Questions

• Reading?
• Quiz?
• Homework?

Concept: 2D - Array

• Declaring

  String[][] words = new String[3][4]

  • Creates a 2D String array with 3 rows and 4 columns
• Alternate declaration

     int[][] array = {
             {1,2,3,-5},
             {3,5,-1,2},
             {-2,7,5,3}
     };

  • How many rows?
  • How many columns?
  • This is an array of arrays
  • 3 arrays, each with 4 elements
    • 1st array {1, 2, 3, -5}
    • 2nd array {3, 5, -1, 2}
    • 3rd array {-2, 7, 5, 3}
  • What is the element at array[0][0]?
    • row 0, column 0
  • At array[2][1]?
• Given a 2d array called matrix of unknown size
  • Number of rows: matrix.length
  • Number of columns: matrix[0].length
  • The last element in the last row: matrix[matrix.length - 1][matrix[0].length - 1]

Concept - Processing a 2D array

• We can look at every element using nested loops
• We can use the same set of nested loops to implement many different algorithms for 2d-arrays

    //do some set up first
    for (int row = 0; row < matrix.length; row ++)
    {
         for (int column = 0; column < matrix[0].length; column++)
         {
             //do something with each element: matrix[row][column]
         }
    }

 

Activity - Processing a 2D Array: sum

Record your participation in Piazza clicker question Lesson18 Q1

We are going to do some 2d array processing. Download this starter file.

• Look at IntArray2DProcessor to see what is there
• Several method stubs
• In IntArray2DProcessor, we want to complete the method that finds the sum of all the elements in the 2D array
• Let's code the sum method together
• We need nested loops
• Since I am lazy, let's copy the loops from the previous slide into the sum method
• What do you think we should do before the loops?
• What needs to happen in the loops?
• What should be returned?

Activity - More 2D array processing: max

Record your participation in Piazza clicker question Lesson18 Q2

We can use basically the same loops to implement other array processing algorithms

• What if we wanted to find the maximum value in matrix in IntArray2DProcessor? Think about what you need to change.
• How are you going to initialize the max?
• Write the code to complete the max method
• Uncomment both calls to max in the code in the tester to test your method

1. Got it
2. Got it, but it was hard
3. Tried but did not get it in the time available
4. Did not know where to start

 

Activity - Processing one row

Record your participation in Piazza clicker question Lesson18 Q3

What if we wanted to process only one row in IntArray2DProcessor?

• You will only need one loop
• Complete the method in IntArray2DProcessor to find the sum of the elements in the r^th row

  public int sum(int r)

• The first index will always be r
• The second index will vary
• Like this
  • matrix[r][column]
• Complete the method
• Uncomment the code in the tester to test your method

1. Got it
2. Got it, but it was hard
3. Tried but did not get it in the time available
4. Did not know where to start

 

Activity - Initializing a 2D Array

Record your participation in Piazza clicker question Lesson18 Q4

Given an 8 × 8 array for a board game:

int[][] board = new int[8][8];

Finish the main method in the Board class and use nested loops to initialize the board so that zeros and ones alternate, like this

0 1 0 1 0 1 0 1
1 0 1 0 1 0 1 0
0 1 0 1 0 1 0 1
. . .
1 0 1 0 1 0 1 0

Hint: Check whether row + column is even.

• if row + column is even the element should be 0
• otherwise it should be 1

1. Got it
2. Got it, but it was hard
3. Tried but did not get it in the time available
4. Did not know where to start

 

Activity - Printing a 2D Array

Record your participation in Piazza clicker question Lesson18 Q5

Add code to the main method of the Board class in the previous activity to print the board.

1. Got it
2. Got it, but it was hard
3. Tried but did not get it in the time available
4. Did not know where to start