if/else 条件分支

代码示例

1if (condition) {
2    // 条件为真时执行的代码
3}

1#include <iostream>
2
3int main() {
4    int score = 85;
5
6    std::cout << "Checking exam results...\n";
7
8    // 简单的 if 语句
9    if (score >= 50) {
10        std::cout << "You passed the exam!\n";
11    }
12
13    std::cout << "Grade check complete.\n";
14    return 0;
15}

代码示例

1if (condition) {
2    // 真分支
3} 
4
5else {
6    // 假分支
7}

1#include <iostream>
2
3int main() {
4    int score = 49;
5
6    std::cout << "Checking exam results...\n";
7
8    // 简单的 if 语句
9    if (score >= 50) {
10        std::cout << "You passed the exam!\n";
11    }
12    else
13    {
14       std::cout << "You failed the exam!\n";
15    }
16
17    std::cout << "Grade check complete.\n";
18    return 0;
19}

1#include <iostream>
2
3int main() {
4    int temperature = 0;
5
6    std::cout << "Analyzing weather...\n";
7
8    if (temperature > 30) {
9        std::cout << "It's a hot day.\n";
10    } 
11    
12    else if (temperature > 20) {
13        std::cout << "It's a nice day.\n";
14    } 
15    
16    else if (temperature > 10) {
17        std::cout << "It's a bit chilly.\n";
18    } 
19    
20    else {
21        std::cout << "It's cold!\n";
22    }
23}

1#include <iostream>
2
3int main() {
4    bool bonusEnabled = true;
5
6    if (bonusEnabled) {
7        int bonusPoints = 50; 
8        std::cout << "Bonus applied: " << bonusPoints << "\n";
9    }
10    
11    std::cout << bonusPoints; // 错误！ 
12}

1#include <iostream>
2
3int main() {
4    bool hasAccount = true;
5    bool knowsPassword = false;
6
7    if (hasAccount) {
8        std::cout << "Account found.\n";
9
10        // 此检查仅在 hasAccount 为真时执行
11        if (knowsPassword) {
12            std::cout << "Login successful!\n";
13        } else {
14            std::cout << "Incorrect password.\n";
15        }
16    } else {
17        std::cout << "Please sign up first.\n";
18    }
19
20    return 0;
21}

1if (initialization; condition) 
2{  
3    ...
4}

1#include <iostream>
2
3int main() {
4    int balance = 500;
5    int cost = 600;
6
7    // 我们计算 'diff' 并立即检查它
8    // 'diff' 仅在 if/else 块内存在
9    if (int diff = balance - cost; diff >= 0) {
10        std::cout << "Purchase successful. Remaining: " << diff << "\n";
11    } else {
12        std::cout << "Insufficient funds. Missing: " << -diff << "\n";
13    }
14    
15    // 'diff' 在此处不可访问，保持我们的工作区整洁。
16    return 0;
17}

1#include <iostream>
2
3int main() {
4    int x = 0;
5
6    // BUG: 赋值而非比较
7    if (x = 5) {
8        std::cout << "Condition is true\n";
9    }
10
11    std::cout << "Value of x after if: " << x << "\n";
12
13    return 0;
14}

1#include <iostream>
2
3int main() {
4    int health = 0;
5
6    // BUG: 分号使 if 语句立即结束
7    if (health > 0); 
8    {
9        std::cout << "You are alive!\n";
10    }
11
12    return 0;
13}