极客笔记C++ map clear()函数
clear()函数是C++中的map容器的一个函数，用于删除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示例来计算清除操作之前和之后地图的大小。
#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
在上面的示例中，一个Map对象被初始化为5个元素。因此，Map的大小为5，但在执行clear操作后，大小变为0。
示例2
让我们看一个简单的示例来清空Map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
在上面的示例中，在清除地图后，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来清除地图的元素。
#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.
在上面的示例中，有三个地图组，根据用户的选择，已经删除了一个组。
示例4
让我们看一个简单的示例：
#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
在上面的示例中，一个水果映射被初始化为三个水果。询问清除映射时，如果你输入0，则水果桶有3个元素，或者如果你输入1，则它将清除水果映射，并且大小变为0。