Chapter 7 Arrays

A 12-element array

Declaring and Creating Arrays Arrays are objects that occupy memory Created dynamically with keyword new int c[] = new int[ 12 ]; Equivalent to int c[]; // declare array variable c = new int[ 12 ]; // create array –We can create arrays of objects too String b[] = new String[ 100 ];

Problem Develop a test case that create an array of 10 integers and print out the initial values of this array’s elements

Solution 1: Using the Conventional for Loop

Solution 2: Using the for-each Loop

Using an Array Initializer int n[] = { 10, 20, 30, 40, 50 }; –Creates a five-element array –Index values of 0, 1, 2, 3, 4

Problem Develop a test case that create an array of the following 10 integers 32, 27, 64, 18, 95, 14, 90, 70, 60, 37 Print out the values of this array’s elements

Solution 1: Using the Conventional for Loop

Exercise Use the for-each loop to rewrite the previous test case

Problem Develop a test case that create an array of 10 even integers, starting from 2 Print out the values of this array’s elements

Problem Develop a test case that create an array of the following 10 integers 87, 68, 94, 100, 83, 78, 85, 91, 76, 87 Calculate the sum of all elements in the array

Solution 1

Question Can we make the sum() method to be the static one?

Solution 2: Using varargs

Problem Using bar charts to display array data (grade distribution) graphically Bar labels: “00-09”, “10-19”, …, “90-99”, and “100” See an example in the next slide

Example Input: Grade distribution data: 0, 0, 0, 0, 0, 0, 1, 2, 4, 2, 1 Output: The bar chart

Exercise Compare the following solution with the previous one

Using the Elements of an Array as Counters

Revisiting the Randomness Verification Problem Problem statement: To show that the numbers produced by nextInt() occur with approximately equal likelihood, develop an object that rolls 6000 times of a die. Thus, each integer from 1 to 6 should appear approximately 1000 times

Solution 1: Without Using Array

Solution 2: Using Arrays

Question Do a comparison between the Solution 1 and Solution 2

Using Arrays to Analyze Survey Results

Problem Develop a method that receives an array of responses about the rating of food quality and returns an array of counts corresponding to each rating 1-10 Rating scale: 1 mean awful, 10 means excellent See an example in the next slide

Example Input: 1, 2, 6, 4, 8, 5, 9, 7, 8, 10, 1, 6, 3, 8, 6, 10, 3, 8, 2, 7, 6, 5, 7, 6, 8, 6, 7, 5, 6, 6, 5, 6, 7, 5, 6, 4, 8, 6, 8, 10 Expected output: 2, 2, 2, 2, 5, 11, 5, 7, 1, 3

Case Study: Card Shuffling and Dealing Simulation

Sub-Problem 1 A card has a face and a suit –A face can be one of the following 13 values: Ace, Deuce, Three, Four, Five, Six, Seven, Eight, Nine, Ten, Jack, Queen, King –A suit can be one of the following 4 values: Hearts, Diamonds, Clubs, and Spades 1.Develop enums for Face and Suit 2.Develop a Card class with the toString() method

Sub-Problem 2 Develop the DeckOfCards class by making the following test pass

Sub-Problem 3 Develop a method that can shuffle a deck of cards by making the following test pass

Sub-Problem 4 Develop a method that can deal one card –Throw an NoSuchElementException if there is no card left

Case Study: Class GradeBook Using an Array/List to Store Grades

Problem Statement A grade book has a course name and a list of grades Develop a grade book class that can perform the following tasks 1.Constructing a grade book object with a given course name and a given array of grades 2.Returning the minimum grade 3.Returning the maximum grade 4.Returning the average grade 5.Drawing the bar chart of grades

Task 1: Constructor

Example Course name: CS101 Introduction to Java Programming Grades: 87, 68, 94, 100, 83, 78, 85, 91, 76, 87

Task 2: Minimum Grade

Q: How can we use Collections.min( ) ? A: Use java.util.List instead of array

Task 3: Maximum Grade

Task 4: Average Grade

Example Input: 87, 68, 94, 100, 83, 78, 85, 91, 76, 87 Output: 84.9

Task 5: Bar Chart

Example Input: 87, 68, 94, 100, 83, 78, 85, 91, 76, 87 Output:

Multidimensional Arrays

Two-dimensional array with three rows and four columns

Multidimensional Arrays (Cont.) Arrays of one-dimensional array –Declaring two-dimensional array b[2][2] int b[][] = { { 1, 2 }, { 3, 4 } }; –1 and 2 initialize b[0][0] and b[0][1] –3 and 4 initialize b[1][0] and b[1][1] int b[][] = { { 1, 2 }, { 3, 4, 5 } }; –row 0 contains elements 1 and 2 –row 1 contains elements 3, 4 and 5

Multidimensional Arrays (Cont.) Two-dimensional arrays with rows of different lengths –Lengths of rows in array are not required to be the same E.g., int b[][] = { { 1, 2 }, { 3, 4, 5 } };

Multidimensional Arrays (Cont.) Creating two-dimensional arrays with array- creation expressions –Can be created dynamically 3 -by- 4 array int b[][]; b = new int[ 3 ][ 4 ]; Rows can have different number of columns int b[][]; b = new int[ 2 ][ ]; // create 2 rows b[ 0 ] = new int[ 5 ]; // create 5 columns for row 0 b[ 1 ] = new int[ 3 ]; // create 3 columns for row 1

Outline Use nested array initializers to initialize array1 Use nested array initializers of different lengths to initialize array2

Outline array[row].length returns number of columns associated with row subscript Use double-bracket notation to access two-dimensional array values

7.9 Multidimensional Arrays (Cont.) Common multidimensional-array manipulations performed with for statements –Many common array manipulations use for statements E.g., for ( int column = 0; column < a[ 2 ].length; column++ ) a[ 2 ][ column ] = 0;

Case Study: Class GradeBook Using a Two-Dimensional Array

Problem Statement A grade book has a course name and a list of grades. Each student has more than one grades Develop a grade book class that can perform the following tasks 1.Constructing a grade book object with a given course name and a given 2D-array of grades 2.Returning the minimum grade 3.Returning the maximum grade 4.Returning a list of average grades 5.Drawing the bar chart of grades

Examples Grades Test1Test2Test3 Student 1: Student 2: Student 3: Student 4: Student 5: Student 6: Student 7: Student 8: Student 9: Student 10:

Task 1: Constructor

Task 2: Minimum Grade

Task 3: Maximum Grade

Task 4: Average Grades

Task 4: Drawing Bar Chart

Case Study: The Sudoku Puzzle

What You Have Learnt? To be filled