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[2][0] = 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" }
      };
 
      // Print the last element in the array!
       System.out.println(arr[2][2]);
    }
 
 }
 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" }
      };
 
       // Change Austin to Athens and print!
       System.out.println("Change Austin to Athens and print!");
       arr[2][0] = "Athens";
       System.out.println(arr[2][0]);
    }
 
 }
 Test.main(null);

Change Austin to Athens and print!
Athens

Nested Loops, Our Beloved:

  • You can use Nested Loops to traverse 2D Arrays
    • for example: to print out an entire array in order, you need to 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 the 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" }
      };

      // Print out the array without using numerical values!
      for (String[] row : arr) {
         for (String place : row) {
            System.out.print(place + " ");
         }
         System.out.println();
      }
   }

}
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 the 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];

        // 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);

Use nested for loops to find the longest string!

HW!

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.

FRQ 4 

public class Position 

{ 

    /** Constructs a Position object with row r and column c. */ 

    public Position(int r, int c) 

    { /* implementation not shown */ } 

    // There may be instance variables, constructors, and methods that are not shown.

}

public class Successors {

    // Part A
    public static Position findPosition(int num, int[][] numArr) {
        for (int i = 0; i < numArr.length; i++) {
            for (int j = 0; j < numArr[i].length; j++) {
                if (num == numArr[i][j]) {
                    return new Position(i, j);
                }
            }
        }
        return null;
    }

    // Part B
    public static Position[][] getSuccessorArray(int[][] numArr) {
        Position[][] successorArray = new Position[numArr.length][numArr[0].length];
        for (int i = 0; i < numArr.length; i++) {
            for (int j = 0; j < numArr[i].length; j++) {
                successoryArray[i][j] = findPosition(numArr[i][j] + 1, numArr);
            }
        }
        return successorArray;
    }
}