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

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

3. 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. Then you work alone for the project.

4. Lab 6: Loops & Debugging
Exercise TA: 2, Scanner
"flushing the buffer"

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

6. Unit Testing (method) Writes method to meet requirements
Verifies that method meets requirements
These are different perspectives and can be challenging to switch between. But the better you can develop this skill of switching between perspectives the more reliable your code will likely be.

7. Requirements /** * 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; }

8. Which are true? 1,2,3,4 String []arr = new String[4];
arr[2] = new String("hello");
arr[3] = arr[2];
arr[0] = "hello";
arr[1] = "hello";
System.out.println( arr[2] == "hello" ); //1
System.out.println( arr[2].equals( "hello")); //2
System.out.println( arr[3] == arr[2]); //3
System.out.println( arr[3].equals(arr[2])); //4
1,2,3,4
1,2,4
2,3,4
2,4
try it

9. Will this set all elements to 3?
final int START_VALUE = 3;
int [] list = new int[5];
for ( int i = 0; i < list.length -1; i++) {
list[i+1] = START_VALUE; }
Yes
Not first
Not last
Error
try it

10. What does this code do? double value = 0.0;
find minimum value
find maximum value
compiler error
logic error
double value = 0.0;
double [] nums= {3.0,5.0,2.3,4.1};
for ( int i = nums.length; i > 0; i--) {
if ( nums[i-1] > value) {
value = nums[i-1]; }
try it.

11. Which swaps the ith and i+1th values?
temp = list[i];
list[i] = list[i+1];
list[i+1] = temp;
temp = list[i+1];
list[i] = temp;
list[i+1] = list[i];
try it

12. What is print out? char [] list1 = new char[]{'a', 'b', 'c'};
true
false
char [] list1 = new char[]{'a', 'b', 'c'};
char [] list2 = new char[]{'b', 'b', 'c'};
System.out.println(
list1.length == list2.length);
System.out.println( list1[1] == list2[1]);
try it.

13. What is print out? System.out.println( list1 == list2);
true
false
char [] list1 = new char[]{'a', 'b', 'c'};
char [] list2 = new char[]{'a', 'b', 'c'};
System.out.println( list1 == list2);
System.out.println( list1.equals( list2));
try it.
Note: equals method depends on which equals method is called. This equals is probably the one inherited from Object and simply compares references
See java.util.Arrays for equals methods to compare the contents of arrays.
See java.util.Arrays for equals methods to compare the contents of arrays.

14. Arrays of Arrays
Also called: Multi-dimensional arrays

15. Which picture of 2-D array is more accurate?
int [][] board = new int[3][2];
Depends on your purpose. The first is simpler conceptually and for many purposes, fine. However, when working with code the 2nd is more technically accurate and important to understand.

16. How many elements will this array hold?5 10 25
Error
int [][] board = new int[5][5];
try it.

17. What is the data type of board[3][1]?
int [][] board = new int[5][5];
int
int []
reference to int
can't access board[3][1]
int element data type

18. What is the data type of board[2]?
int [][] board = new int[5][5];
int
int []
reference to int
can't access board[2]
int []
The declaration is: int [][] board. So board dereferenced once (board[2]) results in: int []

19. What values will elements in this array have, initially?
boolean [][] board;
board = new boolean[5][5];
false
true
none, they must be initialized first.
try it.

20. Which is correct way to set an element value?
int [][] board = new int[5][5];
How to set element 0,0 to 5?
board[0][0] = 5;
board[0,0] = 5;
board{0,0} = 5;
error
try it.

21. How many elements in this array?
int [][] board = {{1,2},{4,5,6}}; 5 6
ragged arrays are not valid
error
try it.

22. How would you access "Hi."? responses[1][1][1]
Error
String [ ][ ][ ] responses = {
{{"hello"},
{ "How do you do.", "Hi."}},
{{"always"},
{ "When?", "Really, always?"}} };
try it.

23. Today
More Arrays & Methods

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

25. Draw a Diagram int [][] board; board = new int[][]{{1,2},{4,5,6}};

26. Will this initialize the array to all 1's?
yes no
don't know
Error
int [][] board;
board = new int[][]{{1,2},{4,5,6}};
for ( int i = 0; i < 2; i++)
for ( int j = 0; j < 3; j++)
board[ i ][ j ] = 1;
try it.

