C++ map clear() 函数

2024年8月30日 | 阅读4分钟

C++ map 的 clear() 函数用于移除 map 容器中的所有元素。它会清空 map 并将其大小设置为 0。

语法

void clear(); //until C++ 11
void clear() noexcept; //since C++ 11

参数

无

返回值

无

示例 1

让我们看一个简单的例子，计算 clear 操作前后 map 的大小。

#include <iostream>
#include <map>

using namespace std;

int main() {

   map<char, int> mymap = {
            {'a', 1},
            {'b', 2},
            {'c', 3},
            {'d', 4},
            {'e', 5},
            };

   cout << "Initial size of map before clear operation = " << mymap.size() << endl;

   mymap.clear();

   cout << "Size of map after clear opearation = " << mymap.size() << endl;

   return 0;
}

输出

Initial size of map before clear operation = 5
Size of map after clear opearation = 0

在上面的例子中，一个 map 被初始化了 5 个元素。因此，map 的大小是 5，但在 clear 操作之后，大小变为 0。

示例 2

让我们看一个简单的例子来清除 map 中的元素。

#include <iostream>
#include <map>
using namespace std;

int main ()
{
  map<int,string> mymap;

  mymap[1] = "Nikita";
  mymap[2] = "Deep";
  mymap[3] = "Ashish";

  cout << "mymap contains:\n";
  for (map<int,string>::iterator it=mymap.begin(); it!=mymap.end(); ++it)
    cout << it->first << " : " << it->second << '\n';

  mymap.clear();
  
  mymap[4] = "Rajni";
  mymap[5] = "Sunil";

  cout << "\nmymap contains:\n";
  for (map<int,string>::iterator it=mymap.begin(); it!=mymap.end(); ++it)
    cout << it->first << " : " << it->second << '\n';

  return 0;
}

输出

mymap contains:
1 : Nikita
2 : Deep
3 : Ashish

mymap contains:
4 : Rajni
5 : Sunil

在上面的例子中，清除 map 后，我们可以无需初始化就添加新元素。

示例 3

让我们看一个简单的例子来清除 map 中的元素。

#include <iostream>
#include <map>
using namespace std;
int main ()
{
  int n;
  map<int,string> m1,m2,m3;

  m1[1] = "Nikita";
  m1[2] = "Deep";
  m1[3] = "Ashish";
  
  m2[1] = "Nidhi";
  m2[2] = "Priya";
  m2[3] = "Gitanjali";
   m3[1] = "Rakesh";
  m3[2] = "Fruti";
  m3[3] = "Kamlesh";
  cout << "m1 group has following members:\n";
  for (map<int,string>::iterator it=m1.begin(); it!=m1.end(); ++it)
  cout << it->first << " : " << it->second << '\n';
  cout << "m2 group has following members:\n";
  for (map<int,string>::iterator it=m2.begin(); it!=m2.end(); ++it)
  cout << it->first << " : " << it->second << '\n';
  cout << "m3 group has following members:\n";
  for (map<int,string>::iterator it=m3.begin(); it!=m3.end(); ++it)
   cout << it->first << " : " << it->second << '\n';
   cout<<"\nWhich group do you want to delete?\n 1.m1\n 2.m2\n 3.m3\n Please enter your choice: ";
  cin>>n;
  if(n==1){
  m1.clear();
  cout<<"\nGroup m1 has been cleared.";
  }
  else if(n==2){
  m2.clear();
  cout<<"\nGroup m2 has been cleared.";
  }
  else if(n==3){
  m3.clear();
  cout<<"\nGroup m3 has been cleared.";
  }
  else
  cout<<"Invalid option!";
  
  return 0;
}

输出

m1 group has following members:
1 : Nikita
2 : Deep
3 : Ashish
m2 group has following members:
1 : Nidhi
2 : Priya
3 : Gitanjali
m3 group has following members:
1 : Rakesh
2 : Fruti
3 : Kamlesh

Which group do you want to delete?
 1. m1
 2. m2
 3. m3
 Please enter your choice: 2

Group m2 has been cleared.

在上面的例子中，有三组 map，根据用户的选择，其中一组被删除。

示例 4

让我们看一个简单的例子

#include <iostream>
#include <map>
 using namespace std;
 int main() {
 int n;
 map<string, int> fruit = {
            {"Banana", 40},
            {"Apple", 190},
            {"Orange", 120},
            };
 cout << "Fruit bucket has following fruits = \n";
 for (map<string,int>::iterator it=fruit.begin(); it!=fruit.end(); ++it)
 cout << it->first << " : " << it->second << '\n';
cout<<"\nDo you want to clear your fruit bucket?\nPress 1 for Yes and 0 for No: ";
   cin>>n;
   if( n==1){
   fruit.clear();
   cout<<fruit.size()<<" fruits in bucket \n";  
   }
   else if(n==0)
   cout <<fruit.size() << " fruits in bucket \n" ;
    
 return 0;
}

输出

1. 
Fruit bucket has following fruits = 
Apple : 190
Banana : 40
Orange : 120

Do you want to clear your fruit bucket?
Press 1 for Yes and 0 for No: 0
3 fruits in bucket



2. 
Fruit bucket has following fruits = 
Apple : 190
Banana : 40
Orange : 120

Do you want to clear your fruit bucket?
Press 1 for Yes and 0 for No: 1
0 fruits in bucket

在上面的例子中，一个水果 map 被初始化了三种水果。程序会询问是否要清除 map，如果您输入 0，则水果篮中有 3 个元素；如果您输入 1，则会清除水果 map，其大小变为 0。

下一个主题C++ Map

C++ map clear() 函数

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C++ map clear() 函数

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