C++ Unordered_map::bucket_count() Function

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The C++ function unordered_map::bucket_count() returns the number of buckets in the container. A bucket is a slot in the container's internal hash table to which elements are assigned based on the hash value of their key. Buckets have numbers ranging from 0 to (bucket_count - 1).

Syntax

Following is the Syntax of std::unordered_map::bucket_count() function.

size_type bucket_count() const noexcept;

Parameters

This function does not accepts any parameter.

Return value

Returns the total number of bucket present in the unordered_map.

Example 1

Consider the following example, where we are going to observe the usage of unordered_map::bucket_count() function.

#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;
}

Output

Following is the output of the above code −

Number of buckets = 13

Example 2

In the following example, we are using the bucket_count() to obtain the total number of buckets along with no.of items in each bucket, from an unordered_map.

#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;
}

Output

Following is the output of the above code −

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: 

Example 3

Consider the following example, where we are creating unordered_map using [] operator and using bucket_count() to count the number of buckets and buckets_size() to count the number of elements in each bucket.

#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;
}

Output

Let us compile and run the above program, this will produce the following result −

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.