2D Array Review

This is a review of 2D Arrays. Please see part 1 of the lesson for more details.

Creation of 2D Array

2D arrays are simply arrays within arrays.

// Declare a 2D array
int[][] array = new int[3][4]; // 3 rows and 4 columns

// Declare and initialize with values
int[][] array = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

Indexing of 2D Array

To index a 2D array, place the row number, followed by the column number.

// Access elements in the array
int value = array[0][1]; // Gets the number in the 1st row and 2nd column
System.out.println(value); // Output: 2

int value2 = array[1][2]; // Gets the number in the 2nd row and 3rd column
System.out.println(value2); // Output: 6

// Whoopsie daisies! Error! This is out of bounds
int value3 = array[20][10];
System.out.println(value3);

Iteration of 2D Array

To iterate through a 2D array:

Row-major order

// For loop: Add 5 to all elements of the array
for (int row = 0; row < array.length; row++) {
    for (int col = 0; col < array[row].length; col++) {
        array[row][col] += 5; // Add 5 to each element
    }
}

// Foreach loop: Output all elements of the array
for (int[] row : array) {
    for (int item : row) {
        System.out.print(item + " ");
    }
    System.out.println(); // Adds a newline after each row
}

Column-major order

for (int col = 0; col < array[0].length; col++) {
    for (int row = 0; row < array.length; row++) {
        System.out.print(array[row][col]);
    }
    System.out.println();
}

Popcorn Hack

Farmer John has 7 cats. They all eat a certain amount of magical grass, measured in lbs, every day. The farmer makes a 2D array with information about how much grass each cat ate each day. Each row represents how many pounds each cat ate that day. Each column represents a single cat. Please return a 1D array, where each element is the total amount of grass the cat ate.

TL;DR: Sum up the values within each column and return a 1D array.

// Sample test case
int[][] grassData = {
    {2, 3, 1, 4, 3, 2, 5},
    {1, 2, 3, 1, 4, 2, 3},
    {3, 4, 2, 5, 1, 3, 2}
};

// Your Code Here

// Expected Output: [6, 9, 6, 10, 8, 7, 10]

import java.util.Arrays;

public class PopcornHack {
    public static void main(String[] args) {
        int[][] grassData = {
            {2, 3, 1, 4, 3, 2, 5},
            {1, 2, 3, 1, 4, 2, 3},
            {3, 4, 2, 5, 1, 3, 2}
        };

        int[] result = calculateTotalGrass(grassData);

        System.out.println(Arrays.toString(result));
    }

    public static int[] calculateTotalGrass(int[][] grassData) {
        int numCats = grassData[0].length;
        
        int[] totalGrass = new int[numCats];

        for (int[] day : grassData) {
            for (int cat = 0; cat < numCats; cat++) {
                totalGrass[cat] += day[cat];
            }
        }

        return totalGrass;
    }
}

PopcornHack.main(null);

[6, 9, 6, 10, 8, 7, 10]

Period 3 2D Arrays Pt 2 - Refresher

Class of 2025