C++ set erase()

2024年8月30日 | 5分钟阅读

C++ set 的 **erase()** 函数用于从 set 容器中移除与给定键关联的单个元素，或移除一个范围 ([first, last)) 的元素。因此，大小将根据移除的元素数量而减少。

语法

void erase (iterator position);                       	  //until C++ 11

size_type erase (const value_type& val);    		  //until C++ 11

void erase (iterator first, iterator last);  		  //until C++ 11

iterator  erase (const_iterator position);		  //since C++ 11

size_type erase (const value_type& val);		  //since C++ 11	

iterator  erase (const_iterator first, const_iterator last); //since C++ 11

参数

**position**：指向要从 set 中移除的单个元素的迭代器。

**val**：要从 set 中移除的值。

first: 要擦除范围的起始。

last: 要擦除范围的结束。

返回值

它返回一个指向被删除元素之后元素的迭代器，或者返回被删除元素的数量。

复杂度

erase(position): 摊销常数时间。

erase (val): 对容器大小取对数时间。

**erase(first,last)**：与 first 和 last 之间的距离呈线性关系。

迭代器有效性

指向被函数移除的元素的迭代器、引用和指针将失效。

所有其他迭代器、指针和引用保持其有效性。

数据竞争

容器被修改。

被移除的元素被修改。在容器中迭代范围是不安全的，尽管并发访问其他元素是安全的。

异常安全

此函数不会抛出异常。

如果指定了无效范围或位置，则会导致未定义行为。

示例 1

让我们看一个使用迭代器擦除元素的简单示例。

#include <iostream>
#include <set>

using namespace std;

int main ()
{
  set<int> myset;
  set<int>::iterator it;

  myset = {10,20,30};
  
  cout<<"Before erasing the element: \n";
   for (it=myset.begin(); it!=myset.end(); ++it)
    cout << *it << '\n';

  it=myset.find('b');
  myset.erase (*it);                   // erasing by iterator

  cout<<"\nAfter erasing the element: \n";
  for (it=myset.begin(); it!=myset.end(); ++it)
    cout << *it << '\n';
    
  return 0;
}

输出

Before erasing the element: 
10
20
30

After erasing the element: 
10
20
30

在上面的示例中，元素通过迭代器 it 被擦除。

示例 2

让我们看一个简单的例子来擦除给定键值的 set 元素

#include <iostream>
#include <set>

using namespace std;

int main ()
{
  set<int> myset;
  set<int>::iterator it;

  myset = {10, 20, 30, 40};
  
  cout<<"Before erasing the element: \n";
   for (it=myset.begin(); it!=myset.end(); ++it)
    cout << *it<< '\n';

   myset.erase (30);                  // erasing by value

  cout<<"\nAfter erasing the element: \n";
  for (it=myset.begin(); it!=myset.end(); ++it)
    cout << *it<< '\n';
  return 0;
}

输出

Before erasing the element: 
10
20
30
40

After erasing the element: 
10
20
40

在上面的例子中，erase(value) 函数从 set 中移除了值 30。

示例 3

让我们看一个使用给定范围擦除元素的简单示例

#include <iostream>
#include <set>

using namespace std;

int main ()
{
  set<int> myset;
  set<int>::iterator it;

  myset = {10, 20, 30};
  
  cout<<"Before erasing the element are: \n";
   cout<<"Size is: "<<myset.size()<<'\n';
   for (it=myset.begin(); it!=myset.end(); ++it)
   cout << *it << '\n';

   myset.erase ( myset.begin () ,  myset.end () );   // erasing by range

  cout<<"\nAfter erasing the element are: \n";
  cout<<"Size is: "<<myset.size();
  for (it=myset.begin(); it!=myset.end(); ++it)
  cout << *it << '\n';
  return 0;
}

