CS 200 Arrays, Loops and Methods Jim Williams, PhD

Transcription

1 CS 200 Arrays, Loops and Methods Jim Williams, PhD

2 This Week 1. Team Lab: Arrays and Hangman a. drawing diagrams (bring paper and pencil) 2. BP1 Milestone 1 due Thursday 3. Lecture: More Arrays and Methods

3 Key Concepts Memory Access Drawing a picture of memory for an array Looping through multidimensional arrays appropriate use of.length Unit Testing Common Array algorithms

4 Question 1 minute to answer - I want to keep the results to analyze later

5 What will print out? public static void methoda(int [] list) { list[0] = 9; public static void main(string []args) { int [] list = new int[]{1,2,3; methoda( list); System.out.println( list[0]); list[0]

6 Memory Access Local variables and parameters (stored on stack) are only visible within the method itself. to share value, return value from method, caller captures and uses Instances/objects (stored on heap) are available anywhere using the reference. Follow the reference from inside or outside the method to shared memory area (heap).

7 Sorting Demo 9, 6, 4, 5, 8, 2

8 4 Questions First 3, 1 minute each Last 1, 2 minutes (sorting)

9 What will print out? public static void methoda(int [] list) { list = new int[3]; list[0] = 9; public static void main(string []args) { int [] list = new int[]{1,2,3; methoda( list); System.out.println( list[0]); list[0]

10 What will print out? public static void methoda(int [] list) { list[0] = 9; list = new int[3]; public static void main(string []args) { int [] list = new int[]{1,2,3; methoda( list); System.out.println( list[0]); list[0]

11 What will print out? public static void methoda(int [] list) { list[0] = 9; public static void main(string []args) { int [] list = new int[]{1,2,3; methoda( list); System.out.println( list[0]); list[0]

12 Add Element At Front of Array TopHat Reorder the lines

13 Draw a picture int [] items = new int []{5,6,7;

14 What does this do? int [] list = new int[4]; for ( int i = 0; i <= list.length; i++) { list[i] = i * 2; set each element to i * 2 0, 2, 4, 6 compiler error runtime error

15 Is it possible for methodc to change the contents of the array? public static void main(string []args) { char [] items = new char[]{'c','b','a'; methodc( items);

16 Where in memory is variable z? public static void main(string []args) { int r = 3; int c = 4; int [][] z = new int[r][c]; stack heap it is local Error

17 Does createlist return a new array? public static int [] createlist(int size) { return new int[size]; public static void main(string[]args) { int [] list = createlist( 10); list[3] = 10; yes no reference lost Error

18 Can main see the changed value within list? public static void changelist(int [] list) { if ( list.length > 0) { list[0] = 10; public static void main(string []args) { int [] z = new int[5]; changelist( z); System.out.println( z[0]); yes no reference lost Error

19 Can you call this method to get the new array? public static void createlist(int [] list) { list = new int[10]; yes no reference lost Error

20 What prints out? static void change( int num, int [] list) { num = 11; list[2] = num; public static void main( String [] args) { int [] list = {1,2,3; int num = 10; change( num, list); System.out.println("num:" + num + "\nlist:" + Arrays.toString( list)); num:10 list:[1, 2, 11] num:11 list:[1, 2, 11] num:10 list:[1, 2, 3] num:10 list:[1, 2, 10]

21 What prints out? static void change( int num, int [] list) { list = new int[]{4,5,6; list[2] = num; public static void main( String [] args) { int [] list = new int[] {1,2,3; int num = 10; change( num, list); System.out.println("num:" + num + "\nlist:" + Arrays.toString( list)); num:10 list:[1, 2, 11] num:10 list:[4,5,10] num:10 list:[1, 2, 3] num:10 list:[1, 2, 10]

22 BP1 Notes Process: 3 milestones then further testing Concepts: Use only material from Chapters 1-8 Teams: Once you submit as a team for Milestone 1 then you are a team for the entire project. If your team is not working you must dismantle your team by notifying an instructor by by 11:59pm on Milestone 1 (tonight). Then you work alone for the project.

23 Draw a Picture int [][] board; board = new int[3][2]; How do you find the number of rows? How do you find the number of columns?

24 Draw a Picture int [][] board; board = new int[][]{{1,2,{4,5,6; board[1] = new int[4];

25 Memory Access Local variables and parameters (stored on stack) are only visible within the method itself. to share value, return value from method, caller captures and uses Instances/objects (stored on heap) are available anywhere using the reference. Follow the reference from inside or outside the method to shared memory area (heap).

26 Can you call this method to get the new array outside the method? static void createboard(int [][] board) { board = new int[3][3]; yes no sometimes Error

27 Can you call this method to get the new array? static int [][] createboard(int r, int c) { return new int[r][c]; yes no reference lost Error

28 What will print out? public static void changelist(int [][] list) { list[0] = new int[3]; list[0][1] = 9; public static void main(string []args) { int [][] z = new int[5][5]; changelist( z); System.out.println( z[0][1]);

29 Will this initialize the array to all 1's? public static void main(string []args) { int [][] board = {{2,3,{4,3,2; initboard( board); static void initboard(int[][] board) { for ( int i = 0; i < 2; i++) for ( int j = 0; j < 3; j++) board[ i ][ j ] = 1; yes no don't know Error

30 Will this initialize the array to all 1's? public static void main(string []args) { int [][] board = {{2,3,{4,3,2; initboard( board); static void initboard(int[][] b) { for ( int i = 0; i < b[i].length; i++) for ( int j = 0; j < b[j].length; j++) b[ i ][ j ] = 1; yes no don't know Error

31 Will this initialize the array to all 1's? public static void main(string []args) { int [][] board = {{2,3,{4,3,2; initboard( board); static void initboard(int[][] b) { for ( int i = 0; i < b.length; i++) for ( int j = 0; j < b[ i ].length; j++) b[ i ][ j ] = 1; yes no don't know Error

32 What will print out? public static void methoda(int [] list) { list = new int[3]; list[0] = 9; public static void main(string []args) { int [] list = new int[]{1,2,3; methoda( list); System.out.println( list[0]); list[0]

33 What will print out? public static void methoda(int [] list) { list[0] = 9; list = new int[3]; public static void main(string []args) { int [] list = new int[]{1,2,3; methoda( list); System.out.println( list[0]); list[0]

34 What will print out? public static void methoda(int [] list) { list[0] = 9; public static void main(string []args) { int [] list = new int[]{1,2,3; methoda( list); System.out.println( list[0]); list[0]

35 Unit Testing (method) Developer Writes method to meet specification Tester Verifies that method meets specification

36 Specification /** * This method returns the number of 9s in the table. table a 2 dimensional array of int the number of 9s in table */ public static int num9s(int [][]table) { return -1;

37 Common Algorithms for Arrays Searching Sorting