# c++入门

- `endl` 用于向流的末尾部位加入换行符
- `cout` 代表显示器
- `cout << x` 操作就相当于把x的值输出到显示器


## 输入 Hello World

```cpp
# include<iostream>
using  namespace std;
int main() {
    cout <<"Hello World!\n";
}
```

## 输入输出

```cpp
#include <iostream>

using namespace std;

int main() {
    int a, b, result;
    cout << "please input two number:\n";
    cin >> a >> b;
    result = 3 * a - 2 * b + 1;
    cout << "result is " << result << endl;
}
```
![](/cpp/c++%E5%85%A5%E9%97%A8/a/image-1731850280498.jpg)
## 使用函数

```cpp
#include<iostream>
#include<cmath>
using namespace std;

double max(double x, double y); 

int main() {
    double a, b, c;
    cout << "input two number:\n";
    cin >> a >> b;
    c = max(a, b);
    cout << "maximum=" << c << "\n";
    cout << "the squart of maximum=" << sqrt(c);
}

double max(double x, double y) {
    if (x > y)
        return x;
    else
        return y;
}

```
![](/cpp/c++%E5%85%A5%E9%97%A8/a/image-1731850497554.jpg)

## 函数定义包含函数声明

```cpp
#include<iostream>
#include<cmath>
using namespace std;

double max(double x, double y) { // 既是函数定义又是函数声明
    if (x > y)
        return x;
    else
        return y;
}

int main() {
    double a, b, c;
    cout << "input two numbers:\n";
    cin >> a >> b;
    c = max(a, b);
    cout << "the squart of maximum=" << sqrt(c);
}
```