Map
========
Introduce
--------
欲使用map需先於程式開頭加上

```c
#include <map>
```

Map通常用於將資料整合成一對一關係

其對關係為map<key, value>

其中一個value可以對應多個key

但是一個key只能對應一個value

例如常用的是用 int 將 string 做編號 ：

```c
map<int, string> m;
int n;
string s;
cin >> n >> s;
m[n] = s;
//Beware of the input order must suit the format of map
```

而欲讀取資料的話需給予他iterator

注意：如果使用iterator時要符合格式.如果有格式不同的map則需另建一個新的iterator不可共用

```c
map <int, string>::iterator it;
map <int, char>::iterator it2;
```

map在資料存取中.key的為.first, value的為.second

例如要讀取所有的map的資料

(.begin

```c
cout << "The datas stored in the map are: ";
for(it = m.begin(); it!= m.end(); it++)
{
    cout << (*it).first << " " << (*it).second << endl;
}
//.begin() stands for the first data stored in the map
```

或是如果只有要讀取"被你編號的某個" string

```c
    int search;
    cout << "Input the number ID of the string you are looking for : ";
    cin >> search;
    cout << m[search] << endl;
```

An fully executable program example :

```c
#include <iostream>
#include <map>
#include <string>

using namespace std;

int main()
{
    map<int, string> m;
    map<int, string>::iterator it;
    int n;
    string s;
    int search;
    cout << "Please insert the datas you want to save into the map :" << endl;
    cout << "(0 0 to quit)" << endl;
    while(1)
    {
        cin >> n >> s;
        if(n == 0 && s == "0")
            break;
        m[n] = s;
    }
    cout << "The datas stored in the map are : " << endl;
    for(it = m.begin(); it!= m.end(); it++)
    {
	cout << (*it).first << " " << (*it).second << endl;
    }
    cout << "Input the number ID of the string you are looking for : ";
    cin >> search;
    cout << m[search] << endl;
}
```

Output:

![](/map1.png)

Member functions
----------------------------

### .size()

可以用來檢測當前map裡有幾筆資料

例如

```c
map<int, string> m;
int n;
string s;
while(1)
    {
        cin >> n >> s;
        if(n == 0 && s == "0")
            break;
        m[n] = s;
        cout << "SIZE: " << m.size() << endl;
    }
```

![](/map2.png)

### .clear() / .empty()

.clear()是用來清空map裡所有資料的一個member function

.empty()是用來察看map裡的資料是否是空的的member function (returns boolean)

```c
cout << "Is it empty ?" << endl;
if(m.empty())
    cout << "Yes~" << endl;
else
    cout << "No~~~~~" << endl;
cout << "Is it empty now?" << endl;
m.clear();
if(m.empty())
    cout << "EMPTY!" << endl;
else
    cout << "Nope" << endl;
```

![](/map3.png)