bucket_count and bucket_size in unordered_map in C++

Unordered_map is an associated container that stores elements formed by the combination of key-value and a mapped value. The key value is used to uniquely identify the element and the mapped value is the content associated with the key. Both key and value can be of any type predefined or user-defined. 

Bucket: Internally, unordered_map is implemented using a hash table so, a bucket is a slot in the internal hash Table to which elements are assigned based on the hash value of their key. Buckets are numbered from 0 to (bucket_count-1). Hence this function returns the bucket no. where the element with key is located in unordered_map. 

Time Complexity: O(1).

Syntax:

unordered_map.bucket(k);
// k is the key corresponds to which
// we want to know bucket number.

Return Value: The order number of the bucket corresponding to key k.

There are two more functions regarding bucket: 

1. std::bucket_count: This function is used to count the total no. of buckets in the unordered_map. No parameter is required to pass into this function. 

Time Complexity: O(1).

Syntax:

unordered_map.bucket_count();

Return Value: The number of the bucket present in the hash table of unordered_map.

2. std::bucket_size: This function count the number of elements present in each bucket of the unordered_map. 

Time Complexity: Linear in the bucket size.

Syntax:

unordered_map.bucket_size(i); 
// 'i' is the bucket number in which we want 
// to find no. of elements. (i < bucket_count)

Return Value: The number of elements present in bucket ‘i’.

CPP

// C++ program to demonstrate the use of std::bucket
#include <bits/stdc++.h>
using namespace std;

// Driver Code
int main()
{
    // Declaring umap to be of <string, double> type
    // key will be of string type and mapped value will
    // be of double type
    unordered_map<string, double> umap;

    // inserting values by using [] operator
    umap["PI"] = 3.14;
    umap["root2"] = 1.414;
    umap["log10"] = 2.302;
    umap["loge"] = 1.0;
    umap["e"] = 2.718;

    // Display bucket no. where key, value pair is located
    // using bucket(key)
    for (auto& x : umap) {
        cout << "(" << x.first << ", " << x.second << ")";
        cout << " is in bucket= " << umap.bucket(x.first)
             << endl;
    }
    cout << endl;

    // Count the no.of buckets in the unordered_map
    // using bucket_count()
    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

(loge, 1) is in bucket= 5
(e, 2.718) is in bucket= 4
(log10, 2.302) is in bucket= 4
(PI, 3.14) is in bucket= 0
(root2, 1.414) is in bucket= 3

umap has 7 buckets.

Bucket 0 has 1 elements.
Bucket 1 has 0 elements.
Bucket 2 has 0 elements.
Bucket 3 has 1 elements.
Bucket 4 has 2 elements.
Bucket 5 has 1 elements.
Bucket 6 has 0 elements.

We can also print all the elements present in each bucket of the unordered_map. 

CPP

// C++ program to print all elements present in each bucket
#include <bits/stdc++.h>
using namespace std;

int main()
{
    // Declaring umap to be of <string, double> type
    // key will be of string type and mapped value
    // will be of double type
    unordered_map<string, double> umap;

    // inserting values by using [] operator
    umap["PI"] = 3.14;
    umap["root2"] = 1.414;
    umap["log10"] = 2.302;
    umap["loge"] = 1.0;
    umap["e"] = 2.718;

    unsigned n = umap.bucket_count();

    // Prints elements present in each bucket
    for (unsigned i = 0; i < n; i++) {
        cout << "Bucket " << i << " contains: ";
        for (auto it = umap.begin(i); it != umap.end(i);
             it++)
            cout << "(" << it->first << ", " << it->second
                 << ") ";
        cout << "\n";
    }
    return 0;
}

Output

Bucket 0 contains: (PI, 3.14) 
Bucket 1 contains: 
Bucket 2 contains: 
Bucket 3 contains: (root2, 1.414) 
Bucket 4 contains: (e, 2.718) (log10, 2.302) 
Bucket 5 contains: (loge, 1) 
Bucket 6 contains: 

Use of bucket in std::unordered_map: There is a number of algorithms that require the objects to be hashed into some number of buckets, and then each bucket is processed. Let say, you want to find duplicates in a collection. You hash all items in the collection, then in each bucket, you compare items pairwise. A bit less trivial example is the Apriori algorithm for finding frequent itemsets.