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

當一個key尚未被指定值時 其對應value為0

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()是map第一筆資料的iterator

>.end()是最後一筆資料的iterator

```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
----------------------------

### .begin() / .end()

>.begin()是map第一筆資料的iterator

>.end()是最後一筆資料的iterator

### .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)

### .find()

.find()是用來搜尋某筆map資料的iterator

在取得該筆map資料的iterator後可對其進行修改value

但是key因為是read-only所以無法對齊進行修改 

>格式： it = map.find(key);

Code 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;
    m[1] = "apple";
    m[2] = "banana";
    m[3] = "water";
    cout << "The datas stored in the map are : " << endl;
    for(it = m.begin(); it!= m.end(); it++)
    {
        cout << (*it).first << " " << (*it).second << endl;
    }
    cout << "Please insert the key of the value you want to change : ";
    cin >> search;
    it = m.find(search);
    cout << "What do you want to change it into ?" << endl;
    cin >> (*it).second;
    cout << "Now the datas stored in the map are : " << endl;
	for(it = m.begin(); it!= m.end(); it++)
    {
        cout << (*it).first << " " << (*it).second << endl;
    }
}
```

Output:

![](/map4.png)

### .erase()

.erase()是用來清除特定iterator所存的map的資料

格式(有兩種)：

Erase by iterator

>配合上述的.find()使用

>it = map.find(key);

>map.erase(it);

Erase by key

>map.erase(key);

>Code 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;
    m[1] = "apple";
    m[2] = "banana";
    m[3] = "water";
    cout << "The datas stored in the map are : " << endl;
    for(it = m.begin(); it!= m.end(); it++)
    {
        cout << (*it).first << " " << (*it).second << endl;
    }
    cout << "Please insert the key of the value "<< endl;
    cout << "you want to erase : ";
    cin >> search;
    m.erase(search);
    cout << "Now the datas stored in the map are : " << endl;
	for(it = m.begin(); it!= m.end(); it++)
    {
        cout << (*it).first << " " << (*it).second << endl;
    }
}
```

![](/map5.png)

A long code for you to look and execute for review(?)
--------
```c
#include <iostream>
#include <map>
#include <string>

using namespace std;

map<int, string> m;
map<int, string>::iterator it;

void action_list()
{
    cout << "Action List : (Input the number ID for the action)" << endl;
    cout << "1. insert" << endl;
    cout << "2. find" << endl;
    cout << "3. size?" << endl;
    cout << "4. erase" << endl;
    cout << "5. clear" << endl;
    cout << "6. emptied?" << endl;
    cout << "7. print_map" << endl;
}

int main()
{
    int n;
    string s;
    int action;
    int search;
    cout << "\033c";
    cout << "Hi, this is a new map, you can now do several things to it" <<endl;
    cout << "What do you want to do? " << endl;

    action_list();
    while(1)
    {
    	cout << "Input action ID : ";
    	cin >> action;
    	if(action == 100)
    	{
    	    cout << endl;
    	    cout << "Program stopped" << endl;
    	    break;
    	}
        switch(action)
        {
    	    case 0:
                cout << "\033c";
                action_list();
                break;
            case 1:
                cout << "\033c";
                cout << "Please insert the datas you want to save into the map : (0 0 to quit)" << endl;
                while(1)
                {
                    cin >> n >> s;
                    if(n == 0 && s == "0")
                        break;
                    m[n] = s;
                }
                break;
            case 2:
                cout << "\033c";
                cout << "Input the number ID of the string you are looking for : ";
                cin >> search;
                cout << m[search] << endl;
                break;
            case 3:
                cout << "\033c";
                cout << "There are " << m.size() << " datas now stored in the map" << endl;
				break;
            case 4:
                cout << "\033c";
                cout << "Please insert the key of the value you want to erase : ";
                cin >> search;
                m.erase(search);
                break;
            case 5:
                cout << "\033c";
                cout << "Map cleared to empty" << endl;
                m.clear();
                break;
            case 6:
                cout << "\033c";
                if(m.empty())
                    cout << "Yeah ~ it's empty now..." << endl;
                else
                    cout << "No ~ not empty yet ~" << endl;
                break;
            case 7:
                cout << "\033c";
                cout << "The datas now stored in the map are : " << endl;
                for(it = m.begin(); it!= m.end(); it++)
                {
                    cout << (*it).first << " " << (*it).second << endl;
                }
                break;
            default:
                cout << "\033c";
                cout << "Error action input, type 0 to look at the action list" << endl;
                break;
    	}
        cout << endl;
        cout << "Anything you also want to do ?" << endl;
        cout << "(To end the program, you can type 100 now)" << endl;
    	cout << "(To review the action list please type 0)" << endl;
    }
}
```