How can I resize a 2D C++ vector?

Mastering C++ Vector Resizing: A Comprehensive Guide

Learn how to effectively resize 2D C++ vectors using various methods, understand their implications, and choose the best approach for your needs.

Resizing a 2D C++ std::vector is a common task when dealing with dynamic data structures like matrices or grids. Unlike fixed-size arrays, std::vector offers the flexibility to change its dimensions during runtime. This article will guide you through different techniques for resizing 2D vectors, explaining the nuances of each method and providing practical code examples.

Understanding 2D Vectors in C++

A 2D vector in C++ is typically implemented as a std::vector of std::vectors. This means you have an outer vector holding rows, and each row is itself a vector holding elements. When resizing, you often need to consider both the number of rows and the number of columns (elements per row).

graph TD
    A[2D Vector] --> B[Outer Vector (Rows)]
    B --> C[Inner Vector 1 (Elements)]
    B --> D[Inner Vector 2 (Elements)]
    B --> E[...]
    C --> F[Element 1]
    C --> G[Element 2]
    D --> H[Element 1]
    D --> I[Element 2]

Conceptual structure of a 2D C++ vector

Method 1: Resizing with resize() for Rows and Columns

The most straightforward way to resize a 2D vector is to use the resize() method on both the outer vector (for rows) and then iterate through each row to resize the inner vectors (for columns). This method allows you to specify a default value for newly added elements.

#include <vector>
#include <iostream>

void resize2DVector(std::vector<std::vector<int>>& vec, size_t newRows, size_t newCols, int defaultValue = 0) {
    // Resize the outer vector (number of rows)
    vec.resize(newRows);

    // Resize each inner vector (number of columns) and initialize new elements
    for (size_t i = 0; i < newRows; ++i) {
        vec[i].resize(newCols, defaultValue);
    }
}

int main() {
    std::vector<std::vector<int>> my2DVec = {{1, 2, 3}, {4, 5, 6}};
    std::cout << "Original vector (2x3):\n";
    for (const auto& row : my2DVec) {
        for (int val : row) {
            std::cout << val << " ";
        }
        std::cout << "\n";
    }

    // Resize to 3x4 with default value 0
    resize2DVector(my2DVec, 3, 4, 0);
    std::cout << "\nResized vector (3x4, default 0):\n";
    for (const auto& row : my2DVec) {
        for (int val : row) {
            std::cout << val << " ";
        }
        std::cout << "\n";
    }

    // Resize to 2x2
    resize2DVector(my2DVec, 2, 2);
    std::cout << "\nResized vector (2x2):\n";
    for (const auto& row : my2DVec) {
        for (int val : row) {
            std::cout << val << " ";
        }
        std::cout << "\n";
    }

    return 0;
}

Resizing a 2D vector using nested resize() calls.

Method 2: Reinitializing the Vector

For scenarios where you need to completely reset the 2D vector to a new size with all elements initialized to a specific value, reinitializing the vector can be a clean and concise approach. This involves creating a new vector of vectors with the desired dimensions and initial value.

#include <vector>
#include <iostream>

void reinitialize2DVector(std::vector<std::vector<int>>& vec, size_t newRows, size_t newCols, int initialValue = 0) {
    vec = std::vector<std::vector<int>>(newRows, std::vector<int>(newCols, initialValue));
}

int main() {
    std::vector<std::vector<int>> my2DVec = {{1, 2}, {3, 4}};
    std::cout << "Original vector (2x2):\n";
    for (const auto& row : my2DVec) {
        for (int val : row) {
            std::cout << val << " ";
        }
        std::cout << "\n";
    }

    // Reinitialize to 3x3 with value 7
    reinitialize2DVector(my2DVec, 3, 3, 7);
    std::cout << "\nReinitialized vector (3x3, value 7):\n";
    for (const auto& row : my2DVec) {
        for (int val : row) {
            std::cout << val << " ";
        }
        std::cout << "\n";
    }

    return 0;
}

Reinitializing a 2D vector to a new size and value.

Choosing the Right Resizing Strategy

The best method depends on your specific requirements:

• Preserving Data: If you need to keep existing elements and only add or remove rows/columns, use the nested resize() approach (Method 1).
• Complete Reset: If you want to clear all data and start with a fresh 2D vector of a new size and initial value, reinitialization (Method 2) is simpler and often more readable.
• Performance Considerations: For very large vectors or frequent resizing operations, consider the overhead of memory reallocations. std::vector's resize() can involve copying elements, which can be costly. If you know the maximum size beforehand, reserve() can help pre-allocate memory, though it doesn't change the logical size.

flowchart TD
    A[Start] --> B{Need to preserve existing data?}
    B -- Yes --> C[Use nested resize()]
    C --> D{Specify default value for new elements?}
    D -- Yes --> E[vec[i].resize(newCols, defaultValue)]
    D -- No --> F[vec[i].resize(newCols)]
    B -- No --> G[Reinitialize vector]
    G --> H[vec = std::vector<std::vector<T>>(rows, std::vector<T>(cols, initialValue))]
    E --> I[End]
    F --> I[End]
    H --> I[End]

Decision flow for choosing a 2D vector resizing strategy