C++ unordered_map - bucket_size() Function

The C++ unordered_map::bucket_size function returns the number of elements in the specified bucket of 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_size ( size_type n ) const;

Parameters

Return Value

Number of elements in the bucket number n of the unordered_map.

Time Complexity

Linear i.e, Θ(n).

Example:

In the example below, the unordered_map::bucket_size function returns the number of elements in the specified bucket of 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<<"\nBucket #"<<i<<" contains: "<<uMap.bucket_size(i)<<" elements.";   

  return 0;
}

The output of the above code will be:

uMap contains 13 buckets:
Bucket #0 contains: 0 elements.
Bucket #1 contains: 0 elements.
Bucket #2 contains: 0 elements.
Bucket #3 contains: 0 elements.
Bucket #4 contains: 0 elements.
Bucket #5 contains: 0 elements.
Bucket #6 contains: 0 elements.
Bucket #7 contains: 0 elements.
Bucket #8 contains: 0 elements.
Bucket #9 contains: 0 elements.
Bucket #10 contains: 1 elements.
Bucket #11 contains: 1 elements.
Bucket #12 contains: 1 elements.