📦 C++ <vector> – Dynamic Arrays with std::vector Explained

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🧲 Introduction – Why Use <vector> in C++

C++ provides dynamic and resizable arrays using the std::vector class from the <vector> header. Unlike static arrays, vectors can grow or shrink in size at runtime, support random access, and integrate seamlessly with STL algorithms and iterators.

🎯 In this guide, you’ll learn:

• How to declare and initialize vectors
• Key operations: push_back(), insert(), erase(), resize()
• How vectors compare to arrays and lists
• Performance tips and common use cases

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📘 What Is <vector> in C++?

The <vector> header provides the std::vector class—a sequence container representing a dynamic array. It stores elements contiguously in memory, offering fast element access by index and efficient memory management.

Include it with:

#include <vector>

Declare a vector:

std::vector<int> numbers;

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💻 Code Examples – With Output

✅ Declare and Add Elements

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

int main() {
    vector<int> v;
    v.push_back(10);
    v.push_back(20);
    v.push_back(30);

    for (int x : v)
        cout << x << " ";
    return 0;
}

🟢 Output:

10 20 30

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✅ Initialize a Vector

vector<int> v1 = {1, 2, 3};
vector<int> v2(5, 100); // 5 elements, all 100

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🔧 Common std::vector Methods

Method	Description
push_back(val)	Adds element to end
pop_back()	Removes last element
insert(pos, val)	Inserts value at position
erase(pos)	Removes element at position
clear()	Removes all elements
size()	Returns number of elements
resize(n)	Resizes vector to n elements
front() / back()	First and last elements
at(index)	Access with bounds checking
[]	Access without bounds checking
empty()	Returns true if vector is empty

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🔁 Traversing Vectors

✅ Using Iterators

for (vector<int>::iterator it = v.begin(); it != v.end(); ++it)
    cout << *it << " ";

✅ Range-Based Loop (C++11+)

for (int x : v)
    cout << x << " ";

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📊 Vector vs Array vs List

Feature	vector	array	list
Size flexibility	✅ Dynamic	❌ Static	✅ Dynamic
Random access	✅ Yes	✅ Yes	❌ No
Insert/remove middle	❌ Slow	❌ N/A	✅ Fast
Memory layout	✅ Contiguous	✅ Contiguous	❌ Non-contiguous

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💡 Best Practices & Tips

📘 Use .reserve(n) to avoid reallocations when size is known
💡 Prefer emplace_back() over push_back() for efficiency with objects
⚠️ Don’t use [] for unchecked access—prefer .at() for safety
📦 Use shrink_to_fit() to reduce capacity after large deletions (non-binding)

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🛠️ Use Cases for Vectors

🧾 Dynamic Lists – Add/remove items at runtime
📊 Matrix Representations – Use nested vector<vector<T>>
📚 Stacks/Heaps – Can act as underlying storage
🎮 Game Engines – Store objects/entities with fast access
📁 File Parsing – Store lines, tokens, or records

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📌 Summary – Recap & Next Steps

🔍 Key Takeaways:

• std::vector is a dynamic array with random access and flexible resizing
• Provides rich methods to add, remove, access, and modify data
• Works with STL algorithms and supports iterators

⚙️ Real-World Relevance:
Vectors are used everywhere—from embedded systems and financial tools to game engines and simulators—due to their simplicity and efficiency.

✅ Next Steps:
Explore ⏰ C++ <ctime> to work with timestamps, clocks, and formatted time outputs.

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❓FAQ – C++ <vector>

❓ Is vector faster than list?
✅ For random access and end insertions—yes. But for frequent insertions/removals in the middle, use list.

❓ Can I access vector elements with indexing?
✅ Yes. Use v[i] or v.at(i) for bounds checking.

❓ Does vector automatically manage memory?
✅ Yes. It grows dynamically and manages memory internally.

❓ What’s the difference between reserve() and resize()?
reserve() allocates memory without changing size; resize() changes the number of accessible elements.

❓ Can I use std::vector with STL algorithms?
✅ Absolutely. Vectors are fully compatible with algorithms like sort(), find(), and accumulate().

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