C++ STL Containers: map, set, unordered_map, queue and stack

The Standard Template Library (STL) gives you ready containers. You already know vector and array. Here are the others.

map: key and value, sorted

#include <iostream>
#include <map>
#include <string>

int main() {
    std::map<std::string, int> marks;
    marks["Riya"] = 88;
    marks["Aman"] = 72;
    marks.insert({"Meera", 95});

    std::cout << marks["Riya"] << ' ' << marks.at("Aman") << ' ' << marks.size() << '\n';
    std::cout << marks.contains("Karan") << ' ' << marks.count("Meera") << '\n';

    for (const auto& [name, m] : marks) {              // sorted by key
        std::cout << name << " = " << m << '\n';
    }
}

Careful: marks["Karan"] on a missing key creates it with the value 0. Use find, contains or at to only look.

unordered_map: faster, no order

#include <iostream>
#include <string>
#include <unordered_map>

int main() {
    std::unordered_map<std::string, int> count;
    for (std::string w : {"to", "be", "or", "not", "to", "be"}) {
        count[w]++;
    }
    std::cout << count["to"] << ' ' << count["be"] << ' ' << count["or"] << '\n';
}

set: unique values

#include <iostream>
#include <set>

int main() {
    std::set<int> s = {5, 3, 9, 3, 1, 5};
    s.insert(7);
    for (int x : s) std::cout << x << ' ';               // sorted and unique
    std::cout << '\n' << s.contains(9) << ' ' << s.size() << '\n';
}

stack, queue and deque

#include <deque>
#include <iostream>
#include <queue>
#include <stack>

int main() {
    std::stack<int> st;
    st.push(1); st.push(2); st.push(3);
    std::cout << st.top() << '\n';                        // last in, first out
    st.pop();
    std::cout << st.top() << '\n';

    std::queue<std::string> q;
    q.push("first"); q.push("second");
    std::cout << q.front() << '\n';                       // first in, first out
    q.pop();
    std::cout << q.front() << '\n';

    std::deque<int> d = {2, 3};
    d.push_front(1);
    d.push_back(4);
    for (int x : d) std::cout << x << ' ';
    std::cout << '\n';
}

Which container?

Need	Use
A list which grows, fast reading by index	std::vector (the default)
Fixed size	std::array
Key and value, sorted	std::map
Key and value, fastest search	std::unordered_map
Unique values	std::set or std::unordered_set
Add and remove at both ends	std::deque
Stack, queue	std::stack, std::queue
Many inserts in the middle with stable iterators	std::list (rare)

• Previous Page
• Next Page

---

Exercises & Assignments
No Content Found.

Interview Questions & Answers
No Content Found.