1. CS 200
Arrays
Jim Williams, PhD

2. This Week Team Lab: Loops & Debugging Exam Review
Come to office hours to review in detail
Big Program 1: Sokoban
Lecture: Arrays

3. Arrays Hold multiple values Access individual values with an index
A reference type (not primitive)
Automatically initialized (allocated on heap)
Contiguous memory area

4. Memory Diagram for Arrays
int [] list;
list = new int[5];
int [] list2 = new int[]{ 2, 4, 6, 8};
terms: element, index, element datatype
reference, braces { }, brackets [ ]

5. Where is element n? single dimension arrays are contiguous in memory
Equally efficient to access any element
nth element's location:
address + (n * elementSize)
Helpful that arrays start with 0.
Why were the 1900's the 20th century?
Because centuries started counting at 1 rather than 0.

6. Question int [] list = new int[]{8,7,6,5,4};
If array begins at address 1000, what is address of the element with value 6?
* 2 is (4 is size of int in bytes, 2 is index of value 6)

7. Which are valid ways to create arrays?
1, 2, 3, 4
1, 2, 4
3, 4 2
int [] list1; //1
int [] list2 = new int[5]; //2
int list3 = new int[]{1,2,3}; //3
int [] list4 = new int[1,2,3,4];//4
try it.

8. Which array initializations are valid?
int [] listA = new int[4]; //1
int [] listB = new int[]{8, 7, 6}; //2
Scanner input = new Scanner( System.in);
int sizeC = input.nextInt();
int [] listC = new int[ sizeC]; //3
1, 2, 3
1,3
1,2
none
try it.

9. What will this print? int [] list = null; list[0] = 10;
list[1] = list[0] + 20;
System.out.print( list[1]); 30
1020
compiler error
runtime error
try it.

10. Will these show the contents of the arrays?
both will
both won't
one but not the other
char [] chars = {'a','b','c','d','e'};
System.out.println(chars);
int [] list = {10, 2, 23, 64};
System.out.println(list);

11. What is the value of sum? int sum; int [] list = {1,3,5};9 3
compiler error
runtime error
int sum;
int [] list = {1,3,5};
sum = list[0] + list[1] + list[2];
try it.

12. Today
P6 Due today
BP1 Milestone 1 due next Thursday
Lecture: Arrays

13. Either syntax works in Java
int [ ] list1; //preferable OR
int list2 [ ];

14. What is the value of z? int [] list = new int[3]; list[1] = 6;
int z = list[3]; 3
compiler error
runtime error
try it

15. What is the data type of listG?
String
array of String
compiler error
JVM error
String [ ] listG = {"first","middle","last"};
Try to draw a memory diagram of this.
Answer: array of String
String is the element data type

16. 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

17. 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

18. 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.

19. 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

20. 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.

21. 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.

22. Multi-Dimensional Arrays
More accurately: multiple single dimensional arrays

23. 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.

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

25. 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

26. 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 []

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

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

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

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