C++ Standard Template Library (STL) pair tutorial

The pair in the C++ Standard Template Library (STL) is a simple and versatile utility to store two related values in a single object.

It is often used in containers, like map or set, where pairs of values are required. pair can store two elements of potentially different data types and can be accessed directly by first and second.

Key Features of pair

1. Two Values: Stores two values, which can be of different types.
2. Easy Access: Access the values using first and second.
3. Comparison Operators: Supports comparison operators (==, <, >, etc.).
4. Versatile: Useful in complex containers like map and multimap.

Basic Syntax

#include <utility>
using namespace std;

pair<Type1, Type2> p;

Let’s explore pair with various examples.

1. Initializing a pair

A pair can be initialized with values directly or by using the make_pair function.

#include <iostream>
#include <utility>
using namespace std;

int main() {
    // Initialize using constructor
    pair<string, int> person1("Alice", 25);

    // Initialize using make_pair
    pair<string, int> person2 = make_pair("Bob", 30);

    // Display pairs
    cout << "Person 1: " << person1.first << ", Age: " << person1.second << endl;
    cout << "Person 2: " << person2.first << ", Age: " << person2.second << endl;

    return 0;
}

Output:

Person 1: Alice, Age: 25
Person 2: Bob, Age: 30

2. Accessing Elements

You can access the two elements in a pair using the first and second members.

#include <iostream>
#include <utility>
using namespace std;

int main() {
    pair<string, double> product("Laptop", 799.99);

    // Accessing elements
    cout << "Product: " << product.first << endl;
    cout << "Price: $" << product.second << endl;

    return 0;
}

Output:

Product: Laptop
Price: $799.99

3. Using make_pair to Create a pair

The make_pair function provides a convenient way to create a pair without specifying the types explicitly.

#include <iostream>
#include <utility>
using namespace std;

int main() {
    // Using make_pair to create a pair
    auto book = make_pair("1984", 12.99);

    // Display pair
    cout << "Book: " << book.first << endl;
    cout << "Price: $" << book.second << endl;

    return 0;
}

Output:

Book: 1984
Price: $12.99

4. Comparing Pairs

Pairs support comparison operators such as ==, <, >, etc., which compare first and second values lexicographically.

#include <iostream>
#include <utility>
using namespace std;

int main() {
    pair<int, int> p1(1, 5);
    pair<int, int> p2(1, 10);

    // Compare pairs
    if (p1 < p2) {
        cout << "p1 is less than p2" << endl;
    } else {
        cout << "p1 is not less than p2" << endl;
    }

    return 0;
}

Output:

p1 is less than p2

5. Swapping Pairs

The swap function exchanges the contents of two pair objects.

#include <iostream>
#include <utility>
using namespace std;

int main() {
    pair<string, int> p1("Alice", 25);
    pair<string, int> p2("Bob", 30);

    // Swapping pairs
    p1.swap(p2);

    // Display swapped pairs
    cout << "After swap:" << endl;
    cout << "p1: " << p1.first << ", " << p1.second << endl;
    cout << "p2: " << p2.first << ", " << p2.second << endl;

    return 0;
}

Output:

After swap:
p1: Bob, 30
p2: Alice, 25

6. Using Pairs in Containers

pair is commonly used with STL containers such as vector, map, and set.

#include <iostream>
#include <vector>
#include <utility>
using namespace std;

int main() {
    vector<pair<string, int>> people;

    // Adding pairs to a vector
    people.push_back(make_pair("Alice", 25));
    people.push_back(make_pair("Bob", 30));
    people.push_back(make_pair("Charlie", 35));

    // Display vector of pairs
    for (const auto &person : people) {
        cout << person.first << ": " << person.second << endl;
    }

    return 0;
}

Output:

Alice: 25
Bob: 30
Charlie: 35

7. Using Pairs with map

map uses pair to store key-value pairs, where first represents the key and second represents the value.