输出

Before erasing the element are: 
Size is: 3
10
20
30

After erasing the element are: 
Size is: 0

在上面的例子中，erase(first, last) 函数用于擦除给定范围（即从 begin 到 end）的元素。

示例 4

让我们看一个简单的例子来擦除 set 中的所有奇数

#include <set>
#include <iostream>

using namespace std;

int main()
{
    set<int> m = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
                          
    // erase all odd numbers from m
    cout<<"After erasing odd numbers,elements are:\n ";
    for(auto it = m.begin(); it != m.end(); )
        if(*it % 2 == 1)
            it = m.erase(it);
        else
            ++it;
    for(auto& p : m)
        cout << p << ", ";
}

输出

After erasing odd numbers, elements are:
 2, 4, 6, 8, 10, 12, 14,

在上面的示例中，所有奇数都被擦除，并显示偶数。

示例 5

让我们看另一个例子

#include <set>  
#include <string>  
#include <iostream>  
#include <iterator> // next() and prev() helper functions  
  
using namespace std;  
  
using myset = set<string>;  
  
void printset(const myset& s) {  
    for (const auto& iter : s) {  
        cout << " [" << iter << "]";  
    }  
    cout << endl << "size() == " << s.size() << endl << endl;  
}  
  
int main()  
{  
    myset s1;  
  
    // Fill in some data to test with, one at a time  
    s1.insert("Bob");  
    s1.insert("Robert");  
    s1.insert("Bert");  
    s1.insert("Rob");  
    s1.insert("Bobby");  
  
    cout << "Starting data of set s1 is:" << endl;  
    printset(s1);  
    // The 1st member function removes an element at a given position  
    s1.erase(next(s1.begin()));  
    cout << "After the 2nd element is deleted, the set s1 is:" << endl;  
    printset(s1);  
  
    // Fill in some data to test with, one at a time, using an intializer list  
    myset s2{ "meow", "hiss", "purr", "growl", "yowl" };  
  
    cout << "Starting data of set s2 is:" << endl;  
    printset(s2);  
    // The 2nd member function removes elements  
    // in the range [First, Last)  
    s2.erase(next(s2.begin()), prev(s2.end()));  
    cout << "After the middle elements are deleted, the set s2 is:" << endl;  
    printset(s2);  
  
    myset s3;  
  
    // Fill in some data to test with, one at a time, using emplace  
    s3.emplace("C");  
    s3.emplace("C#");  
    s3.emplace("D");  
    s3.emplace("D#");  
    s3.emplace("E");  
    s3.emplace("E#");  
    s3.emplace("F");  
    s3.emplace("F#");  
    s3.emplace("G");  
    s3.emplace("G#");  
    s3.emplace("A");  
    s3.emplace("A#");  
    s3.emplace("B");  
  
    cout << "Starting data of set s3 is:" << endl;  
    printset(s3);  
    // The 3rd member function removes elements with a given Key  
    myset::size_type count = s3.erase("E#");  
    // The 3rd member function also returns the number of elements removed  
    cout << "The number of elements removed from s3 is: " << count << "." << endl;  
    cout << "After the element with a key of \"E#\" is deleted, the set s3 is:" << endl;  
    printset(s3);  
}  

输出

Starting data of set s1 is:
 [Bert] [Bob] [Bobby] [Rob] [Robert]
size() == 5

After the 2nd element is deleted, the set s1 is:
 [Bert] [Bobby] [Rob] [Robert]
size() == 4

Starting data of set s2 is:
 [growl] [hiss] [meow] [purr] [yowl]
size() == 5

After the middle elements are deleted, the set s2 is:
 [growl] [yowl]
size() == 2

Starting data of set s3 is:
 [A] [A#] [B] [C] [C#] [D] [D#] [E] [E#] [F] [F#] [G] [G#]
size() == 13

The number of elements removed from s3 is: 1.
After the element with a key of "E#" is deleted, the set s3 is:
 [A] [A#] [B] [C] [C#] [D] [D#] [E] [F] [F#] [G] [G#]
size() == 12

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C++ set erase()

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

示例 5

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C++ set erase()

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

示例 5

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