C++ flat_set::count() Function

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The std::flat_set::count() function in C++, is used to determine the number of elements in a flat_set that match the given key. Since the flat_set is a unique, sorted container, returns either 1 (if the key exists) or 0 (if it does not exists).

This function provides the efficient way to check for key existence without modifying the container. Unlike the std::map, which may stores duplicate keys, flat_set maintains only unique values.

Syntax

Following is the syntax for std::flat_set::find() function.

count( const Key& key ) const;

Parameters

• key − It indicates the key value of the element to count.

Return Value

This function returns the number of elements with keyt that compare the equivalent to key.

Example 1

Let's look at the following example, where we are going to check for an existing element.

#include <iostream>
#include <boost/container/flat_set.hpp>
int main() {
   boost::container::flat_set < int > x = {2,4,6,8};
   int a = 6;
   if (x.count(a)) {
      std::cout << a << " is present in the set.\n";
   } else {
      std::cout << a << " is not present in the set.\n";
   }
   return 0;
}

Output

Output of the above code is as follows −

6 is present in the set.

Example 2

Consider the following example, where we are going to search for the element that does not exist and observing the output.

#include <iostream>
#include <boost/container/flat_set.hpp>
int main() {
   boost::container::flat_set < int > a = {1,3,5,7};
   int x = 15;
   if (a.count(x)) {
      std::cout << x << " is present in the set.\n";
   } else {
      std::cout << x << " is not present in the set.\n";
   }
   return 0;
}

Output

Following is the output of the above code −

15 is not present in the set.

Example 3

In the following example, we are going to use the count() inside the loop.

#include <iostream>
#include <boost/container/flat_set.hpp>
int main() {
   boost::container::flat_set < int > a = {1,2,5,3};
   for (int x = 1; x <= 5; x++) {
      if (a.count(x)) {
         std::cout << x << " is found in the set.\n";
      } else {
         std::cout << x << " is missing from the set.\n";
      }
   }
   return 0;
}

Output

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

1 is found in the set.
2 is found in the set.
3 is found in the set.
4 is missing from the set.
5 is found in the set.