#include <iostream>
#include <map>
using namespace std;

int main() {
    map<string, int> age;

    // Adding pairs to the map
    age.insert(make_pair("Alice", 25));
    age.insert(make_pair("Bob", 30));
    age["Charlie"] = 35;

    // Display map elements
    for (const auto &entry : age) {
        cout << entry.first << ": " << entry.second << endl;
    }

    return 0;
}

Output:

Alice: 25
Bob: 30
Charlie: 35

8. Pair Inside Another Pair

You can nest pairs within pairs, allowing more complex data structures.

#include <iostream>
#include <utility>
using namespace std;

int main() {
    // Pair inside a pair
    pair<int, pair<string, double>> product = make_pair(1, make_pair("Laptop", 799.99));

    // Access nested pair
    cout << "Product ID: " << product.first << endl;
    cout << "Product Name: " << product.second.first << endl;
    cout << "Price: $" << product.second.second << endl;

    return 0;
}

Output:

Product ID: 1
Product Name: Laptop
Price: $799.99

9. Initializing Pair Arrays

You can create arrays or vectors of pairs for storing multiple pairs.

#include <iostream>
#include <utility>
using namespace std;

int main() {
    // Array of pairs
    pair<string, int> students[] = {
        make_pair("Alice", 25),
        make_pair("Bob", 30),
        make_pair("Charlie", 35)
    };

    // Display array of pairs
    for (const auto &student : students) {
        cout << student.first << ": " << student.second << endl;
    }

    return 0;
}

Output:

Alice: 25
Bob: 30
Charlie: 35

10. Pair as Return Type from Functions

A pair can be a convenient return type for functions that need to return two values.

#include <iostream>
#include <utility>
using namespace std;

// Function returning a pair of name and age
pair<string, int> getPerson() {
    return make_pair("Alice", 25);
}

int main() {
    pair<string, int> person = getPerson();

    // Display pair returned by function
    cout << "Name: " << person.first << endl;
    cout << "Age: " << person.second << endl;

    return 0;
}

Output:

Name: Alice
Age: 25

Summary Table of pair Operations

Complete Example

This example demonstrates initializing pairs, using make_pair, nesting pairs, comparing pairs, swapping pairs, and storing pairs in a container.

#include <iostream>
#include <utility>
#include <vector>
using namespace std;

int main() {
    // Initialize pairs
    pair<string, int> p1("Alice", 25);
    pair<string, int> p2 = make_pair("Bob", 30);

    // Compare pairs
    if (p1 < p2) {
        cout << "p1 is less than p2" << endl;
    }

    // Swap pairs
    p1.swap(p2);

    cout << "After swap:" << endl;
    cout << "p1: " << p1.first << ", Age: " << p1.second << endl;
    cout << "p2: " << p2.first << ", Age: " << p2.second << endl;

    // Nested pair
    pair<int, pair<string, double>> product = make_pair(1, make_pair("Laptop", 799.99));
    cout << "Product ID: " << product.first << endl;
    cout << "Product Name: " << product.second.first << endl;
    cout << "Price: $" << product.second.second << endl;

    // Store pairs in a vector
    vector<pair<string, int>> people = {make_pair("Charlie", 35), make_pair("Dave", 28)};
    cout << "People in vector:" << endl;
    for (const auto &person : people) {
        cout << person.first << ": " << person.second << endl;
    }

    return 0;
}

Sample Output:

p1 is less than p2
After swap:
p1: Bob, Age: 30
p2: Alice, Age: 25
Product ID: 1
Product Name: Laptop
Price: $799.99
People in vector:
Charlie: 35
Dave: 28

Key Takeaways

• pair is a versatile utility that stores two related values, making it ideal for key-value pairs in containers.
• Access the values with first and second, and use make_pair for easy initialization.
• Comparisons and swapping make pair useful in containers, and it works well in complex structures like map and nested containers.