C++ unordered_map::bucket_count() 函数

C++ 函数unordered_map::bucket_count()返回容器中桶的数量。桶是容器内部哈希表中的一个槽，元素根据其键的哈希值分配到该槽中。桶的编号范围从 0 到 (bucket_count - 1)。

语法

以下是 std::unordered_map::bucket_count() 函数的语法。

size_type bucket_count() const noexcept;

参数

此函数不接受任何参数。

返回值

返回 unordered_map 中存在的桶的总数。

示例 1

考虑以下示例，我们将观察 unordered_map::bucket_count() 函数的用法。

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void){
   unordered_map<char, int> um = {
      {'a', 1},
      {'b', 2},
      {'c', 3},
      {'d', 4},
      {'e', 5}
   };
   cout << "Number of buckets = " << um.bucket_count() << endl;
   return 0;
}

输出

以下是上述代码的输出：

Number of buckets = 13

示例 2

在下面的示例中，我们使用 bucket_count() 获取 unordered_map 中桶的总数以及每个桶中的项目数量。

#include <iostream>
#include <string>
#include <unordered_map>
using namespace std;
int main () {
   unordered_map<string, string> UnorderMap={
      {"Aman","Akash"},
      {"Gautam","Garav"},
      {"Anil","Sunil"},
      {"Raja","Roja"},
      {"Sarika","Revathi"},
   };
   unsigned n = UnorderMap.bucket_count();
   cout << "map has " << n << " buckets.\n";

   for (unsigned i=0; i<n; ++i) {
      cout << "bucket #" << i << " contains: ";
      for (auto it = UnorderMap.begin(i); it!=UnorderMap.end(i); ++it)
         cout << "[" << it->first << ":" << it->second << "] ";
      cout << "\n";
   }
   return 0;
}

输出

以下是上述代码的输出：

map has 13 buckets.
bucket #0 contains: 
bucket #1 contains: 
bucket #2 contains: 
bucket #3 contains: [Raja:Roja] 
bucket #4 contains: 
bucket #5 contains: [Anil:Sunil] [Gautam:Garav] 
bucket #6 contains: 
bucket #7 contains: 
bucket #8 contains: [Aman:Akash] 
bucket #9 contains: [Sarika:Revathi] 
bucket #10 contains: 
bucket #11 contains: 
bucket #12 contains: 

示例 3

考虑以下示例，我们使用 [] 运算符创建 unordered_map，并使用 bucket_count() 计数桶的数量，使用 buckets_size() 计数每个桶中的元素数量。

#include <iostream>
#include <unordered_map>
using namespace std;
int main() {
   unordered_map<char, int> umap;
   umap['a'] = 1;
   umap['b'] = 2;
   umap['c'] = 3;
   umap['d'] = 4;
   umap['e'] = 5;
 
   int n = umap.bucket_count();
   cout << "umap has " <<  n <<  " buckets.\n\n";
 
   // Count no. of elements in each bucket using
   // bucket_size(position)
   for (int i = 0; i < n; i++) {
      cout <<  "Bucket " <<  i <<  " has "<<  umap.bucket_size(i) <<  " elements.\n";
   }
   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：

umap has 13 buckets.

Bucket 0 has 0 elements.
Bucket 1 has 0 elements.
Bucket 2 has 0 elements.
Bucket 3 has 0 elements.
Bucket 4 has 0 elements.
Bucket 5 has 0 elements.
Bucket 6 has 1 elements.
Bucket 7 has 1 elements.
Bucket 8 has 1 elements.
Bucket 9 has 1 elements.
Bucket 10 has 1 elements.
Bucket 11 has 0 elements.
Bucket 12 has 0 elements.