contrib/cs4hs-lab.txt

# @title{CS4HS Music & Racket}

```@title{CS4HS Music & Racket}

@section{Getting Started}

After logging in, click on the DrRacket symbol in the bottom center of
the screen.

Click on the "Run" button.  If all goes well, the bottom, "interactions"
window should show this text:

Welcome to DrRacket, version 5.1.1
Language: Beginning Student with List Abbreviations [custom]; memory limit: 256 MB.
Teachpack: cs4hs-lib.rkt.
>

If you see something different, ask for help :).

For our first program, type this in the top ("definitions") window:

(rsound-play ding)

Now, click on the "Run" button.

With luck, you heard something! Adjust the computer volume so it's at a
comfortable level.

Okay, let's explore the sets of values in DrRacket.

@section{Values 1: Integers}

In Racket, integers are numbers. Try typing

14

in the interactions window. Evaluate it, by clicking "Run". What did it produce?

-324134127658176587622563578765

?

@section{Operations 1: arithmetic:}

In Racket, there is just one way to use an operator: you write a left paren

(

followed by the name of the operator

+

followed by the arguments, separated by spaces

3 4 5

followed by a right parenthesis

)

Put it all together, and you get

(+ 3 4 5)

Figure out what this program should produce.  *After* doing this, click on
the Run button to see if you agree with Racket.

Naturally, there are other operations as well:

(- 5 4)

(/ 4 9)

... did that one produce what you expected?

Exercise 1: write a program that divides twelve by three.

@section{Operations 2: nested operations:}

Just like in math class, expressions can go inside other expressions:

(* (- 2 1) (- 4 3))

That is: multiply the result of two minus one by the result of four
minus three.

Exercise 2: write a program that divides three plus five by sixteen
divided by four.

@section{Values 2: lists of integers:}

Here's a list of numbers:

(list 60 62 64)

Exercise 3: what will the program (list (+ 60 12) (+ 62 12)) produce? Try it!

@section{Values 3: sounds}

We've already seen one value: ding.

Try running this program:

ding

by itself.  What does it produce?

What about the rsound-play operator... what does it do? Remember, you used
it earlier.

@section{Operations 3: music}

Finally! We're ready to make some music.

Here's another operation: note-num-8ths. It takes a list of numbers, and produces

- try ding; it's a kind of value. A kind of value you can put into rsound-play.

- intro to numbers

- intro to operations; they all have parentheses!

- (list 60 64 67) ; it's another kind of value; a list of numbers.

- Can you put it into rsound-play? Try it! Oh dear. What kind of thing does
rsound-play accept?

- Here's a new operator: note-num-4ths. It takes a list of numbers, and
produces an rsound.

Here's a program that uses rsound-play.

(rsound-play (note-num-4ths (list 60 64 67 64)))

Can you figure out how the numbers are related to the music?

Numbers are hard to remember. Can we give them names? yes! using
"define". So, for instance, if you add

(define c4 60)

To the beginning of your program, then you can use the name "c4" to refer
to the value 60. Substitute c4 for the earlier use of 60, so you get

(rsound-play (note-num-4ths (list c4 64 67 64)))

- give the name "e4" to 64
- give the name "g4" to 67

Then, replace all the numbers in the program with names.

Here are some more:

g3 : 55
a3 : 57
b4 : 59

Exercise: Now, try to write the first line of "happy birthday."

@section{Another value: false.}

Here's another value:

false

it's what's called a "boolean" value. There's another, shorter way to write this:

#f

We're going to want the shorter version, for use in our songs.

We can use the #f value to represent "rests", or gaps in the melody. Like this:

(define notes (list 72 69 72 #f 72 69 72 #f 74 72 70 69 67 69 70 #f))

(rsound-play (note-num-4ths notes))

here's another operator: "reverse". It takes a list, and produces a new list in the opposite order.

Try this program:

(reverse (list 60 70 80))

Exercise: use "reverse" on the notes in the last song.  Is it recognizable?

Songs to identify:

60 #f 60 #f 60 62 64 #f
64 62 64 65 67 #f #f #f

72 #f 72 #f 72 #f 72 71
69 #f 69 #f 69 #f 69 67
65 #f 65 #f 65 #f 65 69
67 #f 67 #f 67 #f 67 67

#f 64 64 64 64 62 60 62
64 #f 55 57 #f #f 60 62
60 #f #f 57 #f #f 60 62
60 #f #f 57 #f #f #f #f

@section{More operators:}

append: list list -> list
rsound-append: rsound rsound -> rsound
rsound-overlay: rsound rsound -> rsound
transpose : number list -> list

Can you use rsound-overlay to play two melodies at once?

```