1. Week 7 - Monday
CS 121

2. Last time
What did we talk about last time?
Loop examples

3. Questions?

4. Project 2

5. Arrays

6. What good is a loop without something to loop over?
Loops are great
But, without a way to talk about a list of variables, we can’t get the full potential out of a loop
Enter: the array

7. Definition of an array
An array is a homogeneous, static data structure
Homogeneous means that everything in the array is the same type: int, double, String, etc.
Static (in this case) means that the size of the array is fixed when you create it

8. Where have you seen arrays before?
The args variable passed into the main() method is an array of type String
This array has a set number of Strings (maybe zero) that you can use as input to your program
Now, we are giving you the ability to create and manipulate your own arrays

9. Array Syntax

10. Declaration of an array
To declare an array of a specified type with a given name:
Example with a list of type int:
Just like any variable declaration, but with []
type[] name;
int[] list;

11. Instantiation of an array
When you declare an array, you are only creating a variable that can hold an array
At first, it holds nothing, also know as null
To use it, you have to create an array, supplying a specific size:
This code creates an array of 100 ints
int[] list;
list = new int[100];

12. Accessing elements of an array
You can access an element of an array by indexing into it, using square brackets and a number
Once you have indexed into an array, that variable behaves exactly like any other variable of that type
You can read values from it and store values into it
Indexing starts at 0 and stops at 1 less than the length
list[9] = 142;
System.out.println(list[9]);

13. Length of an array
When you instantiate an array, you specify the length
Sometimes (like in the case of args) you are given an array of unknown length
You can use its length member to find out
int[] list = new int[42];
int size = list.length;
System.out.println("List has " + size +
" elements"); //prints 42

14. Automatic initialization
When you create an int, double, char, or boolean array, the array is automatically filled with certain values
For other types, including Strings, each index in the array must be filled explicitly
Type
Value
int
double
0.0
char
'\0'
boolean
false

15. Explicit initialization
Explicit initialization can be done with a list:
Or, a loop could be used to set all the values:
String[] days = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};
String[] numbers = new String[100];
for(int i = 0; i < numbers.length; i++)
numbers[i] = "" + (i + 1);

16. Array Memory

17. Memory
An array takes up the size of each element times the length of the array
Each array starts at some point in computer memory
The index used for the array is actually an offset from that starting point
That’s why the first element is at index 0

18. A look at memory
We can imagine that we have an array of type int of length 10
Java decides what address in memory is going to be used, but let’s say it starts at 524
Addresses 12 43 -9 6
789
-23 23 10
Indexes

19. Connection to for-loops

20. for loops + arrays = power
Arrays are a fixed size list of a single kind of data
A for loop is ideal for iterating over every item and performing some operation
for loops and arrays will crop up again and again
Of course, a while loop can be used with an array, but it rarely is

21. for loop for summing an array
Imagine that we have an array of ints called list
Let’s use a for loop to sum up those ints
Super easy, just like before
We don’t even need to know how big the array is ahead of time
int sum = 0;
for( int i = 0; i < list.length; i++ )
sum += list[i];

22. Array example Ask the user how many words he or she wants to enter
Make an array of String values of that length
Read in all of the words and store them in the array
Print all the words back out

23. Statistics
Recently, we showed you how to add a set of numbers together as they were input by a user
Although this is a useful technique, not every operation is possible
We can find the sum or the average of the numbers because we only need to see the numbers once
What operation needs to see the numbers more than once?

24. Variance
Variance is a measurement of how spread out numbers are from their mean
To calculate it, you have to calculate the mean first
The formula is:
Where N is the number of elements, xi is the ith element, and is the mean

25. Code for variance
Given an array of doubles called numbers, here’s the code for finding their variance
double average = 0;
double variance = 0;
double temp;
for( int i = 0; i < numbers.length; i++ )
average += numbers[i];
average /= numbers.length;
for( int i = 0; i < numbers.length; i++ ) {
temp = numbers[i] – average;
variance += temp*temp; }
variance /= numbers.length;

26. Mid-Semester Evaluations

27. Upcoming

28. Next time… More on arrays Representing sound inside of a computer
StdAudio

29. Reminders Keep reading Chapter 6 of the textbook Finish Project 2
Due tonight before midnight!