Unit 8 2D Array¶
8.1 2D Arrays¶
Goals
Represent collections of related primitive or object reference data using two-dimensional (2D) array objects.
Write program code to create elements in 2D array objects.
In Java, a 2D array is organized in rows and columns much like a spreadsheet. Sometimes it is referred to as an array of arrays. It is an object that contains a set of similar variable data types. Real life examples of 2D Arrays can be a traditional classroom seating chart, a chess board, or the game Connect Four.
Creating a 2D array is easy. First you must declare the type of an array that you want to create. In the example below, we are using int. Then you use 2 sets of open and closed brackets. Create a name for the array… in this case it is newArray. From there, you will declare it as a new array and define the size of the array. The code creates a 2D array with 3 rows and 3 columns, and it initializes all the elements to the default value of 0.
NOTE: When you initialize a 2D array, you must always specify the first dimension(rows), but you do not need to specify the second dimension(columns).
// my preferred initialization method of a 2D array
int[][] newArray = new int[3][5];
// The minimum required is declaring the data type,
// 2 open/close brackets, and the array name.
int newArray [][];
Much like one-dimensional arrays:
the size of the array cannot be changed after being initialized
all elements in the array must be the same type
a 2D array can hold primitive types as well as objects.
int[][] integer2DArray; // 2D integer array
double[][] double2DArray; // 2D double array
boolean[][] boolean2DArray; // 2D boolean array
String[][] string2DArray; // 2D String array
Another way to initialize a 2D array is as follows:
int[][] newArray = {
{1, 2, 3, 4, 5},
{6, 7, 8, 9, 10},
{11, 12, 13, 14, 15}
};
NOTE It is possible for 2D arrays to have different row lengths.
This statement creates a variable num and assigns it the value of the number inside the second array and in the third column, so the number 6. Remember that arrays are zero-indexed, so the first element’s index is 0.
In this Java program, we’ll represent collections of related data using a two-dimensional (2D) array object. We’ll create a 2D array to store information about students and their scores in different subjects.
public class TwoDArrayExample {
public static void main(String[] args) {
// Create a 2D array to store student scores
int[][] studentScores = new int[3][5]; // 3 students and 5 subjects
// Assign values to the elements in the 2D array
studentScores[0][0] = 90; // Student 1, Subject 1
studentScores[0][1] = 85; // Student 1, Subject 2
studentScores[0][2] = 78; // Student 1, Subject 3
studentScores[0][3] = 95; // Student 1, Subject 4
studentScores[0][4] = 88; // Student 1, Subject 5
studentScores[1][0] = 75; // Student 2, Subject 1
studentScores[1][1] = 92; // Student 2, Subject 2
studentScores[1][2] = 84; // Student 2, Subject 3
studentScores[1][3] = 80; // Student 2, Subject 4
studentScores[1][4] = 88; // Student 2, Subject 5
studentScores[2][0] = 82; // Student 3, Subject 1
studentScores[2][1] = 78; // Student 3, Subject 2
studentScores[2][2] = 90; // Student 3, Subject 3
studentScores[2][3] = 85; // Student 3, Subject 4
studentScores[2][4] = 92; // Student 3, Subject 5
// Display the 2D array
System.out.println("Student Scores:");
for (int i = 0; i < studentScores.length; i++) {
for (int j = 0; j < studentScores[i].length; j++) {
System.out.print(studentScores[i][j] + "\t");
}
System.out.println();
}
}
}
Explanation:
We create a 2D array studentScores to store student scores for 3 students (rows) and 5 subjects (columns).
We assign values to the elements in the 2D array to represent the scores of each student in each subject.
We use nested loops to display the 2D array in a tabular format, showing the scores for each student in each subject.
When you run this program, it will output:
Student Scores:
90 85 78 95 88
75 92 84 80 88
82 78 90 85 92
This program demonstrates how to represent collections of related data using a 2D array in Java. It creates a 2D array studentScores and assigns values to its elements to store student scores in different subjects. The program then displays the 2D array to show the scores for each student in each subject.
8.2 Traversing 2D Arrays¶
Goals
Traverse 2D arrays using nested for and enhanced for loops.
Write program code to create, traverse, and manipulate elements in 2D array objects.
Use test cases to find errors or validate results.
