Working with Pairs in C++
In this article, we will learn what are pairs in C++, and how to initialize pairs in C++. Some common C++pairs STL functions. Later in the article, we will also discuss some examples to get a better understanding of pairs in C++.
The C++ pair is similar to a Python tuple and consists of two elements, “first” and “second”, in a fixed order. The “first” element is the first element in the pair, and the “second” element is the second. The header file for the pair in C++ is located in the <utility> header and it is also a container class in the Standard Template Library (STL). In this tutorial, we will go in-depth into the representation and implementation of pairs in C++. So, without further ado, let’s get started!
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What are Pairs in C++?
The C++ pair container is a simple header file that holds two distinct elements or objects in a specific order. The first element is referred to as “first” and the second element is referred to as “second.” This container allows for the storage of two values as a single entity, making it easier to access and manipulate the values. The values stored in a pair can be of different or the same data types. By default, pair objects are used in maps or HashMap as the default list type, where the “first” component serves as a unique key for the “second” component.
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Syntax
pair <dtype1, dtype2> pair_name;
The parameters are:
- dtype1 : data type of the first element.
- dtype2 : data type of the second element.
How to Initialize Pairs in C++?
There are several ways to initialize a C++ pair:
- Using make_pair function:
#include <utility>#include <iostream>
int main(){ auto p = std::make_pair(1, "hello"); std::cout << p.first << " " << p.second << std::endl; return 0;}
Output:
1 hello
2. Using the pair constructor:
#include <utility>#include <iostream>
int main(){ std::pair<int, std::string> p(1, "hello"); std::cout << p.first << " " << p.second << std::endl; return 0;}
Output:
1 hello
3. Using uniform initialization:
#include <utility>#include <iostream>
int main(){ std::pair<int, std::string> p{1, "hello"}; std::cout << p.first << " " << p.second << std::endl; return 0;}
Output:
1 hello
4. Using direct assignment:
#include <utility>#include <iostream>
int main(){ std::pair<int, std::string> p; p.first = 1; p.second = "hello"; std::cout << p.first << " " << p.second << std::endl; return 0;}
Output:
1 hello
In each of these examples, a pair object is created with first set to 1 and second set to “hello”. The output of each program will be 1 hello.
Common C++ Pairs STL Functions
Here is a table of some commonly used C++ pair Standard Template Library (STL) functions:
| Function | Description |
| make_pair | Constructs a pair object with the given values. |
| pair.first | Accesses the first element of the pair. |
| pair.second | Accesses the second element of the pair. |
| swap(pair1, pair2) | Swaps the elements of two pair objects. |
| operator==(pair1, pair2) | Compares two pair objects for equality. |
| operator!=(pair1, pair2) | Compares two pair objects for inequality. |
| operator<(pair1, pair2) | Determines if the first pair object is less than the second pair object. |
| operator>(pair1, pair2) | Determines if the first pair object is greater than the second pair object. |
| operator<=(pair1, pair2) | Determines if the first pair object is less than or equal to the second pair object. |
| operator>=(pair1, pair2) | Determines if the first pair object is greater than or equal to the second pair object. |
Please note that this is not an exhaustive list of all pair functions available in the C++ STL.
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Example of C++ Pairs
Example 1:
#include <iostream>#include <utility>#include <vector>
int main(){ // Initializing a pair using make_pair std::pair<int, std::string> p1 = std::make_pair(1, "one"); std::cout << "First element of pair p1: " << p1.first << std::endl; std::cout << "Second element of pair p1: " << p1.second << std::endl;
// Initializing a pair using constructor std::pair<int, std::string> p2(2, "two"); std::cout << "First element of pair p2: " << p2.first << std::endl; std::cout << "Second element of pair p2: " << p2.second << std::endl;
// Initializing a vector of pairs std::vector<std::pair<int, std::string>> v; v.push_back(std::make_pair(3, "three")); v.push_back(std::make_pair(4, "four")); v.push_back(std::make_pair(5, "five"));
// Printing the vector of pairs for (const auto &p : v) { std::cout << "First element of pair: " << p.first << std::endl; std::cout << "Second element of pair: " << p.second << std::endl; }
return 0;}
Output 1
First element of pair p1: 1 Second element of pair p1: one First element of pair p2: 2 Second element of pair p2: two First element of pair: 3 Second element of pair: three First element of pair: 4 Second element of pair: four First element of pair: 5 Second element of pair: five
This program creates three pair objects and adds them to a vector. The program then iterates through the vector and prints the first and second elements of each pair.
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Example 2:
#include <iostream>#include <utility>#include <map>#include <string>
int main(){ // Defining a map of employees and their respective ages std::map<std::string, int> employees = { {"John", 25}, {"Jane", 30}, {"Bob", 35} };
// Iterating through the map and printing employee names and their ages for (const auto &employee : employees) { std::cout << "Employee name: " << employee.first << std::endl; std::cout << "Employee age: " << employee.second << std::endl; }
// Adding a new employee to the map employees.insert(std::make_pair("Samantha", 28));
// Finding the age of an employee using find function auto it = employees.find("Jane"); if (it != employees.end()) { std::cout << "Jane's age is: " << it->second << std::endl; } else { std::cout << "Jane not found." << std::endl; }
return 0;}
Output 2:
Employee name: Bob Employee age: 35 Employee name: Jane Employee age: 30 Employee name: John Employee age: 25 Jane's age is: 30
In this example, a map of employees and their respective ages is defined. The program then iterates through the map and prints the name and age of each employee. A new employee is added to the map using the insert function. Finally, the program uses the find function to search for the age of an employee, and if found, prints the age.
Nested C++ Pairs
Nested C++ pairs are pairs that contain other pairs as their elements. In other words, a pair can have its first or second element as another pair, allowing you to create more complex data structures. For example, you can create a pair that contains two pairs, and each of those pairs contains two integers. This allows you to represent a two-dimensional structure as a single entity. Nested pairs can be used to create complex data structures or to return multiple values from a function, which can be useful in certain situations.
Here’s an example of nested pairs in C++:
#include <iostream>#include <utility>
int main(){ std::pair<std::pair<int, int>, std::pair<int, int>> nested_pair;
nested_pair.first.first = 10; nested_pair.first.second = 20; nested_pair.second.first = 30; nested_pair.second.second = 40;
std::cout << "First pair: " << nested_pair.first.first << " " << nested_pair.first.second << std::endl; std::cout << "Second pair: " << nested_pair.second.first << " " << nested_pair.second.second << std::endl;
return 0;}
Output:
First pair: 10 20 Second pair: 30 40
In this example, a nested pair is created and initialized with four integers. The program then accesses and prints the values of the nested pairs.
Endnotes
Hope this article helped you grasp the concept of pairs in C++. The C++ pair container is a simple yet powerful data structure that allows you to store two disparate elements as a single entity.
Explore our C++ articles to find out more about the language and consolidate your knowledge of the fundamentals.
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