C++语法基础--关联容器--map(二）--查找并读取map中的元素count(),find()与从map对象中删除元素erase()

古城微笑少年丶 2022-01-10 15:19 248阅读 0赞

**1.查找并读取map中的元素count(),find()  
![20130710111636171][]  
  
 \*对于map对象，count成员的返回值只能是0或1  
原型：  
iterator find (const key\_type& k);  
const\_iterator find (const key\_type& k) const  
size\_type count (const key\_type& k) const;  
eg:  
int main ()  
\{  
map<string,int> mymap;  
map<string,int>::iterator it;  
mymap\["jack"\]=18;**

**cout<<mymap\["rose"\]<<endl; //直接使用下标的弊端：mymap并不存在"rose"的键，**

**//但是却把"rose"插入了mymap**

**if(mymap.count("tom"))  //mymap并不存在"tom"的键,返回0  
\{   
cout<<mymap\["tom"\]<<endl;;  
\}  
it=mymap.find("jack"); //返回"jack"在mymap中的迭代器  
cout<<it->first<<": "<<it->second<<endl; //验证返回的迭代器  
 mymap.find("lily"); //mymap并不存在"lily"的键,返回end()迭代器，但并不会插入"lily"的键值   
  
for(it=mymap.begin();it!=mymap.end();it++)  
\{   
 cout<<it->first<<": "<<it->second<<endl;  
\}  
  
  
 return 0;  
\}  
  
  
运行结果：  
![Center][]  
  
  
  
  
2. 从map对象中删除元素  
![20130710112234484][]  
  
原型：  
void erase (iterator position);  
size\_type erase (const key\_type& k);  
void erase (iterator first, iterator last);  
eg：  
int main ()  
\{  
map<string,int> mymap;  
map<string,int>::iterator it;  
mymap\["jack"\]=18;  
mymap\["tom"\]=19;  
mymap\["jany"\]=17;  
mymap\["rose"\]=20;  
mymap\["lily"\]=16;  
cout<<"before erase: "<<endl;  
for (it=mymap.begin(); it!=mymap.end(); ++it)  
\{  
 cout << it->first << " => " << it->second << '\\n';  
 \}  
  
  
it=mymap.find("tom");  
mymap.erase (it); // erasing by iterator  
 mymap.erase ("rose"); // erasing by key  
 it=mymap.find ("jany");  
 mymap.erase ( it, mymap.end() );  // erasing by range  
cout<<"after erase: "<<endl;  
 for (it=mymap.begin(); it!=mymap.end(); ++it)  
 \{   
 cout << it->first << " => " << it->second << '\\n';  
 \}  
  
  
 return 0;  
 \}  
运行结果：  
![Center 1][]**

[20130710111636171]: /images/20220110/f1b49a2d396d441c9df4fbe30490ed88.png
[Center]: /images/20220110/6200a5631543492191d7d45def817545.png
[20130710112234484]: /images/20220110/64e6564b64dc4fb487ece93d41ac912e.png
[Center 1]: /images/20220110/ef12f64c455045b6a51952a5214d6eb9.png