unordered_map rehash in C++ STL

The std::unordered_map::rehash() is a built in function C++ STL which which sets the number of buckets in container to n or more.
Syntax

void rehash( size_type s );
  • If s is greater than the current buckets into the container then rehashed is done. The new bucket count can be greater than or equal to n.
  • If s is less than current bucket count then there may or may not be any effect of function. It totally depends upon compiler

Parameters : It takes New number of buckets into the container.

Return type : Its return type is none.

Example-1:

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// C++ code to illustrate the method
// unordered_map rehash
#include <bits/stdc++.h>
  
using namespace std;
  
int main()
{
    unordered_map<char, int> sample;
  
    // Map initialization
    sample = { { 'a', 2 }, { 'b', 4 }, { 'c', 6 } };
  
    cout << " Size of container : " << sample.size() << endl;
    cout << " Initial bucket count : " << sample.bucket_count() << endl;
  
    // changing rehash
  
    sample.rehash(30);
    cout << " Size of container : " << sample.size() << endl;
    cout << " Now bucket count is :  " << sample.bucket_count() << endl;
    return 0;
}

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Output:

Size of container : 3
 Initial bucket count : 5
 Size of container : 3
 Now bucket count is :  31

Example-2:

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// C++ code to illustrate the method
// unordered_map rehash
#include <bits/stdc++.h>
  
using namespace std;
  
int main()
{
    unordered_map<int, int> sample;
  
    // Map initialization
    sample = { { 2, 2 }, { 3, 4 }, { 4, 6 }, { 5, 8 } };
  
    cout << " Size of container : " << sample.size() << endl;
    cout << " Initial bucket count : " << sample.bucket_count() << endl;
  
    // changing rehash
  
    sample.rehash(20);
    cout << " Size of container : " << sample.size() << endl;
    cout << " Now bucket count is :  " << sample.bucket_count() << endl;
    return 0;
}

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Output:

Size of container : 4
 Initial bucket count : 7
 Size of container : 4
 Now bucket count is :  23


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