【C++】map容器的用法

检测map容器是否为空：

 1 #include <iostream>
 2 #include<map>
 3 #include<string>
 4 using namespace std;
 5 int main()
 6 {
 7     //检测容器是否为空
 8     map<string, string>mapText;
 9     if (mapText.empty())
10     {
11         cout << "mapText为空" << endl;
12     }
13     else
14     {
15         cout << "mapText不为空" << endl;
16     }
17
18     //向容器中添加元素
19     mapText["小A"] = "A"; //赋值
20     mapText["小B"] = "B"; //赋值
21     mapText["小C"] = "C"; //赋值
22
23     if (mapText.empty())
24     {
25         cout << "mapText为空" << endl;
26     }
27     else
28     {
29         cout << "mapText不为空" << endl;
30     }
31
32     system("pause");
33     return 0;
34 }

---

 重复赋值，值被替换：

 1 #include <iostream>
 2 #include<map>
 3 #include<string>
 4 using namespace std;
 5 int main()
 6 {
 7     map<string, string>mapText;
 8     mapText["小A"] = "A"; //赋值
 9     mapText["小A"] = "B"; //重复赋值
10     cout << mapText["小A"] << endl;
11
12     system("pause");
13     return 0;
14 }

---

 判断键是否存在，如果不存在再赋值：

 1 #include <iostream>
 2 #include<map>
 3 #include<string>
 4 using namespace std;
 5 int main()
 6 {
 7     map<string, string>mapText;
 8     mapText["小A"] = "A";     //赋值
 9      //先检测键是否存在，如果存在则不赋值
10     if (mapText.count("小A") == 0)  //count==0不存在 count==1存在
11     {
12         mapText["小A"] = "B"; //重复赋值
13     }
14     cout << mapText["小A"] << endl;
15
16     system("pause");
17     return 0;
18 }

---

 map循环遍历：

map.begin()指向map的第一个元素

map.end()指向map的最后一个元素之后的地址

 1 #include <iostream>
 2 #include<map>
 3 #include<string>
 4 using namespace std;
 5 int main()
 6 {
 7     map<string, string>mapText;
 8     mapText["小A"] = "A";     //赋值
 9     mapText["小B"] = "B";     //赋值
10     mapText["小C1"] = "C";     //赋值
11     mapText["小C2"] = "C";     //赋值
12     for (map<string, string>::iterator itor = mapText.begin(); itor != mapText.end(); ++itor)
13     {
14         cout << "key = " << itor->first << ", value = " << itor->second << endl;
15
16     }
17     system("pause");
18     return 0;
19 }

---

 map 通过“键”删除键值对：

 1 #include <iostream>
 2 #include<map>
 3 #include<string>
 4 using namespace std;
 5 int main()
 6 {
 7     map<string, string>mapText;
 8     mapText["小A"] = "A";     //赋值
 9     mapText["小B"] = "B";     //赋值
10     mapText["小C1"] = "C";     //赋值
11     mapText["小C2"] = "C";     //赋值
12     cout << "删除前：" << endl;
13     for (map<string, string>::iterator itor = mapText.begin(); itor != mapText.end(); ++itor)
14     {
15         cout << "key = " << itor->first << ", value = " << itor->second << endl;
16
17     }
18     //删除小B
19     mapText.erase("小B");
20     cout << "删除后：" << endl;
21     for (map<string, string>::iterator itor = mapText.begin(); itor != mapText.end(); ++itor)
22     {
23         cout << "key = " << itor->first << ", value = " << itor->second << endl;
24
25     }
26     mapText.erase("小B"); //小B不存在，erase也不会报错
27     system("pause");
28     return 0;
29 }

---

  map 通过“值”删除键值对，先写一个错误用法，这里要注意：

 1 #include <iostream>
 2 #include<map>
 3 #include<string>
 4 using namespace std;
 5 int main()
 6 {
 7     map<string, string>mapText;
 8     mapText["小A"] = "A";     //赋值
 9     mapText["小B"] = "B";     //赋值
10     mapText["小C1"] = "C";     //赋值
11     mapText["小C2"] = "C";     //赋值
12     cout << "删除前：" << endl;
13     for (map<string, string>::iterator itor = mapText.begin(); itor != mapText.end(); ++itor)
14     {
15         cout << "key = " << itor->first << ", value = " << itor->second << endl;
16
17     }
18     //删除值为C的元素
19     //错误用法：
20     for (map<string, string>::iterator itor = mapText.begin(); itor != mapText.end(); ++itor)
21     {
22         if ((itor->second) == "C")
23         {
24             mapText.erase(itor);
25         }
26     }
27
28     cout << "删除后：" << endl;
29     for (map<string, string>::iterator itor = mapText.begin(); itor != mapText.end(); ++itor)
30     {
31         cout << "key = " << itor->first << ", value = " << itor->second << endl;
32
33     }
34     system("pause");
35     return 0;
36 }

错误原因：itor指针在元素被删除后失效了，回到for语句中与mapText.end()进行比较出现错误。

---

 map 通过“值”删除键值对，正确的用法:

 1 #include <iostream>
 2 #include<map>
 3 #include<string>
 4 using namespace std;
 5 int main()
 6 {
 7     map<string, string>mapText;
 8     mapText["小A"] = "A";     //赋值
 9     mapText["小B"] = "B";     //赋值
10     mapText["小C1"] = "C";     //赋值
11     mapText["小C2"] = "C";     //赋值
12     cout << "删除前：" << endl;
13     for (map<string, string>::iterator itor = mapText.begin(); itor != mapText.end(); ++itor)
14     {
15         cout << "key = " << itor->first << ", value = " << itor->second << endl;
16
17     }
18     //删除值为C的元素
19     //正确用法：
20     for (map<string, string>::iterator itor = mapText.begin(); itor != mapText.end(); /*++itor*/)
21     {
22         if ((itor->second) == "C")
23         {
24             itor = mapText.erase(itor);
25         }
26         else
27         {
28             ++itor;
29         }
30     }
31
32
33     cout << "删除后：" << endl;
34     for (map<string, string>::iterator itor = mapText.begin(); itor != mapText.end(); ++itor)
35     {
36         cout << "key = " << itor->first << ", value = " << itor->second << endl;
37
38     }
39     system("pause");
40     return 0;
41 }

---

删除map的第一个元素

mapText.erase(mapText.begin());