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C++ Unordered_multimap Library - rehash() Function
Description
The C++ function std::unordered_multimap::rehash() sets the number of buckets in the container to n or more.
If n is greater than the current number of buckets in the container, a rehash is forced. The new bucket count can either be equal or greater than n.
If n is lower than the current number of buckets in the container, the function may have no effect on the bucket count and may not force a rehash.
Declaration
Following is the declaration for std::unordered_multimap::rehash() function form std::unordered_map() header.
C++11
void rehash(size_type n);
Parameters
n − New number of buckets
Return value
None
Time complexity
Linear i.e. O(n) in average case.
Quadratic i.e. O(n2) in worst case.
Example
The following example shows the usage of std::unordered_multimap::rehash() function.
#include <iostream> #include <unordered_map> using namespace std; int main (void) { unordered_multimap<char, int> umm; cout << "Initial bucket_count: " << umm.bucket_count() << endl; umm.rehash(20); cout << "Current bucket_count: " << umm.bucket_count() << endl; return 0; }
Let us compile and run the above program, this will produce the following result −
Initial bucket_count: 11 Current bucket_count: 23