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C++ Unordered_set::reserve() function
The C++ unordered_set::reserve() function is used to change the capacity of the bucket. It sets the number of buckets in the container (bucket_count) to the most appropriate to contain at least n elements without exceeding maximum load factor and rehashes the container.
A rehash is forced when n is greater than the current bucket_count multiplied by the max_load_factor, and increase in the container bucket count, causes a rehash.
Syntax
Following is the syntax of std::unordered_set::reserve() function.
void reserve ( size_type n );
Parameters
- n − It inidicates the minimum number of buckets.
Return Value
This function does not returns anything.
Example 1
Let's look at the following example, where we are going to demonstrate the usage of unordered_set::reserve() function.
#include <iostream> #include <string> #include <unordered_set> using namespace std; int main () { unordered_set<string> uSet; uSet.reserve(3); uSet.insert("android"); uSet.insert("java"); uSet.insert("html"); cout << "uSet contains:"; for (const string& x: uSet) cout << " " << x; cout << endl; cout<<"After reserve bucket count is: "<< uSet.bucket_count(); return 0; }
Output
If we run the above code it will generate the following output −
uSet contains: html java android After reserve bucket count is: 3
Example 2
In the following example, we are going to count the number of buckets before and after the use of the unordered_set::reserve() function.
#include <iostream> #include <string> #include <unordered_set> using namespace std; int main () { unordered_set<string> uSet = {"android", "java", "Html", "CSS"}; cout<<"Initial bucket count is: "<< uSet.bucket_count() << endl; uSet.reserve(3); cout << "uSet contains:"; for (const string& x: uSet) cout << " " << x; cout << endl; cout<<"After reserve bucket count is: "<< uSet.bucket_count(); return 0; }
Output
Following is the output of the above code −
Initial bucket count is: 13 uSet contains: android java Html CSS After reserve bucket count is: 5
Example 3
Consider the following example, where we are going to display the buckets and their elements before and after the use of the unordered_set::reserve() function.
#include <iostream> #include <unordered_set> using namespace std; int main () { unordered_set<string> uSet={"Hyderabad", "Delhi", "Bangalore"}; cout<<"unordered_set contains "<<uSet.bucket_count()<<" buckets:"; for(unsigned int i = 0; i < uSet.bucket_count(); i++) { cout<<"\nThe bucket "<<i<<" contains: "; for(auto it = uSet.begin(i); it != uSet.end(i); ++it) { cout<<*it<<" "; } } cout<<"\n***Capacity is changed using reserve function***\n"; uSet.reserve(5); cout<<"unordered_set contains "<<uSet.bucket_count()<<" buckets:"; for(unsigned int i = 0; i < uSet.bucket_count(); i++) { cout<<"\nThe bucket "<<i<<" contains: "; for(auto it = uSet.begin(i); it != uSet.end(i); ++it) { cout<<*it<<" "; } } return 0; }
Output
Output of the above code is as follows −
unordered_set contains 13 buckets: The bucket 0 contains: Bangalore The bucket 1 contains: The bucket 2 contains: Hyderabad The bucket 3 contains: Delhi The bucket 4 contains: The bucket 5 contains: The bucket 6 contains: The bucket 7 contains: The bucket 8 contains: The bucket 9 contains: The bucket 10 contains: The bucket 11 contains: The bucket 12 contains: ***Capacity is changed using reserve function*** unordered_set contains 5 buckets: The bucket 0 contains: The bucket 1 contains: Bangalore The bucket 2 contains: The bucket 3 contains: Delhi The bucket 4 contains: Hyderabad