Lab 1: Getting Started
1 Introduction(s)
You’ll work in labs in pairs. Find someone to work with for this first lab and introduce yourself.
Make sure at least one of you have a laptop to work on for this first lab.
The two of you will work as a team to solve problems. At any time, one of you will be the Head and the other will be the Hands. The Head does the thinking and the Hands does the typing. Hands type only what the Head tells them to, but you’re free to discuss any issues that pop up. We’ll have you switch off during the lab to make sure each of you get practice problem solving, dealing with syntax, and getting finger exercises on the keyboard.
You both should install DrRacket, but only one instance should be used during the lab.
2 Install DrRacket
Download and install DrRacket, part of the Racket platform, available for download from https://download.racket-lang.org/.
3 Meet DrRacket
You should already have DrRacket installed on your computer, but now is the chance to get help (or catch up) if you don’t yet. DrRacket is the program you’ll use to design and run your programs. Download, install, and run DrRacket to get started.
Help your partner install it on their machine if you’re done first. Then, pick the first Head and Hands and get continue on only one machine.
Explore DrRacket’s interface. First, find out how to set the current Language to the Beginning Student Language (BSL). We’ll tell you which language to use at the beginning of each lab, exercise, or assignment.
(Note: if you change the language, you’ll have to hit the <Run> button for it to take effect.)
Next, look through the documentation for the BSL. (Hint: also check the Help menu.) You should get comfortable searching and reading the documentation for anything you need to know about the language and its libraries.
Locate the definitions window and the interactions window.
The interactions window lets you quickly make simple calculations. You can type in some expression and hit <Enter> to run it. Test out a few expressions, try to do some arithmetic with big numbers and fractions (or anything else you want to test out).
The definitions window is where you’ll define and develop programs. You execute the code in the definitions window by hitting the <Run> button.
There’s a whole bunch more to explore, but that’s all you need for now. Feel free to try out the Stepper or other features as you program.
4 Finger exercises
Now that you’ve had a chance to install and look around in DrRacket, here are a few exercises to help you get a first glance at programming in BSL.
Use the interactions window to try some simple calculations. For each of these BSL expressions, make a prediction about what you think will happen when its evaluated. Discuss it with your partner. Then type it in the interactions window and press enter. If something different than expected occurs, discuss possible explainations with your partner. Try some variations of these expressions if it’s helpful.
Lab problem 1: A fancy calculator
In the interactions window, type each of the following and predict what happens when evaluated:
25
#true
"Fred"
(+ 4 8)
(* 4 8)
(string-append "Dear" "Fred")
(string-append "Dear" (string-append " " "Fred"))
(+ 3 4 6)
(string-append "Dear" " " "Fred")
(string-length "Dear Fred")
(string-length (string-append "Dear" " " "Fred"))
(string-length 352)
(string-length "352")
(widget 42)
(8 + 7)
(+ (8) 7)
(not #false)
The interactions window gives you an ephemeral interaction with computation. Once you press Run, all your work is wiped out and you get a fresh interaction prompt.
To write programs that persist, type expressions in to the Definitions window.
Lab problem 2: A fancy calculator that persists
Copy and paste all of the examples above into the Definitions window. Press Run. What happens?
You’ll notice that BSL prints out a read message in the interactions window saying there is a problem with this program.
Technically speaking, it’s not a program, it’s just junk.
To check whether some text is a grammatically well-formed program, press the Check Syntax button. This will highlight the offending peice of text and give you a message (the same one we saw when running).
Read the message and try to understand what it’s telling you. Once you understand, comment out that expression by putting a semicolon ";" in front of it. This tells BSL to treat the rest of the line after the ";" as prose intended only for the human reading and writing this code.
Try Check Syntax again. Rinse and repeat until Check Syntax succeeds. When it does, you have a well-formed BSL program. Hit Run. What happens?
Lab problem 3: A fancy calculator that persists (w/o errors)
Comment out the expressions that are ill-formed. Press Run. What happens?
Now you should see some values printed in the Interactions window, followed by another red message. Some of the expressions in the Definitions window will be colored in Halloween colors (orange and black). What do you think that means?
You’ve now seen two kinds of errors. Syntax errors are about grammar. They include things like putting parentheses in the wrong place or putting operators somewhere other than the front of an expression or using names that aren’t defined. The other kind of error here is a run-time error. It happens when you run a well-formed program that does something non-sensical, like divide by zero or uses a primitive operation on a kind of data that it doesn’t understand.
Lab problem 4: A fancy calculator that persists (w/o errors)
Comment out the expressions that produce run-time errors. Press Run. What happens?
Now you’ve got a fancy calculator that does extended "arithmetic"; it works not just for numbers, but for strings, booleans, and other kinds of values as we’ll see.
Press the Save button to save your Definitions window as a file on your computer. If you want to revist this program, you can just open it in DrRacket.
Switch Head and Hands. Whoever was typing should now be guiding the thinking and conversation. Whoever was guiding the thinking and conversation should now be typing.
In grade school, you were often asked to do the kinds of calculations we have just asked DrRacket to do. We are using a different notation (BSL) and some of our operations are different (e.g. you probably didn’t see string-append in 3rd grade). But essentially it’s the same process.
Another thing you might’ve been asked to do was to go beyond just producing the "right answer," but to justify your calculation by showing your work at each step.
You can ask DrRacket to show you exactly that information with the Step button. This will open up a new window, The Stepper, which let’s you explore step-by-step each part of the computation.
Lab problem 5: Show your work
Press the Step button. Step through the computation. Try to build a mental model of what’s happening. After a few steps, start making predictions about what will happen the next time you press Step. Work your way through all of the steps.
Now that you can write expressions, let’s move on to functions.
Lab problem 6: From Arithmetic to Algebra
Define a function double-plus1 that take a number and produces twice that number, plus one. So (double-plus1 5) should produce 11.
Define a function greeting that takes two arguments: a first and last name and produces a greeting for use in the opening of a letter or email. For example, (greeting "David" "Van Horn") should produce "Dear David Van Horn".
You’ve now got the basics covered. If you’ve made it this far and have time to spare, engage in some thoughtful experimentation.