# 黑魔法
## 資料流
### Intro
```cpp=
#include <iostream>
```
回過頭來看看這行
這什麼東西？
$\text{iostream}$ 為 $\text{Input/Output Stream}$ 的縮寫，即是輸入/輸出流。
「流」是一連串從 $\text{I/O}$ 裝置讀寫的字元。($\text{From Wiki}$)
$\text{iostream}$ 分成 $\text{istream}$ 和 $\text{ostream}$
但今天重點不是這個
### stringstream
用處： 整行輸入時能拿來用
```cpp=
#include <sstream>
using namespace std;
int main()
{
string tmp;
stringstream ss;
getline(cin, tmp);
ss << tmp;
vector<int> v;
int a;
while (ss >> a)
{
v.push_back(a);
}
return 0;
}
```
也許有更精簡的寫法
### fstream
讀/寫檔
```cpp=
ifstream fin;
fin.open("input.txt");
vector<int> v(10);
for (int i = 0; i < 10; i++)
{
fin >> v[i];
}
ofstream fout("out.txt");
for (int i = 9; i >= 0; i--)
{
fout << v[i] << endl;
}
fin.close();
fout.close();
```
拿來生測資 非常好
## using
有三種用法
但這裡只介紹兩種
### using namespace
```cpp=
#include <bits/stdc++.h>
struct node
{
int value;
node *ln, *rn;
};
void swap(node &x)
{
using std::swap;
swap(x->ln, x->rn);
return;
}
int main()
{
using std::string;
string s;
std::cout << s << endl;
}
```
### as typedef
```cpp=
#define LL long long
typedef long long LL;
using LL = long long;
```
## max/min
```cpp=
cout << max({1, 2, 3, 4, 5}) << endl;
cout << min({1, 2, 3, 4, 5}) << endl;
```
## reference
就是別名
```cpp=
string s;
string &ref = s;
```
此時 $\text{ref = s}$
```cpp=+
s = "Hi";
cout << ref << endl;
ref += "Yes";
cout << s << endl;
cout << ref << endl;
```
```cpp=
#include <bits/stdc++.h>
using namespace std;
void Add_B(string &ref)
{
ref += "B";
return;
}
int main()
{
string s = "AAA";
Add_B(s);
cout << s << endl;
return 0;
}
```
## 在 if 裡面宣告變數
```cpp=
set<LL> s;
if (auto it = s.find(10); it != s.end())
{
cout << *it << endl;
}
```
## 關於字串...
懶得用跳脫字元？
```cpp=
string s = R"(\n"'""')";
string a = "\\n\"'\"\"'";
cout << s << endl;
cout << a << endl;
```
執行結果:
```
\n"'""'
\n"'""'
```
## decltype
```cpp=
int a = 10;
auto b = 10; // b is an interger
decltype(b) c; // c is an interger
priority_queue<int> pq;
decltype(pq) pq2;
```
釋放空間
```cpp=
#include <bits/stdc++.h>
using namespace std;
vector<int> v;
void reset()
{
decltype(v)().swap(v);
}
int main()
{
return 0;
}
```
## 傳入多個參數
```cpp=
template <typename T>
void ls(T &v)
{
for (auto i : v)
cout << i << " ";
cout << endl;
}
template <typename T, typename... Args>
void ls(T &v, const Args &...args)
{
for (auto i : v)
cout << i << " ";
cout << endl;
ls(args...);
}
int main()
{
vector<int> a, b;
set<int> s1, s2;
ls(a, b, s1, s2);
return 0;
}
```
## 隨機數
```cpp=
mt19937 rng(time(NULL));
for (int i = 0; i < 10; i++)
{
cout << rng() << endl;
}
```
[**梅森旋轉算法**](https://zh.wikipedia.org/zh-tw/%E6%A2%85%E6%A3%AE%E6%97%8B%E8%BD%AC%E7%AE%97%E6%B3%95)
## 三元運算
```cpp=
bool f = false;
int a;
cin >> a;
if (a & 1)
{
f = true;
}
cout << (f == true ? "odd" : "even") << endl;
```
```cpp=
x = (y < 0) ? 1 : 2; // if y < 0, x = 1; else x = 2
(c < 0 ? a : b) = 1; // if c < 0, a = 1; else b = 1
```
## 這標題該怎麼下
```cpp=
cout << "ABCD"[2] << endl;
```
```cpp=
int n;
vector<int> v(n, 10);
for (int i = 0; i < n; i++)
{
cout << v[i] << " \n"[i == n - 1];
}
```
## switch
[**A. Division?**](https://codeforces.com/problemset/problem/1669/A)
```cpp=
void sol()
{
LL n;
cin >> n;
cout << "Division ";
switch (n)
{
case -5000 ... 1399:
cout << 4 << endl;
break;
case 1400 ... 1599:
cout << 3 << endl;
break;
case 1600 ... 1899:
cout << 2 << endl;
break;
default:
cout << 1 << endl;
}
}
```
## define for debug
```cpp=
#include <bits/stdc++.h>
using namespace std;
#define ch cout << "Hi" << endl
int main()
{
ch;
return 0;
}
```
```cpp=
#include <bits/stdc++.h>
using namespace std;
#define ch cout << "Hi at line" << __LINE__ << endl
int main()
{
ch;
ch;
ch;
return 0;
}
```
```cpp=
#include <bits/stdc++.h>
using namespace std;
#define ch(x) cout << #x << ": " << x << endl
int main()
{
int a = 10;
ch(a);
return 0;
}
```
[**宏定义的黑魔法 - 宏菜鸟起飞手册**](https://onevcat.com/2014/01/black-magic-in-macro/)
## 蝌蚪(?)
```cpp=
int x = 5;
cout << ~-x << endl;
cout << -~x << endl;
cout << x << endl;
```
## 沒了
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