C++ unordered_multimap::equal_range() 函数

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C++ 的 std::unordered_multimap::equal_range() 函数用于返回一对迭代器，它们表示键与指定键等效的元素范围。

众所周知，equal_range() 函数返回一对迭代器，其中第一个迭代器指向范围内与指定键等效的第一个元素，第二个迭代器指向该范围的最后一个元素之后的位置。如果 unordered_multimap 中没有与指定键等效的元素，则返回对中的两个迭代器（下界和上界）都将等于容器末尾之后的位置，即 unordered_multimap.end()。

语法

以下是 std::unordered_multimap::equal() 函数的语法。

std::pair<iterator, iterator> equal_range(const key_type& key);

参数

• k − 表示要比较或搜索的键值。

返回值

此函数返回一对迭代器。

示例 1

在下面的示例中，我们演示了 unordered_multimap::equal_range() 函数的用法。

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> um = {
      {'a', 1},
      {'b', 2},
      {'c', 3},
      {'e', 4},
      {'a', 5},
      {'d', 6},
      {'e', 5}
   };
   auto ret = um.equal_range('e');
   cout << "Lower bound is " << ret.first->first 
       << " = "<< ret.first->second << endl;
   cout << "Upper bound is " << ret.second->first
       << " = " << ret.second->second << endl;
   return 0;
}

输出

让我们编译并运行上面的程序，这将产生以下结果：

Lower bound is e = 5
Upper bound is c = 3

示例 2

考虑下面的示例，我们将访问与范围内键关联的值。

#include <iostream>
#include <unordered_map>
using namespace std;
int main() {
   unordered_multimap<int, string> um = {
      {1, "one"},
      {2, "two"},
      {4, "four"},
      {3, "three"},
      {4, "four"},
      {5, "five"},
      {3, "three"},
   };
   auto range = um.equal_range(3);

   // Check if the key was found
   if (range.first != um.end()) {
      cout << "Key 3 found in the multimap!" << endl;
      // Access the values associated with keys in the range
      for (auto it = range.first; it != range.second; ++it) {
          cout << "The value associated with key " << it->first << " is: " << it->second << endl;
      }
   } else {
      cout << "Key 3 not found in the multimap!" << endl;
   }
   return 0;
}

输出

如果我们运行上面的代码，它将生成以下输出：

Key 3 found in the multimap!
The value associated with key 3 is: three
The value associated with key 3 is: third

示例 3

让我们看一下下面的示例，我们将获取重复键的值以及指定键的上界和下界。

#include <iostream>
#include <string>
#include <unordered_map>
using namespace std;
typedef unordered_multimap<string,string> stringmultimap;
int main () {
   stringmultimap umm = {
      {"orange","FL"},
      {"strawberry","LA"},
      {"strawberry","OK"},
      {"pumpkin","NH"}
   };
   cout << "Entries with strawberry: "<<endl;
   auto range = umm.equal_range("strawberry");
   for (auto it = range.first; it != range.second; ++it){
      cout<< it->first << " is: " << it->second<<endl;
   }
   cout << "Lower bound is " << range.first->first
      << " = "<< range.first->second << endl;
   cout << "Upper bound is " << range.second->first
      << " = " << range.second->second << endl;
   return 0;
}

输出

以下是上述代码的输出：

Entries with strawberry: 
strawberry is: OK
strawberry is: LA
Lower bound is strawberry = OK
Upper bound is pumpkin = NH

示例 4

以下示例将使用 multimap 并迭代指向范围的键值对。

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

int main() {
   unordered_multimap<int, int> umm;    
   umm.insert({ 1, 2 });
   umm.insert({ 1, 2 });
   umm.insert({ 2, 3 });
   umm.insert({ 3, 4 });
   umm.insert({ 2, 6 });
   auto range = umm.equal_range(1);
   cout << "Elements with Key 1: ";
   for (auto it = range.first; it != range.second; it++) {
      cout << it->second << " ";
   }
   
   cout << endl;
   range = umm.equal_range(2);
   cout << "Elements with Key 2: ";
   for (auto it = range.first; it != range.second; it++) {
      cout << it->second << " ";
   }
   return 0;
}

输出

上述代码的输出如下：

Elements with Key 1: 2 2 
Elements with Key 2: 6 3