C++ Unordered_map::bucket_size() Function

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The C++ function unordered_map::bucket_size() function returns the number of elements present in the n^th bucket. 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. Buckets have numbers ranging from 0 to (bucket_count - 1).

This function return the number of elements in each bucket which is always lessthan the count.

Syntax

Following is the Syntax of unordered_map::bucket_size() function.

unordered_map.bucket_size(n);

Parameters

• n − It indicates the bucket number. that is an unsigned integral and should be lower than bucket_count.

Return value

This returns an unsigned integral that is the total number of elements in the current bucket.

Example 1

Following is the example, where we are going to use unordered_map::bucket_size().

#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 (int i = 0; i < um.bucket_count(); ++i)
      cout << "Bucket " << i << " contains "<< um.bucket_size(i) << " elements." << endl;
   return 0;
}

Output

Following is the output of the above code −

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 1 elements.
Bucket 7 contains 1 elements.
Bucket 8 contains 1 elements.
Bucket 9 contains 1 elements.
Bucket 10 contains 1 elements.
Bucket 11 contains 0 elements.
Bucket 12 contains 0 elements.

Example 2

Consider the following example, where we are going to count the number of buckets.

#include <iostream>
#include <string>
#include <unordered_map>
using namespace std;
int main () {
   std::unordered_map<std::string,std::string> map = {
      {"us","United States"},
      {"uk","United Kingdom"},
      {"fr","France"},
   };
   unsigned totalbuckets = map.bucket_count();
   unsigned element = map.bucket_size(9);
   cout<<"total number of buckets"<<": "<<totalbuckets<<endl;
   cout << "bucket 9 has " << element << " element:\n";
   return 0;
}

Output

Output of the above code is as follows −

total number of buckets: 13
bucket 9 has 1 element:

Example

In the following example, we are obtaining the number of buckets and their element sizes that have at least one element in the current unordered_map.

#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 (int i = 0; i < um.bucket_count(); ++i) {
      if(um.bucket_size(i)>0) {
         cout << "Bucket " << i << " contains "<< um.bucket_size(i) << " elements." << endl;
      }
   }
   return 0;
}

Output

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

Bucket 6 contains 1 elements.
Bucket 7 contains 1 elements.
Bucket 8 contains 1 elements.
Bucket 9 contains 1 elements.
Bucket 10 contains 1 elements.