C++ unordered_map - bucket_count() Function

The C++ unordered_map::bucket_count function returns the number of buckets in the unordered_map.

As an unordered_map is implemented using hash table where 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. The number of buckets directly influences the load_factor of the container's hash table. The container automatically increases the number of buckets to keep the load_factor below its max_load_factor which causes rehash whenever the number of buckets is increased.

Syntax

C++11

size_type bucket_count() const noexcept;

Parameters

No parameter is required.

Return Value

Number of buckets in the unordered_map.

Time Complexity

Constant i.e, Θ(1).

Example:

In the example below, the unordered_map::bucket_count function returns the number of buckets in uMap.

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_map<string, string> uMap;
  uMap = {{"CAN", "Ottawa"}, {"USA", "Washington"}, {"IND", "Delhi"}};

  cout<<"uMap contains "<<uMap.bucket_count()<<" buckets:";

  for(unsigned int i = 0; i < uMap.bucket_count(); i++) {
    cout<<"\nThe bucket #"<<i<<" contains: ";   
    for(auto it = uMap.begin(i); it != uMap.end(i); ++it) {
      cout<<it->first<<":"<<it->second<<" ";
    } 
  } 
  return 0;
}

The output of the above code will be:

uMap contains 13 buckets:
The bucket #0 contains: 
The bucket #1 contains: 
The bucket #2 contains: 
The bucket #3 contains: 
The bucket #4 contains: 
The bucket #5 contains: 
The bucket #6 contains: 
The bucket #7 contains: 
The bucket #8 contains: 
The bucket #9 contains: 
The bucket #10 contains: USA:Washington 
The bucket #11 contains: IND:Delhi 
The bucket #12 contains: CAN:Ottawa

❮ C++ <unordered_map> Library