C++ Unordered_map::bucket() Function

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The C++ function unordered_map::bucket() function returns the bucket number where element with key k is located. Bucket is a memory space in the container's hash table to which elements are assigned based on the hash value of their key. Valid range of buckets is from 0 to bucket_count - 1.

Syntax

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

size_type bucket(const key_type& k) const;

Parameters

• k − It indicates the key whose bucket is to be located.

Return value

Returns the unsigned integral type that is the order number of the bucket corresponding to key k.

Example 1

Following is the example, where we are going to demonstrate the usage of unordered_map::bucket() 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}
   };
   for (auto it = um.begin(); it != um.end(); ++it) {
      cout << "Element " << "[" << it->first  << " : "<< it->second << "] " << "is in "<< um.bucket(it->first) << " bucket." << endl; 
   }
   return 0;
}

Output

Following is the output of the above code −

Element [e : 5] is in 3 bucket.
Element [d : 4] is in 2 bucket.
Element [c : 3] is in 1 bucket.
Element [b : 2] is in 0 bucket.
Element [a : 1] is in 6 bucket.

Example 2

In the following example, we are creating an unordered_map that stores the only string value and counting the number of buckets assigned to each name in the current unordered_map.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_map<string, string> um = {
      {"Aman", "Ranchi"},
      {"Vivek", "Kanpur"},
      {"Akash", "Daltonganj"},
      {"Revathi", "Wrangle"},
      {"Sarika", "Banaras"}
   };
   for (auto it = um.begin(); it != um.end(); ++it) {
      cout << "Element " << "[" << it->first  << " : "
          << it->second << "] " << "is in " 
          << um.bucket(it->first) << " bucket." << endl;
   }
   return 0;
}

Output

Following is the output of the above code −

Element [Sarika : Banaras] is in 9 bucket.
Element [Revathi : Wrangle] is in 10 bucket.
Element [Akash : Daltonganj] is in 5 bucket.
Element [Vivek : Kanpur] is in 4 bucket.
Element [Aman : Ranchi] is in 8 bucket.

Example 3

Consider the following example, where we are going to displaying the number of buckets of the iteration pointing to the first element of the container from unordered_map.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_map<string, string> um = {
      {"Aman", "Ranchi"},
      {"Vivek", "Kanpur"},
      {"Akash", "Daltonganj"},
      {"Revathi", "Wrangle"},
      {"Sarika", "Banaras"}
   };
    // prints the bucket number of the beginning element
    auto it = um.begin();
    // stores the bucket number of the key k
    int number = um.bucket(it->first);
    cout << "The bucket number of key " << it->first << " is " << number;
   return 0;
}

Output

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

The bucket number of key Sarika is 9