Vector

Programming

std::vector is a dynamic array container from the C++ Standard Library that automatically manages memory and can grow and shrink in size. It is one of the most commonly used containers in modern C++.

Basic Usage

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    numbers.push_back(6); // Add an element
    for (int num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;
    return 0;
}

Common Operations

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> vec;
    
    // Adding elements
    vec.push_back(10);
    vec.push_back(20);
    vec.push_back(30);
    
    // Accessing elements
    std::cout << "First element: " << vec[0] << std::endl;
    std::cout << "Size: " << vec.size() << std::endl;
    std::cout << "Capacity: " << vec.capacity() << std::endl;
    
    // Safe access with bounds checking
    if (!vec.empty()) {
        std::cout << "Last element: " << vec.back() << std::endl;
    }
    
    // Inserting at specific position
    vec.insert(vec.begin() + 1, 15);
    
    // Removing elements
    vec.pop_back();
    
    // Iterating
    for (size_t i = 0; i < vec.size(); ++i) {
        std::cout << vec[i] << " ";
    }
    std::cout << std::endl;
    
    return 0;
}

Initialization Methods

#include <vector>

int main() {
    // Empty vector
    std::vector<int> vec1;
    
    // With initial size
    std::vector<int> vec2(5);  // 5 elements, all zero-initialized
    
    // With initial size and value
    std::vector<int> vec3(5, 10);  // 5 elements, all set to 10
    
    // From initializer list
    std::vector<int> vec4 = {1, 2, 3, 4, 5};
    
    // From another vector (copy)
    std::vector<int> vec5(vec4);
    
    // From range
    std::vector<int> vec6(vec4.begin(), vec4.begin() + 3);
    
    return 0;
}

Performance Considerations

Time Complexity:
- Access: O(1)
- Insertion at end: O(1) amortized
- Insertion at beginning/middle: O(n)
- Deletion: O(n) for beginning/middle, O(1) for end
Memory: Vectors may allocate more memory than needed (capacity vs size) to avoid frequent reallocations
Reserve: Use reserve() when you know the approximate size to avoid reallocations

Best Practices

Use at() for bounds-checked access in debug builds, [] for performance-critical code
Prefer range-based for loops: for (const auto& elem : vec)
Use reserve() when you know the size in advance
Use emplace_back() instead of push_back() for complex types to avoid unnecessary copies
Consider std::array for fixed-size containers

Common Pitfalls

Accessing elements with [] without checking bounds (use at() for safety)
Using size() in loops with signed integers (use size_t or compare with size())
Invalidating iterators by modifying the vector during iteration
Not reserving capacity when you know the size, causing multiple reallocations

Advantages over C-style Arrays

Dynamic size (can grow/shrink)
Automatic memory management
Rich set of member functions
Bounds checking with at()
Iterator support for STL algorithms
RAII (automatic cleanup when going out of scope)

Output

1 2 3 4 5 6