C++ unordered_set - bucket_count() Function

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

As an unordered_set 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. 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_set.

Time Complexity

Constant i.e, Θ(1).

Example:

In the example below, the unordered_set::bucket_count function returns the number of buckets in uSet.

#include <iostream>
#include <unordered_set>
using namespace std;
 
int main (){
  unordered_set<int> uSet{55, 25, 128, 5, 72, -500, 45, 24, -20};

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

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

The output of the above code will be:

uSet contains 11 buckets:
The bucket #0 contains: -500 55 
The bucket #1 contains: 45 
The bucket #2 contains: 24 
The bucket #3 contains: 25 
The bucket #4 contains: 
The bucket #5 contains: 5 
The bucket #6 contains: 72 
The bucket #7 contains: -20 128 
The bucket #8 contains: 
The bucket #9 contains: 
The bucket #10 contains:

❮ C++ <unordered_set> Library