Pair in C++

Map can be Converted to pairs

vector<vector<int>> vec{{1,2},{3,4},{5,6}}
unordered_map<int,int> mp{{1,2},{3,4},{5,6}}

for(pair<int,int> v: vec){ } ❌ Vector Can’t be converted into pairs.
for(pair<int,itn> m: mp){ } ✅ Map Can be converted into pairs.

m.first access first element of pair
m.second access second element of pair

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Pair

In C++, a pair is a simple container defined in the <utility> header that holds two values, which may be of different types. The types of these values are specified as template parameters. Here’s a quick guide on using pair:

Create Pair ⭐

#include <utility>

// Using std::make_pair
std::pair<int, std::string> p1 = std::make_pair(1, "Hello");

// Direct initialization
std::pair<int, std::string> p2(2, "World");

Access Elements

std::cout << "First element: " << p1.first << std::endl;
std::cout << "Second element: " << p1.second << std::endl;

Modifying elements

p1.first = 10;
p1.second = "Changed";

Comparing Pairs

std::pair<int, int> p3(10, 20);
std::pair<int, int> p4(10, 30);

if (p3 < p4) {
    std::cout << "p3 is less than p4" << std::endl;
}

*Note: *Comparisons are lexicographical, meaning they compare the first elements first, and if they are equal, they compare the second elements.

You can use pairs as elements in STL containers like std::vector, std::map, etc.

#include <vector>

std::vector<std::pair<int, std::string>> vec;
vec.push_back(std::make_pair(1, "One"));
vec.push_back(std::make_pair(2, "Two"));

A pair can hold two values of different types, which can be useful in various contexts.

std::pair<int, double> p5(10, 3.14);
std::pair<std::string, char> p6("Example", 'A');

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you cannot directly access the elements of a std::pair in C++ using the syntax p[0] and p[1]. The std::pair class does not overload the operator[], so trying to use indexing like this will result in a compilation error.

pair<char,char> p  = { 'a', 'b'}; ✅
// or
pair<char,char> p('a', 'b'); ✅

p.first; // ='a' ✅
p.second; // ='b' ✅

p[0]; // ='a'? ❌ Compilation error
p[1]; // ='b'? ❌ Compilation error