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Introduction to C++ Vectors
What is a Vector?
Vectors in C++ are part of the Standard Template Library (STL) and provide a dynamic array that can resize itself automatically when an element is inserted or deleted, while maintaining the stored data.
Why Use Vectors?
Vectors are preferred over arrays when you need a resizable array, as they provide the flexibility of dynamic memory allocation, ease of insertion and deletion, and built-in functions for manipulation.
#include <iostream>
#include <vector>
int main() {
std::vector<int> numbers;
numbers.push_back(10);
numbers.push_back(20);
for(int i : numbers) {
std::cout << i << " ";
}
return 0;
}
Console Output:
10 20
Vector Initialization
Initializing Vectors
Vectors can be initialized in several ways, including default initialization, with a specific size, or using an initializer list.
#include <iostream>
#include <vector>
int main() {
std::vector<int> emptyVector;
std::vector<int> sizedVector(5, 0);
std::vector<int> initList = {1, 2, 3, 4, 5};
for(int i : initList) {
std::cout << i << " ";
}
return 0;
}
Console Output:
1 2 3 4 5
Accessing Vector Elements
Element Access Methods
Elements in a vector can be accessed using the subscript operator [], the at() method, or iterators.
#include <iostream>
#include <vector>
int main() {
std::vector<int> numbers = {10, 20, 30};
std::cout << numbers[0] << " ";
std::cout << numbers.at(1) << " ";
return 0;
}
Console Output:
10 20
Modifying Vector Elements
Inserting and Deleting Elements
Vectors allow easy modification of elements through functions like push_back(), pop_back(), insert(), and erase().
#include <iostream>
#include <vector>
int main() {
std::vector<int> numbers = {10, 20, 30};
numbers.push_back(40);
numbers.pop_back();
numbers.insert(numbers.begin(), 5);
numbers.erase(numbers.begin() + 1);
for(int i : numbers) {
std::cout << i << " ";
}
return 0;
}
Console Output:
5 20 30
Vector Capacity and Size
Understanding Capacity and Size
Vectors have both a size (number of elements) and a capacity (allocated storage). Functions like size(), capacity(), and resize() help manage these attributes.
#include <iostream>
#include <vector>
int main() {
std::vector<int> numbers(10);
std::cout << "Size: " << numbers.size() << " ";
std::cout << "Capacity: " << numbers.capacity() << " ";
numbers.resize(20);
std::cout << "New Size: " << numbers.size();
return 0;
}
Console Output:
Size: 10 Capacity: 10 New Size: 20
Iterating Over Vectors
Using Iterators
Iterators provide a way to traverse vectors. They are similar to pointers and can be used with loops to access elements.
#include <iostream>
#include <vector>
int main() {
std::vector<int> numbers = {10, 20, 30, 40};
for(std::vector<int>::iterator it = numbers.begin(); it != numbers.end(); ++it) {
std::cout << *it << " ";
}
return 0;
}
Console Output:
10 20 30 40
Vector of Vectors
Creating 2D Vectors
Vectors can contain other vectors, allowing the creation of multi-dimensional arrays, such as 2D vectors.
#include <iostream>
#include <vector>
int main() {
std::vector<std::vector<int>> matrix = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9}
};
for(const auto& row : matrix) {
for(int val : row) {
std::cout << val << " ";
}
std::cout << std::endl;
}
return 0;
}
Console Output:
1 2 3
4 5 6
7 8 9
Vector Sorting
Sorting Vectors
Vectors can be sorted using the sort() function from the algorithm library, which arranges elements in ascending order by default.
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> numbers = {40, 10, 30, 20};
std::sort(numbers.begin(), numbers.end());
for(int i : numbers) {
std::cout << i << " ";
}
return 0;
}
Console Output:
10 20 30 40
Advanced Vector Operations
Using Algorithms with Vectors
Vectors can be used with various STL algorithms for operations such as searching, reversing, and more, enhancing their utility.
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> numbers = {10, 20, 30, 40};
std::reverse(numbers.begin(), numbers.end());
for(int i : numbers) {
std::cout << i << " ";
}
return 0;
}
Console Output:
40 30 20 10
Vector Performance Considerations
Efficiency and Best Practices
While vectors are versatile, understanding their underlying performance characteristics, such as reallocation costs and iterator invalidation, is crucial for optimal use.
// Example code omitted for brevity
// Consider using reserve() to preallocate memory
Best Practice:
Use the reserve() function to minimize reallocations when the number of elements is known in advance.
