How do you create a vector of vectors?

To create a vector of vectors (often representing a 2D dynamic array), you essentially create a vector where each element is itself another vector. This is commonly used in C++ for representing matrices or grids where the size can change during runtime.

Creating a Vector of Vectors in C++

Here's how you can create a vector of vectors of integers in C++:

#include <iostream>
#include <vector>

int main() {
  // Declare a vector of vectors of integers.
  std::vector<std::vector<int>> my2DVector;

  // You can optionally initialize it with a specific size:
  int rows = 3;
  int cols = 4;
  std::vector<std::vector<int>> initializedVector(rows, std::vector<int>(cols, 0)); // Creates a 3x4 vector initialized with 0s

  // Add elements to the 2D vector (if not initialized with sizes):
  std::vector<int> row1 = {1, 2, 3};
  std::vector<int> row2 = {4, 5, 6};
  my2DVector.push_back(row1);
  my2DVector.push_back(row2);


  // Accessing elements:
  std::cout << "Element at row 0, column 1: " << my2DVector[0][1] << std::endl;  // Output: 2
  std::cout << "Element at row 1, column 2: " << my2DVector[1][2] << std::endl;  // Output: 6

  // Accessing elements of initialized vector:
  std::cout << "Element at row 0, column 1 of initialized vector: " << initializedVector[0][1] << std::endl;  // Output: 0
  std::cout << "Element at row 2, column 3 of initialized vector: " << initializedVector[2][3] << std::endl;  // Output: 0

  return 0;
}

Explanation

1. Declaration: std::vector<std::vector<int>> my2DVector; declares a vector named my2DVector. The outer vector<> signifies that this vector will hold other vectors. The inner vector<int> indicates that the vectors it will hold will contain integers. You can replace int with any other data type (e.g., float, std::string, etc.) to create a vector of vectors of that type.

2. Initialization (Optional): std::vector<std::vector<int>> initializedVector(rows, std::vector<int>(cols, 0)); This initializes a 2D vector with rows number of rows and cols number of columns, with each element initialized to 0. This is useful when you know the dimensions of your "matrix" beforehand.

3. Adding Rows: If you don't initialize with specific sizes, you can add rows to the outer vector using push_back(). Each row you add is itself a std::vector<int>.

4. Accessing Elements: Use the [] operator twice: once to access the row, and again to access the element within that row. For example, my2DVector[0][1] accesses the element at the first row (index 0) and the second column (index 1).

Key Considerations

• Memory Management: Vectors handle memory management automatically, so you don't need to manually allocate or deallocate memory.
• Dynamic Size: Vectors can grow or shrink in size dynamically, allowing you to add or remove rows and columns as needed.
• Flexibility: This approach allows you to create "jagged" arrays, where each row can have a different number of columns (although they should typically contain same data type).