Now that you know how to create a 2D array, it is important to know how to navigate a 2D array. One of the ways that you can do this is to reference a specific element within the 2D array. To access the variables within a 2D array, you can use the syntax newArray[i][j], where i is the index of the row and j is the index of the column.
index[0] |
index[1] |
index[2] |
index[3] |
index[4] |
|
|---|---|---|---|---|---|
index[0] |
1 |
2 |
3 |
4 |
5 |
index[1] |
6 |
7 |
8 |
9 |
10 |
index[2] |
11 |
12 |
13 |
14 |
15 |
System.out.println(newArray[1] [1]);
Sample output
7
What if you wanted to see all of the elements within a 2D array. You can use a nested for loop to iterate through each element in the array. See the program below:
class Main {
public static void main(String[] args) {
int[][] newArray = {
{1,2,3,4,5},
{6,7,8,9,10},
{11,12,13,14,15}
};
for(int i = 0; i < newArray.length; i++) { // row
for(int j = 0; j < newArray[i].length; j++) { // column
System.out.print(newArray[i][j] + "\t");
}
System.out.println(); // Move to the next line after each row
}
}
}
Sample Output
1 2 3 4 5
6 7 8 9 10
11 12 13 14 15
In this Java program, we’ll traverse a 2D array using nested for and enhanced for loops. We’ll also create the 2D array, traverse it, and manipulate its elements. Finally, we’ll use test cases to find errors and validate the results.
public class TwoDArrayTraversalExample {
public static void main(String[] args) {
// Create a 2D array to store student scores
int[][] studentScores = {
{90, 85, 78, 95, 88},
{75, 92, 84, 80, 88},
{82, 78, 90, 85, 92}
};
// Traverse the 2D array using nested for loops
System.out.println("Student Scores using nested for loop:");
for (int i = 0; i < studentScores.length; i++) {
for (int j = 0; j < studentScores[i].length; j++) {
System.out.print(studentScores[i][j] + "\t");
}
System.out.println();
}
// Traverse the 2D array using enhanced for loop
System.out.println("\nStudent Scores using enhanced for loop:");
for (int[] row : studentScores) {
for (int score : row) {
System.out.print(score + "\t");
}
System.out.println();
}
// Manipulate elements in the 2D array
studentScores[1][3] = 88; // Change the score of Student 2 in Subject 4 to 88
// Display the modified 2D array
System.out.println("\nModified Student Scores:");
for (int[] row : studentScores) {
for (int score : row) {
System.out.print(score + "\t");
}
System.out.println();
}
// Test case to validate the results
validateScores(studentScores);
}
// Test case to validate the results
public static void validateScores(int[][] scores) {
// Calculate the total score for each student
int[] totalScores = new int[scores.length];
for (int i = 0; i < scores.length; i++) {
for (int j = 0; j < scores[i].length; j++) {
totalScores[i] += scores[i][j];
}
}
// Calculate the average score for each student
double[] averageScores = new double[scores.length];
for (int i = 0; i < scores.length; i++) {
averageScores[i] = (double) totalScores[i] / scores[i].length;
}
// Display the validation results
System.out.println("\nValidation Results:");
for (int i = 0; i < scores.length; i++) {
System.out.println("Student " + (i + 1) + ": Total Score = " + totalScores[i] + ", Average Score = " + averageScores[i]);
}
}
}
Explanation:
We create a 2D array studentScores and initialize it with student scores for different subjects.
We traverse the 2D array using nested for loops and enhanced for loop to display the student scores in tabular format.
We manipulate the element studentScores[1][3] to change the score of Student 2 in Subject 4 to 88.
We display the modified 2D array to show the changes.
We use a test case (validateScores()) to calculate the total and average scores for each student and validate the results.
When you run this program, it will output:
Student Scores using nested for loop:
90 85 78 95 88
75 92 84 80 88
82 78 90 85 92
Student Scores using enhanced for loop:
90 85 78 95 88
75 92 84 80 88
82 78 90 85 92
Modified Student Scores:
90 85 78 95 88
75 92 84 88 88
82 78 90 85 92
Validation Results:
Student 1: Total Score = 436, Average Score = 87.2
Student 2: Total Score = 417, Average Score = 83.4
Student 3: Total Score = 427, Average Score = 85.4
This program demonstrates how to create, traverse, and manipulate elements in a 2D array using nested for loops and enhanced for loop. It also shows the use of test cases to validate the results, ensuring that the array elements are correctly modified and the calculated total and average scores are accurate.