1. Week 7 - Wednesday
CS 121

2. Last time What did we talk about last time? Introduction to arrays
Computing variance

3. Questions?

4. Project 3

5. Arrays

6. 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];

7. 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]);

8. 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

9. Array Examples

10. Cards We can represent a deck of cards as an array of 52 items
One easy way is to make each item a String giving the name of the card
We can extend last time's lab and store each of these names in an array

11. Array swap
Swapping the values of two variables is a fundamental operation in programming
It is going to become more important in arrays because now the order of variables has become important
The simplest way to swap two variables involves using a third variable as a temporary location

12. Swap code
Here is an example of swapping two Strings indexed i and j in an array of Strings called array
int i = in.nextInt();
int j = in.nextInt();
String temp = array[i];
array[i] = array[j];
array[j] = temp;

13. Shuffling Cards
Using the swap code, we can do a random shuffling of a deck
To do so, we go through each element of the array, and randomly swap it with any of the later elements
for( int i = 0; i < n; i++ ) {
exchange = i + (int)(Math.random() * (n - i));
temp = deck[i];
deck[i] = deck[exchange];
deck[exchange] = temp; }

14. Searching Searching through an array is an important operation
The simplest way to do so is just linear search: check every element in the array
Searching and sorting are really key to all kinds of problems
We’ll cover both topics in depth in a few weeks

15. Sound

16. Sound Like light, sound is a wave
For those physics buffs here, sound is usually transmitted as a compression wave
In contrast, light is a transverse wave
It doesn’t really matter, we can pretend that sound is a transverse wave

17. Frequency The human ear can hear between about 12 Hz and 20,000 Hz
The higher the frequency of the wave, the higher the frequency of the note
Note (ha, ha) that the A an octave
above A440 has twice the
frequency
Each half-step is an increase in the
frequency by a factor of about 1.06
Note
Frequency A
440 B
493.88 C
523.25 D
587.33 E
659.26 F
698.46 G
783.99
880

18. Example of frequency change
We can take a sound:
And reproduce that sound at double the frequency:
Notice that we have to add twice as much information to have the sound fill the same amount of time

19. Amplitude
The amplitude of a wave is the distance from the trough of a wave to its peak
In sound, amplitude is a measure of volume
The larger the amplitude, the louder the sound
Amplitude

20. Example of amplitude change
We can take a sound:
And make the sound with half the amplitude:
The frequency is exactly the same, but the sound is half as loud

21. Real sounds
Something that looks like a sine wave is called a pure tone
No real instruments play anything like that
Even the purest real sound has overtones and harmonics
Real sound is the result of many messy waves added together:

22. Digital Sampling

23. Sampling On a computer, we cannot record a wave form directly
As usual, we have to figure out a way to store a wave as a series of numbers
We are going to use these numbers to approximate the heights of the wave at various points

24. Sample rate
As we all know by now, Hertz (Hz) is a unit that means a number of times per second
Equivalent to Hz is s-1
We are going to break down the wave into lots of slices
We are going to have 44,100 slices in a second
Thus, we are slicing at 44,100 Hz

25. Sample values We slice up a wave and record the height of the wave
Each height value is called a sample
By getting 44,100 samples per second, we get a pretty accurate picture of the wave

26. Sample format There are many different formats for sampling audio
In our system, each sample will be recorded as a double
The minimum value of a sample will be -1.0 and the maximum value of a sample is 1.0
A series of samples with value 0.0 represents silence
Our samples will be stored in an array

27. StdAudio Class

28. Purpose of the StdAudio class
Audio data on Windows machines is sometimes stored in a WAV file
A WAV file is much simpler than an MP3 because it has no compression
Even so, it contains two channels (for stereo) and can have many different sample rates and formats for recording sound
The StdAudio class lets you read and write a WAV file easily and always deal with a single array of sound, sampled at 44,100 Hz

29. StdAudio methods Everything you’d want to do with sound:
To do interesting things, you have to manipulate the array of samples
Make sure you added StdAudio.java to your project before trying to use it
Method
Use
double[] read(String file)
Read a WAV file into an array of doubles
void save(String file, double[] input)
Save an array of doubles (samples) into a WAV file
void play(String file)
Play a WAV file
void play(double[] input)
Play an array of doubles (samples)

30. StdAudio example Let’s load a file into an array:
If the song has these samples:
Perhaps samples will contain:
String file = "song.wav";
double[] samples = StdAudio.read(file);
-.9
-.7
-.6
-.4
-.2
-.1 .1 .2 .3 .4 .5 .6

31. StdAudio example
With the audio samples loaded into the array named samples, we can play them as follows:
StdAudio.play(samples);

32. Generating sound with StdAudio
Or, we could generate sound from scratch with StdAudio
This example from the book creates 1 second of the pitch A440:
double[] sound = new double[StdAudio.SAMPLE_RATE + 1];
for( int i = 0; i < sound.length; i++ )
sound[i] = Math.sin(2 * Math.PI * i * 440 / StdAudio.SAMPLE_RATE);
StdAudio.play(sound);

33. Breaking a sound into parts
What if we wanted to play the second half of a sound followed by the first half?
I know, why would we want to do that?
double[] samples = StdAudio.read(file);
double[] switched = new double[samples.length];
for(int i = 0; i < samples.length/2; i++ )
switched[i + samples.length/2] = samples[i];
for(int i = samples.length/2;
i < samples.length; i++ )
switched[i - samples.length/2] = samples[i];
StdAudio.play(switched);

34. Upcoming

35. Next time…
More array examples
Lab 7

36. Reminders Keep reading Chapter 6 of the textbook
Start working on Project 3