📅 Date: Feb 6, 2026
🔥 Topic: STL, Vector Basics, Capacity vs Size

📦 What is STL?

STL (Standard Template Library) is a collection of pre-written algorithms and data structures. Vector is the most used container in STL.

🚀 Vector: The Dynamic Array

A Vector is an array that grows automatically.
When it gets full, it doubles its capacity!

Key Functions:

  • v.push_back(x): Adds element to end.
  • v.pop_back(): Removes last element.
  • v.size(): How many elements are currently stored.
  • v.capacity(): How much space is actually reserved.

💻 Day 36 Code: Vector Playground

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

int main() {
    // Initialization
    vector<int> v = {1, 2, 3};

    cout << "Capacity: " << v.capacity() << endl;
    cout << "Size: " << v.size() << endl;

    // Adding element
    v.push_back(4);
    
    cout << "New Capacity: " << v.capacity() << endl; // Likely doubles to 6
    cout << "New Size: " << v.size() << endl;

    return 0;
}

💭 Thoughts

The magic of capacity doubling is efficient but costs memory. Using vector feels like cheating because it's so easy compared to arrays!