Learning Objective!

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``

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);

arr[0][0] = 1
arr[1][2] = 6
arr[2][1] = 8

Accessing and Updating Elements of a 2D Array:

  • nameOf2DArray[r][c]

Hack 1: Access the last element of the 2D Array List: 

public class Test {

   public static void main(String[] args) {

     String[][] arr = {
        { "a", "f", "g" },
        { "b", "e", "h" },
        { "c", "d", "i" }
     };

     System.out.println(arr[arr.length - 1][arr[0].length - 1]);
   }

}
Test.main(null);

i
  • a quick tip for the future: list[list.length - 1][list[0].length - 1]
  • Updating an element:
    • list[r][c] = value;

Hack 2: Changing a Value: 

public class Test {

    public static void main(String[] args) {
 
      String[][] arr = {
         { "Atlanta", "Baltimore", "Chicago" },
         { "Australia", "Boston", "Cincinnati" },
         { "Austin", "Beaumont", "Columbus" }
      };

      
     arr[2][0]  = "Athens";
     System.out.println(arr[2][0]);
 
       // Change Austin to Athens and print!
       System.out.println("Successfully changed Austin to Athens and printed!");
       
    }
 
 }
 Test.main(null);

Athens
Successfully changed Austin to Athens and printed!

Nested Loops:

  • You can use Nested Loops to traverse 2D Arrays
    • For ex: to print out an entire array in order, you needt o use these
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);

a f g l  
b e h k  
c d i j
  • Make sure your columns for loop is always inside your rows for loop!
    • Unless you want it printed thte other way of course
  • Print spaces and lines in between to make a neater matrix

Hack 3: Unknown Dimensions: 

public class Test {

    public static void main(String[] args) {
 
       String[][] arr = {
          { "Atlanta", "Baltimore", "Chicago" },
          { "Australia", "Boston", "Cincinnati" },
          { "Austin", "Beaumont", "Columbus" }
       };

       for (int row = 0; row < 3; row++) {
         for (int col = 0; col < 3; col++) {
            System.out.print(arr[row][col] + " ");
         }
        System.out.println(" ");
      }
       // Print out the array without using numerical values!
       
    }
 
 }
 Test.main(null);

Atlanta Baltimore Chicago  
Australia Boston Cincinnati  
Austin Beaumont Columbus

Searching for a Value in a 2D Array:

  • Here's a quick example to illustrate:
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);

Boston
  • Note: in the code, you see hte use of : which essentially means within

Hack 4: Finding the Min/Max Value: 

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 i = 0; i < arr.length; i++){
          for (int j = 0; j < arr[i].length; j++){
            if (longest.length() < arr[i][j].length()) {
              longest = arr[i][j]; 
            }
          }
        }
        System.out.println(longest);
        
        String shortest = arr[0][0];
        for (int i = 0; i < arr.length; i++){
          for (int j = 0; j < arr[i].length; j++){
            if (shortest.length() > arr[i][j].length()) {
              shortest = arr[i][j]; 
            }
          }
        }
        System.out.println(shortest);
    }
 
 }
Test.main(null);

Cincinnati
Boston

HW

Please submit screenshots of your 4 hacks up and running to our Google Form, which also has a few 2D array related multiple choice questions.

Additionally, Complete and send a screenshot of your code for 2017 FRQ Q4: Successor Array in that same google form.

Please submit as a pair or group.

2017 FRQ 4 A-B 

public static Position findPosition(int num, int[][] intArr)
{
    for (int row=0; row < intArray.length; row++)
    {
        for (int col=0; col < intArray[0].length; col++)
        {
            if (intArray[row][col] == num)
            {
                return new Position(rol, col);
            }
        }
    }
    return.null;
}

public static Position[][] getSuccessorArray(int[][] intArr)
{
    Position[][] newArray = new Position[intArray.length][intArray[0].length];

    for (int row=0; row < intArray.length; row++)
    {
        for (int col=0; col < intArray[0].length; col++)
        {
        newArray[row][col] = findPosition(intArray[row][col]+1, intArray);
        }
    }
    return newArray;

}

Extra Credit -- Christmas Tree 

public class ChristmasTree {

    public static void main(String[] args) {
   
     for (int i = 0; i < 10; i++) {
      for (int j = 0; j < 10 - i; j++)
       System.out.print(" ");
      for (int k = 0; k < (2 * i + 1); k++)
       System.out.print("*");
      System.out.println();
     }
    }
   }
   ChristmasTree.main(null);

          *
         ***
        *****
       *******
      *********
     ***********
    *************
   ***************
  *****************
 *******************