STL vector in C++ (Standard Template Library) 🚀

 

The STL vector is a dynamic array that automatically resizes when elements are added or removed. Unlike std::array, it is resizable and more flexible.

✅ Key Features:
✔ Dynamic Size – Grows/shrinks as needed.
✔ Supports Iterators – Easy traversal.
✔ Random Access – vector[i] is fast.
✔ Built-in Functions – push_back(), pop_back(), sort(), etc.

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1️⃣ Syntax & Declaration

#include <iostream>
#include <vector>  // Include vector library
using namespace std;

int main() {
    vector<int> vec = {10, 20, 30, 40, 50};  // Declare and initialize

    cout << "First Element: " << vec[0] << endl;
    cout << "Last Element: " << vec.back() << endl;

    return 0;
}

🔹 Output

First Element: 10
Last Element: 50

✅ Unlike std::array, vector size is flexible!

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2️⃣ Adding & Removing Elements

🔹 Adding Elements

vector<int> vec;
vec.push_back(10);  // Add 10
vec.push_back(20);
vec.push_back(30);

🔹 Removing Elements

vec.pop_back();  // Removes last element

🔹 Insert Elements

vec.insert(vec.begin() + 1, 15);  // Insert 15 at index 1

🔹 Erase Elements

vec.erase(vec.begin() + 1);  // Removes element at index 1

✅ vector automatically manages memory! 🚀

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3️⃣ Traversing a Vector

🔹 Using a Simple Loop

for (int i = 0; i < vec.size(); i++) {
    cout << vec[i] << " ";
}

🔹 Using a Range-Based Loop

for (int val : vec) {
    cout << val << " ";
}

🔹 Using Iterators

for (auto it = vec.begin(); it != vec.end(); it++) {
    cout << *it << " ";
}

✅ Vectors support iterators, making traversal easy!

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4️⃣ Common Vector Functions

🔹 Size & Capacity

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

🔹 Accessing Elements

cout << vec.front();   // First element
cout << vec.back();    // Last element
cout << vec.at(2);     // Safe access

🔹 Checking if Empty

if (vec.empty()) cout << "Vector is empty";

🔹 Resizing Vector

vec.resize(10);  // Increases size to 10

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5️⃣ Sorting a Vector

#include <iostream>
#include <vector>
#include <algorithm>  // Sorting functions
using namespace std;

int main() {
    vector<int> vec = {50, 10, 40, 20, 30};

    sort(vec.begin(), vec.end());  // Sort in ascending order

    cout << "Sorted Vector: ";
    for (int val : vec) cout << val << " ";

    return 0;
}

🔹 Output

Sorted Vector: 10 20 30 40 50

✅ Uses sort() function from <algorithm>.

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6️⃣ Swapping Two Vectors

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

int main() {
    vector<int> vec1 = {1, 2, 3};
    vector<int> vec2 = {7, 8, 9};

    vec1.swap(vec2);  // Swap contents

    cout << "After swapping:\n";
    for (int val : vec1) cout << val << " ";  // 7 8 9
    cout << endl;
    for (int val : vec2) cout << val << " ";  // 1 2 3

    return 0;
}

✅ swap() exchanges contents efficiently.

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7️⃣ Clearing a Vector

vec.clear();  // Removes all elements

✅ Memory remains allocated. To free memory, use:

vector<int>().swap(vec);

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8️⃣ Summary

✔ std::vector<T> – Dynamic, flexible array.
✔ Supports push_back(), pop_back(), insert(), erase().
✔ Random access using vec[i], at(i), front(), back().
✔ Sorting via sort(vec.begin(), vec.end()).
✔ Resizable with resize(), clear(), swap().

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