控制结构

条件语句

条件语句允许程序根据不同条件做出决策。

if 语句

基本 if 语句

#include <iostream>
using namespace std;

int main() {
    int age = 18;
    
    if (age >= 18) {
        cout << "You are an adult." << endl;
    }
    
    if (age < 18) {
        cout << "You are a minor." << endl;
    }
    
    return 0;
}

if-else 语句

if-else 示例

#include <iostream>
using namespace std;

int main() {
    int score = 85;
    
    if (score >= 90) {
        cout << "Grade: A" << endl;
    } else if (score >= 80) {
        cout << "Grade: B" << endl;
    } else if (score >= 70) {
        cout << "Grade: C" << endl;
    } else if (score >= 60) {
        cout << "Grade: D" << endl;
    } else {
        cout << "Grade: F" << endl;
    }
    
    return 0;
}

switch 语句

switch 语句在需要将变量与多个值进行比较时很有用。

switch 语句示例

#include <iostream>
using namespace std;

int main() {
    char grade = 'B';
    
    switch (grade) {
        case 'A':
            cout << "Excellent!" << endl;
            break;
        case 'B':
            cout << "Good job!" << endl;
            break;
        case 'C':
            cout << "Well done!" << endl;
            break;
        case 'D':
            cout << "You passed!" << endl;
            break;
        case 'F':
            cout << "Better try again!" << endl;
            break;
        default:
            cout << "Invalid grade!" << endl;
            break;
    }
    
    return 0;
}

重要提示：

始终使用 break; 防止穿透行为
default 情况是可选的，但建议使用
switch 适用于整数、字符和枚举

循环

循环允许你高效地重复执行代码。

for 循环

for 循环示例

#include <iostream>
using namespace std;

int main() {
    // Basic for loop
    cout << "Counting from 1 to 5:" << endl;
    for (int i = 1; i <= 5; i++) {
        cout << i << " ";
    }
    cout << endl;
    
    // Loop with step of 2
    cout << "Even numbers from 2 to 10:" << endl;
    for (int i = 2; i <= 10; i += 2) {
        cout << i << " ";
    }
    cout << endl;
    
    // Reverse loop
    cout << "Countdown from 5 to 1:" << endl;
    for (int i = 5; i >= 1; i--) {
        cout << i << " ";
    }
    cout << endl;
    
    return 0;
}

while 循环

while 循环示例

#include <iostream>
using namespace std;

int main() {
    int number = 1;
    
    cout << "Numbers from 1 to 5 using while:" << endl;
    while (number <= 5) {
        cout << number << " ";
        number++;  // Important: increment to avoid infinite loop
    }
    cout << endl;
    
    // User input validation
    int userInput;
    cout << "Enter a number between 1 and 10: ";
    cin >> userInput;
    
    while (userInput < 1 || userInput > 10) {
        cout << "Invalid input! Please enter a number between 1 and 10: ";
        cin >> userInput;
    }
    
    cout << "Thank you! You entered: " << userInput << endl;
    
    return 0;
}

do-while 循环

do-while 循环示例

#include <iostream>
using namespace std;

int main() {
    int choice;
    
    do {
        cout << "\n=== Menu ===" << endl;
        cout << "1. Say Hello" << endl;
        cout << "2. Say Goodbye" << endl;
        cout << "3. Exit" << endl;
        cout << "Enter your choice: ";
        cin >> choice;
        
        switch (choice) {
            case 1:
                cout << "Hello there!" << endl;
                break;
            case 2:
                cout << "Goodbye!" << endl;
                break;
            case 3:
                cout << "Exiting..." << endl;
                break;
            default:
                cout << "Invalid choice!" << endl;
                break;
        }
    } while (choice != 3);
    
    return 0;
}

循环控制语句

这些语句允许你控制循环的流程。

break 和 continue

#include <iostream>
using namespace std;

int main() {
    cout << "Using break - stop at 7:" << endl;
    for (int i = 1; i <= 10; i++) {
        if (i == 7) {
            break;  // Exit the loop when i equals 7
        }
        cout << i << " ";
    }
    cout << endl;
    
    cout << "Using continue - skip even numbers:" << endl;
    for (int i = 1; i <= 10; i++) {
        if (i % 2 == 0) {
            continue;  // Skip the rest of this iteration
        }
        cout << i << " ";
    }
    cout << endl;
    
    // Nested loops with labeled break (using goto - not recommended)
    cout << "Finding first pair that sums to 10:" << endl;
    bool found = false;
    for (int i = 1; i <= 5 && !found; i++) {
        for (int j = 1; j <= 5; j++) {
            if (i + j == 10) {
                cout << "Found: " << i << " + " << j << " = 10" << endl;
                found = true;
                break;  // Break inner loop
            }
        }
    }
    
    return 0;
}

嵌套控制结构

你可以将循环和条件语句相互嵌套。

乘法表

#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    cout << "Multiplication Table (1-5):" << endl;
    cout << "   ";
    
    // Print header
    for (int i = 1; i <= 5; i++) {
        cout << setw(4) << i;
    }
    cout << endl;
    
    // Print table
    for (int i = 1; i <= 5; i++) {
        cout << setw(2) << i << " ";
        for (int j = 1; j <= 5; j++) {
            cout << setw(4) << (i * j);
        }
        cout << endl;
    }
    
    return 0;
}

数字图案

#include <iostream>
using namespace std;

int main() {
    int rows = 5;
    
    cout << "Number Triangle:" << endl;
    for (int i = 1; i <= rows; i++) {
        // Print spaces for alignment
        for (int space = 1; space <= rows - i; space++) {
            cout << " ";
        }
        
        // Print numbers
        for (int j = 1; j <= i; j++) {
            cout << j << " ";
        }
        cout << endl;
    }
    
    return 0;
}

最佳实践

使用有意义的条件

编写清晰、可读的条件来表达意图

避免深度嵌套

使用早期返回或 break 语句来减少嵌套层级

初始化循环变量

始终初始化循环计数器以防止未定义行为

尽可能使用 const

如果循环变量不应改变，将其设为 const