C++ flat_set::contains() Function

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The std::flat_set::contains() function in C++, is used to check whether the specified element in the flat_set. It returns a Boolean value true if the element is found, or else it returns false.

Unlike the find(), which returns an iterator , contains() offers a more direct and readable approach for testing. The time complexity of this function is O(log n).

Syntax

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

bool contains( const Key& key ) const;

Parameters

• key − It indicates the key value of the element to search for.

Return Value

This function returns the true if the element is found otherwise false.

Example 1

Let's look at the following example, where we are going to consider the basic usage of the contains() function.

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

Output

Output of the above code is as follows −

Element is found 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 > x = {1,3,5,7};
   int a = 2;
   if (x.contains(a)) {
      std::cout << "Element exists in the set.\n";
   } else {
      std::cout << "Element is not found.\n";
   }
   return 0;
}

Output

Following is the output of the above code −

Element is not found.

Example 3

In the following example, we are going to iterate over multiple numbers and use teh contains() to verfity their presence in the flat_set.

#include <iostream>
#include <boost/container/flat_set.hpp>
int main() {
   boost::container::flat_set < int > x = {2,4,6,8};
   int test_values[] = {1,2,3,4};
   for (int a: test_values) {
      std::cout << "Checking " << a << ": " <<
         (x.contains(a) ? "Found\n" : "Not Found\n");
   }
   return 0;
}

Output

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

Checking 1: Not Found
Checking 2: Found
Checking 3: Not Found
Checking 4: Found