;
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for (; i < 5;) {
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    std::cout << i << " ";
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    ++i;
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}

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#include <iostream>
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#include <vector>
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#include <string>
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int main() {
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    int arr[]{10, 20, 30, 40};
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    for (int x : arr)                    // 遍历数组
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        std::cout << x << " ";           // 10 20 30 40
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    std::cout << "\n";
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    std::vector<std::string> langs{"C++", "Python", "Java"};
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    for (const auto& lang : langs)       // 只读遍历用 const auto&（免拷贝）
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        std::cout << lang << " ";
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    std::cout << "\n";
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    std::string s = "Cyber";
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    for (char c : s)                     // 遍历字符串
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        std::cout << c << "-";
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    return 0;
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}

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#include <iostream>
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#include <cmath>
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bool is_prime(int n) {                 // bool 函数，第五篇详讲
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    if (n < 2) return false;
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    for (int i = 2; i <= std::sqrt(n); ++i) {
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        if (n % i == 0) return false;
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    }
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    return true;
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}
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int main() {
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    int n;
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    std::cin >> n;
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    std::cout << n << (is_prime(n) ? " 是" : " 不是") << "质数\n";
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}

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#include <iostream>
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#include <cstdlib>      // rand/srand
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#include <ctime>
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int main() {
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    std::srand(std::time(nullptr));        // 播种子
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    int answer = std::rand() % 100 + 1;    // 1~100
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    int guess, tries = 0;
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    do {
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        std::cout << "猜(1-100): ";
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        if (!(std::cin >> guess)) {        // 输入校验（上篇）
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            std::cin.clear();
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            std::cin.ignore(10000, '\n');
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            std::cout << "请输入数字！\n";
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            continue;
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        }
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        ++tries;
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        if (guess < answer)      std::cout << "小了\n";
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        else if (guess > answer) std::cout << "大了\n";
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        else                     std::cout << "对了！用了 " << tries << " 次\n";
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    } while (guess != answer);
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    return 0;
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}