Day 37: The Matrix Reloaded | 2D Vectors

📅 Date: Feb 7, 2026
🔥 Topic: 2D Vectors & Initialization

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⬛ Vector of Vectors

A 2D Array is static. A 2D Vector is a dynamic grid where even the number of columns in each row can be different!

Syntax: vector<vector<int>> mat;

💻 Day 37 Code: 2D Vector Input

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

int main() {
    // 3 Rows, 4 Columns, initialized with 0
    int rows = 3, cols = 4;
    vector<vector<int>> matrix(rows, vector<int>(cols, 0));

    // Taking Input
    cout << "Enter 12 numbers: " << endl;
    for(int i=0; i<rows; i++) {
        for(int j=0; j<cols; j++) {
            cin >> matrix[i][j];
        }
    }

    cout << "You entered element at [1][2]: " << matrix[1][2];
    return 0;
}

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💭 Thoughts

The syntax vector<vector<int>> looks scary, but it's just a vector where every bucket holds another vector.