Unit 8 - 2D Arrays
Notes, Hacks, and HW for 2D Arrays.
- Learning Objective!
- 2D Array Vocab:
- The Basics:
- Accessing and Updating Elements of a 2D Array:
- Nested Loops, Our Beloved:
- Searching for a Value in a 2D Array:
- HW!
2D Array Vocab:
- Array = a data structure used to implement a collection (list) of primitive or object reference data
- Element = a single value in the array
- Index = the position of the element in the array (starts from 0)
-
Array Length = the number of elements in the array
- Is public, so can be accessed in any class
- Is also final, so can’t change it after array has been created
The Basics:
- A 2D array is an array of arrays, and can be a better way to store data
- Declaring a 2D array:
DataType[][] nameOf2DArray
- Initializing a 2D array
-
DataType[][] nameOf2DArray = new DataType[r][c];
- r = # of rows
- The # of arrays in the array
-
r = list.length
- c = # of columns
- The # of elements in the inner arrays
c = list[0].length
- r = # of rows
-
Initializing a Sample Array:
public class Test {
public static void main(String[] args) {
int[][] arr = {
{ 1, 2, 3 },
{ 4, 5, 6 },
{ 7, 8, 9 }
};
System.out.println("arr[0][0] = " + arr[0][0]);
System.out.println("arr[1][2] = " + arr[1][2]);
System.out.println("arr[2][1] = " + arr[2][1]);
}
}
Test.main(null);
public class Test {
public static void main(String[] args) {
String[][] arr = {
{ "a", "f", "g" },
{ "b", "e", "h" },
{ "c", "d", "i" }
};
// Print the last element in the array!
}
}
Test.main(null);
- a quick tip for the future:
list[list.length - 1][list[0].length - 1]
- Updating an element:
list[r][c] = value;
public class Test {
public static void main(String[] args) {
String[][] arr = {
{ "Atlanta", "Baltimore", "Chicago" },
{ "Australia", "Boston", "Cincinnati" },
{ "Austin", "Beaumont", "Columbus" }
};
// Change Austin to Athens and printn!
System.out.println("Change Austin to Athens and print!");
if(arr[2][0] == "Austin") {
arr[2][0] = "Athens";
}
System.out.println(arr[2][0]);
}
}
Test.main(null);
public class Test {
public static void main(String[] args) {
String[][] arr = {
{ "a", "f", "g", "l" },
{ "b", "e", "h", "k" },
{ "c", "d", "i", "j" }
};
for (int row = 0; row < 3; row++) {
for (int col = 0; col < 4; col++) {
System.out.print(arr[row][col] + " ");
}
System.out.println(" ");
}
}
}
Test.main(null);
public class Test {
public static void main(String[] args) {
String[][] arr = {
{ "Atlanta", "Baltimore", "Chicago" },
{ "Australia", "Boston", "Cincinnati" },
{ "Austin", "Beaumont", "Columbus" }
};
// Print out the array without using numerical values!
for(int row = 0; row < arr.length; row++) {
for(int column = 0; column < arr.length; column++) {
System.out.print(arr[row][column] + " ");
}
System.out.println(" ");
}
}
}
Test.main(null);
public class Test {
public static void main(String[] args) {
String[][] arr = {
{ "Atlanta", "Baltimore", "Chicago" },
{ "Australia", "Boston", "Cincinnati" },
{ "Austin", "Beaumont", "Columbus" }
};
String match = "";
String name = "Boston";
for (String[] row : arr) {
for (String item : row) {
if (item.equals(name)) {
match = name;
}
}
}
if (match.length() == 0) {
System.out.println("No Match!");
} else {
System.out.println(name);
}
}
}
Test.main(null);
public class Test {
public static void main(String[] args) {
String[][] arr = {
{ "Atlanta", "Baltimore", "Chicago" },
{ "Australia", "Boston", "Cincinnati" },
{ "Austin", "Beaumont", "Columbus" }
};
String longest = arr[0][0];
for(int row = 0; row < arr.length; row++) {
for(int column = 0; column < arr[row].length; column++) {
if (arr[row][column].length() > longest.length()) {
longest = arr[row][column];
}
}
}
System.out.println(longest);
// Use nested for loops to find the longest or shortest string!
System.out.println("Use nested for loops to find the longest or shortest string!");
}
}
Test.main(null);
HW!
Google Form, which also has a few 2D array related multiple choice questions.
Please submit screenshots of your 4 hacks up and running to our
2017 FRQ Q4: Successor Array
in that same google form.
Additionally, Complete and send a screenshot of your code for Please submit as a pair or group.
import java.util.*;
class Tree {
private String [][] tree;
public Tree () {
tree = new String[][]{ //2D array above is just a name, "new" makes a container ("object")
{ " * " },
{ " *** " },
{ " ***** " },
{ " ******* " },
{ " ********* " },
{ " *********** " },
{ "************* " },
{ " || " },
{ " || " }
};
}
public void printASCII () {
for (int row = 0; row < tree.length;row++) {
for (int column = 0; column < tree[row].length; column++) {
System.out.println(tree[row][column]);
}
}
}
}
Tree myTree = new Tree();
myTree.printASCII();