Check if an element is present in a set in C++

This post will discuss how to check if an element is present in a set in C++.

1. Using `find()` function

The standard solution to check for existence of an element in the set container (std::set or std::unordered_set) is to use its member function find(). If the specified element is found, an iterator to the element is returned; otherwise, an iterator to the end of the container is returned.

#include <iostream>

#include <unordered_set>

using namespace std;

int main()

{

std::unordered_set<int> s = { 1, 2, 3, 4, 5 };

int key = 3;

if (s.find(key) != s.end()) {

std::cout << "Element is present in the set" << std::endl;

}

else {

std::cout << "Element not found" << std::endl;

}

return 0;

}

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Output:

Element is present in the set

2. Using `count()` function

Another good alternative is to use the count() function of the set container. It returns value 1 if the element is found in the set container, otherwise 0 is returned.

#include <iostream>

#include <unordered_set>

using namespace std;

int main()

{

std::unordered_set<int> s = { 1, 2, 3, 4, 5 };

int key = 3;

if (s.count(key)) {

std::cout << "Element is present in the set" << std::endl;

}

else {

std::cout << "Element not found" << std::endl;

}

return 0;

}

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Output:

Element is present in the set

3. Naive Solution

We can also write our own routine for this. The idea is to iterate through the contents of the set using a range-based for-loop and compare each element against the given key.

#include <iostream>

#include <unordered_set>

using namespace std;

int main()

{

std::unordered_set<int> s = { 1, 2, 3, 4, 5 };

int key = 3;

bool found = false;

for (auto const &e: s)

{

if (e == key)

{

found = true;

break;

}

if (found) {

std::cout << "Element is present in the set" << std::endl;

}

else {

std::cout << "Element not found" << std::endl;

}

return 0;

}

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Output:

Element is present in the set

That’s all about determining whether an element is present in a set in C++.

Check if an element is present in a set in C++

1. Using find() function

2. Using count() function

3. Naive Solution

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1. Using `find()` function

2. Using `count()` function