C++ STL: Containers and Algorithms

What is the STL?

The Standard Template Library (STL) provides generic, reusable containers and algorithms. The main components are containers, iterators, and algorithms.

vector — Dynamic Array

#include <vector>
using namespace std;

vector<int> v = {3, 1, 4, 1, 5};
v.push_back(9);          // add to end
v.pop_back();            // remove from end
cout << v[0];            // 3
cout << v.size();        // 5

for (auto x : v) cout << x << " ";  // 3 1 4 1 5

map — Key-Value Store

#include <map>

map<string, int> scores;
scores["Alice"] = 95;
scores["Bob"]   = 87;

cout << scores["Alice"]; // 95
scores.erase("Bob");

for (auto& [key, val] : scores) {  // C++17 structured binding
    cout << key << ": " << val << "
";
}

set — Unique Sorted Elements

#include <set>

set<int> s = {5, 2, 8, 2, 1};
// s contains: {1, 2, 5, 8}  — sorted, no duplicates
s.insert(3);
s.erase(2);
cout << s.count(5); // 1 (exists), 0 (not exists)

queue and stack

#include <queue>
#include <stack>

queue<int> q;
q.push(1); q.push(2); q.push(3);
cout << q.front(); // 1
q.pop();

stack<int> st;
st.push(10); st.push(20);
cout << st.top(); // 20
st.pop();

Sorting and Algorithms

#include <algorithm>

vector<int> v = {5, 2, 8, 1, 9};
sort(v.begin(), v.end());              // ascending
sort(v.begin(), v.end(), greater<int>()); // descending

auto it = find(v.begin(), v.end(), 8);    // find element
int cnt = count(v.begin(), v.end(), 1);   // count occurrences
int mx = *max_element(v.begin(), v.end()); // maximum

unordered_map — O(1) Hash Map

#include <unordered_map>

unordered_map<string, int> freq;
string word;
while (cin >> word) freq[word]++;

// O(1) average lookup vs O(log n) for map