27. Will this initialize the array to all 1's?
yes no
don't know
Error
int [][] board;
board = new int[][]{{1,2},{4,5,6}};
for ( int i = 0; i < b[i].length; i++)
for ( int j = 0; j < b[j].length; j++)
b[ i ][ j ] = 1;
try it.

28. Will this initialize the array to all 1's?
yes no
don't know
Error
int [][] board;
board = new int[][]{{1,2},{4,5,6}};
for ( int i = 0; i < b.length; i++)
for ( int j = 0; j < b[ i ].length; j++)
b[ i ][ j ] = 1;
try it.

29. Is array of int at row 1 accessible?
yes no
don't know
Error
int [][] board;
board = new int[][]{{1,2},{4,5,6}};
board[1] = null;

30. Question 1 minute to answer
I want to keep the results to analyze later

31. What will print out? public static void methodA(int [] list) {}
public static void main(String []args) {
int [] list = new int[]{1,2,3};
methodA( list);
System.out.println( list[0]); 1 9
list[0]
try it.

32. 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).

33. Is it possible for methodC to change the elements in the array?
public static void main(String []args) {
final char [] items = new char[]{'c','b','a'};
methodC( items); }
It is NOT possible for methodC to change the local variable items, regardless of whether items is final or not.
It IS possible for methodC to change the array contents since the reference of the array is passed to methodC. Both main and methodC, having the reference, can access and change the array contents.

34. 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
Other
Error
A is correct. Local variables are those declared within methods and are stored in the stack.

35. 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
try it.

36. Does main use the array from createList?
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
try it.

37. What will print out? z[0] 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]);
z[0] 10
Error
try it.

38. What prints out? num:10 list:[1, 2, 11] num:11 list:[1, 2, 3]
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, 3]
list:[1, 2, 10]
try it.

39. What prints out? num:10 list:[1, 2, 11] list:[4,5,10] list:[1, 2, 3]
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]
list:[4,5,10]
list:[1, 2, 3]
list:[1, 2, 10]
try it.

40. 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]); 3 5 9
try it.

41. 3 Questions
1 minute each

42. 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]); 1 9
list[0]
try it.
In method, array reference changed prior to accessing array contents. Changes to array contents are only visible within the method since the new array is not returned.

43. What will print out? public static void methodA(int [] list) {
list = new int[3]; }
public static void main(String []args) {
int [] list = new int[]{1,2,3};
methodA( list);
System.out.println( list[0]); 1 9
list[0]
try it.
In method, array reference followed and contents changed, prior to changing array reference. Change in array visible outside the method

44. What will print out? public static void methodA(int [] list) {}
public static void main(String []args) {
int [] list = new int[]{1,2,3};
methodA( list);
System.out.println( list[0]); 1 9
list[0]
try it.
In method, array reference followed and contents changed. Change visible anywhere reference to array is accessed since array contents are on the heap.

45. Common Algorithms for Arrays
Searching
Sorting
Palindrome?

46. Sorting Demo 9, 6, 4, 5, 8, 2 import java.util.Arrays;
public class BubbleSort {
public static void swapInts(int a, int b) {
int temp = a;
a = b;
b = temp; }
public static void main(String[] args) {
int x = 1;
int y = 2;
System.out.println("x = " + x + ", y = " + y);
swapInts(x, y);
int[] list = new int[]{10, 5, 3, 7, 9};
printArray(list);
swapInts(list[0], list[4]);
swapElements(list, 0, 4);
bubbleSort(list);
addElementAtEnd(list, 12);
list = addElementAtEnd(list, 4);
public static void printArray( int [] arr) {
System.out.println( Arrays.toString( arr));
public static void swapElements(int[] arr, int i, int j) {
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
public static void bubbleSort(int[] arr) {
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr.length -1; j++) {
if (arr[j] > arr[j+1]) {
swapElements(arr, j, j+1);
public static int[] addElementAtEnd( int[] arr, int elem) {
int newLength = arr.length + 1;
int[] newArray = new int[newLength];
newArray[i] = arr[i];
newArray[newArray.length - 1] = elem;
return newArray;