C++ Unordered_multimap::bucket() Function

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The C++ std::unordered_multimap::bucket() function is used to 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. The size of the bucket is in the range of 0 to bucket_count-1.

Syntax

Following is the syntax of std::unordered_multimap::bucket() function.

size_type bucket(const Key& keyval) const;

Parameters

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

Return value

This function returns the unsigned integral type that is the order number of the bucket corresponding to key.

Example 1

In the following example let's see the usage of unordered_multimap::bucket() function.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> umm = {
      {'a', 1},
      {'b', 2},
      {'c', 3},
      {'d', 4},
      {'e', 5}
   };
   cout << "Unordered multimap contains following elements" << endl;
   for (auto it = umm.begin(); it != umm.end(); ++it) {
      cout << "Element " << "[" << it->first  << " : "<< it->second << "] " << "is in " << umm.bucket(it->first) << " bucket." << endl; 
   }
   return 0;
}

Output

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

Unordered multi-map contains following elements
Element [e : 5] is in 1 bucket.
Element [d : 4] is in 0 bucket.
Element [c : 3] is in 4 bucket.
Element [b : 2] is in 3 bucket.
Element [a : 1] is in 2 bucket.

Example 2

Let's look at the following example, where we are going to create a unordered_multimap that stores the string values and applying the bucket() function to count the number of buckets assigned to each other.

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

Output

If we run the above code it will generate the following output −

Element [Akash : Daltonganj] is in 4 bucket.
Element [Vivek : Kanpur] is in 4 bucket.
Element [Vivek : Kanpur] is in 4 bucket.
Element [Aman : Ranchi] is in 3 bucket.
Element [Aman : Ranchi] is in 3 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_multimap.

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

Output

Following is the output of the above code −

The bucket number of key Akash is 4

Example 4

Following is the example, where we are going to display the bucket number of the specified key from the current unordered_multimap.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> umm = {
      {'A', 1},
      {'B', 2},
      {'C', 3},
      {'A', 1},
      {'D', 4},
      {'A', 1},
      {'E', 5},
      {'D', 4},
   };
   cout<<umm.bucket('A')<<endl;
   cout<<umm.bucket('D')<<endl;
   return 0;
}

Output

Output of the above code is as follows −

10
2