C++ multimap clear() 函数

2024年8月30日 | 阅读4分钟

C++ multimap clear() 函数用于移除 multimap 容器中的所有元素。它会清空 multimap 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

复杂度

大小呈线性。

迭代器有效性

与此容器相关的所有迭代器、引用和指针都将失效。

数据竞争

容器被修改。

所有包含的元素都将被修改。

异常安全

此函数从不抛出异常。

示例 1

让我们看一个简单的例子来计算 clear 操作前后 multimap 的大小

#include <iostream>
#include <map>

using namespace std;

int main() {

   multimap<char, int> mymultimap = {
            {'a', 1},
            {'b', 2},
            {'b', 3},
            {'d', 4},
            {'d', 5},
            };

   cout << "Initial size of multimap before clear operation = " << mymultimap.size() << endl;

   mymultimap.clear();

   cout << "Size of multimap after clear operation = " << mymultimap.size() << endl;

   return 0;
}

输出

Initial size of multimap before clear operation = 5
Size of multimap after clear operation = 0

在上面的例子中，一个 multimap 初始化了 5 个元素，因此大小为 5，但在 clear 操作后，大小变为 0。

示例 2

让我们看一个简单的例子来清空 multimap 的元素

#include <iostream>
#include <map>

using namespace std;

int main ()
{
  multimap<int,string> mymultimap;

  mymultimap = {
               {1, "Nikita"},
               {2, "Divya" },
               {1, "Ashish"} };

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

  mymultimap.clear();
  
  mymultimap = {
               {1, "Aman"},
               {2, "Deep"}
               };
               
  cout << "\nmymultimap contains:\n";
  for (multimap<int,string>::iterator it=mymultimap.begin(); it!=mymultimap.end(); ++it)
    cout << it->first << " : " << it->second << '\n';

  return 0;
}

输出

mymultimap contains:
1 : Nikita
1 : Ashish
2 : Divya

mymultimap contains:
1 : Aman
2 : Deep

在上面的例子中，清空 multimap 后，我们可以在不重新初始化的情况下添加新元素。

示例 3

让我们看一个简单的例子来清空 multimap 的元素

#include <iostream>
#include <map>

using namespace std;

int main ()
{
  int n;
  multimap<int,string> m1,m2,m3;

  m1 = { {1, "Nikita"},
         {2, "Deep"},
         {1, "Ashish"}  };
  
  m2 = { {1, "Nidhi"},
        {2, "Priya"},
        {2, "Gitanjali"} };
  
  m3 = { {1, "Manas"},
       {3, "Fruti"},
       {3, "Kamlesh"} };
  

  cout << "m1 group has following members:\n";
  for (multimap<int,string>::iterator it=m1.begin(); it!=m1.end(); ++it)
    cout << it->first << " : " << it->second << '\n';
  

  cout << "m2 group has following members:\n";
  for (multimap<int,string>::iterator it=m2.begin(); it!=m2.end(); ++it)
    cout << it->first << " : " << it->second << '\n';

  cout << "m3 group has following members:\n";
  for (multimap<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
1 : Ashish
2 : Deep

m2 group has following members:
1 : Nidhi
2 : Priya
2 : Gitanjali

m3 group has following members:
1 : Manas
3 : 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 multimap，根据用户的选择，其中一组已被删除。

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;
   
   multimap<string, int> fruit = {
            {"Banana", 40},
            {"Apple", 190},
            {"Orange", 120},
            };

   cout << "Fruit bucket has following fruits = \n";
   for (multimap<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 multimap 初始化了三种水果。询问是否清除 multimap，如果您输入 0，则水果桶有 3 个元素；如果您输入 1，它将清除水果 multimap，大小变为 0。

下一个主题C++ multimap

C++ multimap clear() 函数

语法

参数

返回值

复杂度

迭代器有效性

数据竞争

异常安全

示例 1

示例 2

示例 3

示例 4

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C++ multimap clear() 函数

语法

参数

返回值

复杂度

迭代器有效性

数据竞争

异常安全

示例 1

示例 2

示例 3

示例 4

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