Determine if a key exists in a map in C++

This post will discuss how to determine if a key exists in a map in C++.

1. Using `std::map::find`

The standard way to use the std::map::find function that searches a map for a key and returns an iterator to it, or the std::map::end if the key is not present in the map. The following code example shows invocation for this function:

#include <iostream>

#include <string>

#include <map>

int main()

{

std::map<std::string, int> map = {

{"one", 1}, {"two", 2}, {"three", 3}

};

std::string key = "two";

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

std::cout << "Key found" << std::endl;

} else {

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

}

return 0;

}

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Output:

Key found

2. Using `std::map::count`

Another option is to use std::map::count to get the total number of elements in the map with a particular key. If a key is present in the map, the count would be exactly 1 since all keys in the map container are unique. If the key is not found, the count function returns zero.

#include <iostream>

#include <string>

#include <map>

int main()

{

std::map<std::string, int> map = {

{"one", 1}, {"two", 2}, {"three", 3}

};

std::string key = "two";

if (map.count(key) != 0) {

std::cout << "Key found" << std::endl;

} else {

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

}

return 0;

}

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Output:

Key found

3. Using `std::map::contains`

Another option to check whether a particular key exists in the map is using the std::map::contains member function. This function is available since C++20. It returns true if the map contains an element with a specific key, false otherwise.

#include <iostream>

#include <string>

#include <map>

int main()

{

std::map<std::string, int> map = {

{"one", 1}, {"two", 2}, {"three", 3}

};

std::string key = "two";

if (map.contains(key)) {

std::cout << "Key found" << std::endl;

} else {

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

}

return 0;

}

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Output:

Key found

That’s all about determining if a key exists in a map in C++.

Determine if a key exists in a map in C++

1. Using std::map::find

2. Using std::map::count

3. Using std::map::contains

Share this:

1. Using `std::map::find`

2. Using `std::map::count`

3. Using `std::map::contains`