2D Array Vocab:

  • Array = a data structure used to implement a collection (list) of primitive or object reference data
    • an example:
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
  • 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
  • Nested Loops = A nested loop is a loop within a loop, an inner loop within the body of an outer one
    • an example:
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);

FRQ 4: 

public class LightBoard {
 /** The lights on the board, where true represents on and false represents off.
 */
 private boolean[][] lights;
 /** Constructs a LightBoard object having numRows rows and numCols columns.
 * Precondition: numRows > 0, numCols > 0
 * Postcondition: each light has a 40% probability of being set to on.
 */
 public LightBoard(int numRows, int numCols)
 { /* to be implemented in part (a) */ }
 /** Evaluates a light in row index row and column index col and returns a status
 * as described in part (b).
 * Precondition: row and col are valid indexes in lights.
 */
 public boolean evaluateLight(int row, int col)
 { /* to be implemented in part (b) */ }
 // There may be additional instance variables, constructors, and methods not shown.
}

part a 

public LightBoard(int numRows, int numCols) {
  
    lights = new boolean[numRows][numCols];

    // use of nested loops to iterate through the array elements
    for(int r = 0; r < lights.length; r++) {
        for(int c = 0; c < lights[0].length; c++) {
            if(Math.random() <= 0.4)
                lights[r][c] = true;
        }
    }

}

part b 

public boolean evaluateLight(int row, int col) {
    
    // sets initial to 0
    int onInColumn = 0;

    // finds how many lights are on
    for(int r = 0; r < lights.length; r++) {
        if(lights[r][col] == true) {
            onInColumn++;
        }
    }
        
    // decides if the conditions match true or false
    if(lights[row][col]) {
        if(onInColumn % 2 == 0) {
            return false;
        }
    } else {
        if(onInColumn % 3 == 0) {
            return true;
        }
    }
    
        
    return lights[row][col];
    
}