Computer Science 252
Problem Solving with Java

Spring 2015, The College of Saint Rose

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Agenda

• Announcements
  • exams not graded yet, hope to get them back to you on Tuesday
  • get Lab 9: Final Project designs in soon (no late penalties will apply before early next week, but the sooner you submit the sooner I can give you feedback)
• More two-dimensional array examples
• Matrices
• In-class Exercise 23 - (20 lecture assignment points) due before the start of our next class.

  Your task is to add some functionality to the Matrix2D example and write code to test it in the main method there. The functionality you are to add:

  • A method max that returns the largest number in the matrix (5 points)
  • A destructive method, that is, a method that modifies the matrix on which it is called, named scale that multiplies each entry in the matrix by a number (5 points)
  • A non-destructive method, that is, one that does not change the matrix on which it is called, named multiply that works much like add but computes the matrix-matrix multiplication result. If you don't remember how to multiply matrices, see the "Matrix Product" section of the Wikipedia entry to Matrix multiplication. (10 points)

  Please submit only the file Matrix2D.java in Submission Box under "InClass23" before you leave class.

Lecture 23 Assignment

Due at the start of class, Tuesday, April 21.

Please submit answers to these questions in Submission Box under "LA23" or in hard copy by the start of our next class. We will discuss these questions at the start of class, so no late submissions are accepted. Please be sure that your name is clearly indicated in all submissions.

Note: this is a somewhat longer lecture assignment than usual.

First, some review since so many seemed to have trouble with writing methods on the exam.

• Write a method that takes an ArrayList of objects that implement the Drawable2DInterface as its parameter (the parameter will have type ArrayList<Drawable2DInterface>) and returns the number of the objects in the array whose x-coordinates are less than 200. (5 points)
• Write a method that takes an int and an ArrayList of Integer as parameters and returns another ArrayList of Integer that includes only those values from the original ArrayList that are greater than the int parameter's value. (5 points)

The popular Sudoku puzzle game (for example, see here) consists of a board that has 9 rows and 9 columns of numbers ranging from 1-9. Suppose we are writing a class to store a Sudoku board. We will assume that filled-in entries contain a number 1 through 9, and blank entries are represented by 0.

• Write a declaration of the instance variable board that would store the board using a two-dimensional array of int. Show also the construction of the array. (3 points)
• Write a method checkRowValidity that takes a row number (let's use rows numbered 0-8 for simplicity) and checks that no two filled-in entries contain the same number within that row (this would be illegal in the game). It should return true if the row is legal, false if not. (6 points)
• Assume you have a similar method checkColValidity (no need to write it). Using your checkRowValidity and checkColValidity methods as building blocks, write a method checkRowsColsValidity that checks that all rows and columns are valid in the current board. (6 points)

Terminology

• matrices

Examples

• Breakout
• Matrix2D