Period 3 2D Arrays Pt 2 - Refresher

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]

Popcorn Hack Response ⬇️⬇️

public static int[] totalGrassEaten(int[][] grassData) {
    // Step 1: Get the number of cats (columns)
    int numCats = grassData[0].length;

    // Step 2: Create a 1D array to store the total grass eaten by each cat
    int[] totalGrass = new int[numCats];

    // Step 3: Loop through each row (day)
    for (int i = 0; i < grassData.length; i++) {
        // Step 4: Loop through each column (cat) in the row
        for (int j = 0; j < grassData[i].length; j++) {
            // Add the amount of grass eaten by the current cat to their total
            totalGrass[j] += grassData[i][j];
        }
    }

    // Step 5: Return the final array with totals for each cat
    return totalGrass;
}

// Sample test case
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}
    };

    // Call the function and print the result
    int[] result = totalGrassEaten(grassData);
    System.out.println(Arrays.toString(result)); // Expected Output: [6, 9, 6, 10, 8, 7, 10]
}


main(null);

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