Multiple Choice Questions on 2D Arrays in Java: AP Computer Science A

📚 Quick Study Guide

• 🧠 A 2D array in Java is an array of arrays, organized in rows and columns. Think of it like a table or a grid.
• 💻 Declaration: `int[][] myArray = new int[rows][columns];`
• 🧮 Accessing elements: `myArray[rowIndex][columnIndex]` (remember that indexing starts at 0).
• 📐 The `length` property: `myArray.length` gives the number of rows, and `myArray[0].length` gives the number of columns (assuming all rows have the same number of columns).
• 💡 Iterating through a 2D array typically involves nested loops. The outer loop iterates through the rows, and the inner loop iterates through the columns.
• 💾 Common operations include traversing, searching, inserting, deleting, and modifying elements.
• 📊 2D arrays are useful for representing matrices, game boards, and images.

🧪 Practice Quiz

1. Which of the following is the correct way to declare a 2D array of integers with 3 rows and 4 columns in Java?
   1. `int array = new int[3][4];`
   2. `int[][] array = new int[3, 4];`
   3. `int[3][4] array = new int[][];`
   4. `int[][] array = new int[3][4];`
2. Given a 2D array `int[][] matrix = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};`, what is the value of `matrix[1][2]`?
   1. 1
   2. 5
   3. 6
   4. 8
3. What does `array.length` return for a 2D array `int[][] array = new int[5][10];`?
   1. 5
   2. 10
   3. 50
   4. 15
4. How do you access the number of columns in the first row of a 2D array `int[][] array` (assuming it has at least one row)?
   1. `array.length`
   2. `array[0].length`
   3. `array.columns`
   4. `array.size()`
5. Which of the following loops correctly iterates through all elements of a 2D array `int[][] array` with `rows` rows and `cols` columns?
   1. `for (int i = 0; i < array.length; i++) { for (int j = 0; j < array[i].length; j++) { System.out.println(array[i][j]); } }`
   2. `for (int i = 0; i < rows; i++) { for (int j = 0; j < cols; j++) { System.out.println(array[j][i]); } }`
   3. `for (int i = 0; i < cols; i++) { for (int j = 0; j < rows; j++) { System.out.println(array[i][j]); } }`
   4. `for (int i = 0; i < array.length; i++) { for (int j = 0; j < array.length; j++) { System.out.println(array[i][j]); } }`
6. What is the purpose of a 2D array?
   1. To store a list of elements.
   2. To store key-value pairs.
   3. To store data in a tabular format with rows and columns.
   4. To perform mathematical operations.
7. Given `int[][] array = new int[2][3];`, what are the initial values of all elements in the array?
   1. Null
   2. Random integers
   3. 0
   4. Undefined