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C++ multimap clear()

C++ multimap clear()函数

C++ multimap clear()函数用于删除multimap容器的所有元素。它将清除 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

返回值

复杂度

大小线性。

迭代器有效性

与此容器相关的所有迭代器,引用和指针均无效。

数据竞争

容器已修改。
所有包含的元素均已修改。

异常安全性

此函数从不抛出异常。

示例1

让我们看一个简单的示例,在清除之前和之后计算 multimap 的大小操作:
#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
在上面的示例中,使用5个元素初始化了一个 multimap ,因此大小为5,但是在清除操作之后,大小变为0。

示例2

让我们看一个清除 multimap 元素的简单示例:
#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
在上面的示例中,清除 multimap 后,我们可以在不初始化的情况下添加新元素。

示例3

让我们看一个简单的示例清除 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.
在上面的示例中,有3组 multimap ,并且根据用户的选择删除了一组。

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
在上面的示例中,一个水果 set 映射用三个水果初始化。要求清除 multimap ,如果您输入0则水果桶有3个元素,或者如果输入1则将清除水果 multimap 并且大小变为0。
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