Unit 8 - 2D Arrays P3
8.1 Info
Main Concepts
Representing Collections with 2D Arrays
- 2D arrays: Arrays of arrays, representing collections of related primitive or object reference data.
- Creation and Indexing: Created and indexed similarly to 1D arrays, but each element is an array.
- Initializer Arrays: Can be initialized using initializer lists, which represent 1D arrays.
Vocabulary
Array
- A data structure used to implement a list of primitive or object reference data.
Element
- A single value in an array.
Index
- The position of an element in an array. The first element is at index
0in Java.
Length of an Array
- The number of elements in the array. It’s a
public finaldata member of an array.- Public: Accessible in any class.
- Final: The length of the array cannot change after it is created.
- The last element of an array is the length of the index - 1.
Example
- 1D Array: Roster of students with strings.
- 2D Array: Student grades (array of arrays).
- A student’s grades:
- Collection of arrays representing student grades, where each entry is an array of a student’s grades.
- This is a rectangular 2D array since each row array has the same number of entries.
- Example: 7 arrays (students), each with 4 grades (tests). Total: 28 elements.
Declaring and Initializing 2D Arrays in Java
Declaring Arrays
To declare a 2D array in Java, use the following format:
int[][] 2DArrayName = new datatype[# of rows][# of columns]
Example
int[][] grades = new int[3][4];
public class Main {
public static void main(String[] args) {
int[][] grades = {
{90, 85, 88, 92},
{75, 80, 78, 85},
{60, 65, 70, 75}
};
int rows = grades.length; // Attribute 1
int columns = grades[0].length;
System.out.println("Number of rows: " + rows);
System.out.println("Number of columns: " + columns);
int element = grades[1][2]; // Accessing an element
System.out.println("Element at row 2, column 3: " + element);
}
}
Main.main(null)
Number of rows: 3
Number of columns: 4
Element at row 2, column 3: 78
Popcorn Hack #1
How would you write the code to access the last score? MC
- A. grades[6][3]
- B. grades[7][4]
- C. grades[grades.length - 1] [grades[0].length - 1]
- D. A and C
Learning Objectives:
- Traverse 2D arrays using nested for loops.
- Traverse 2D arrays using nested enhanced for loops.
Needed Knowledge:
- Nested iteration refers to statements that iterate over elements in a 2D array.
- 2D arrays are arrays of arrays and are traversed using for loops, similar to enhanced for loops for 1D arrays.
Key Concept:
- Nested iteration statements can be written to traverse the 2D array in row-major order or column-major order.
Traversing with Nested Enhanced For Loops:
- The outer loop in a nested enhanced for loop traverses the rows.
- The enhanced for loop variable must be a 1D array type since the row itself is a 1D array.
- The inner loop traverses a single row, and the inner enhanced for loop variable must be the same type as the elements in the 1D array.
Example Code: Nested For Loops
public class NestedLoops {
public static void main(String[] args) {
for (int outer = 1; outer < 5; outer++) {
for(int inner = 1; inner < 3; inner++) {
System.out.print(inner + " ");
}
System.out.println();
}
}
}
NestedLoops.main(null);
1 2
1 2
1 2
1 2
How this applies to 2D Arrays:
public class Main {
public static void main(String[] args) {
String[][] grid = new String[4][5];
for (int row = 0; row < 4; row++) {
for (int col = 0; col < 5; col++) {
System.out.print(grid[row][col] + " "); // Look at the Double For loops, this is used for 2D-arrays, not 1D
}
System.out.println();
}
}
}
Main.main(null);
null null null null null
null null null null null
null null null null null
null null null null null
Popcorn Hack 2
- Reference code below
Row Order
- Given the code below:
Output is:
- Another way of writing this is by using colons (:)
Homework
- Create a program using 2D-Arrays
Requirements:
- It must use row order
- Use the length attributes to go through the double for loop
- Create one program for integers and one using string