Lab Exercise 1: Equipping yourself for the projects
The purpose of this lab time is to give you an introduction to the basic tools we will be using this semester. These tools include a terminal, a text editor, and the Python interpreter.
Labs and projects are tightly coupled. Each project is initiated with a set of lab exercises. In the projects, you will write code that expands on the code written in lab. For each project, you will be writing Python code and running it. The programs typically create images. You will be posting some of these images, along with a description of the project, on a Wiki page. You will then turn in your code by putting it in a special directory on one of the campus computer servers. Today's lab will allow you to practice each of these tasks.
We'll be going through a lot of little things today. Here are some short tutorial videos if you need to revisit any of them as you start your project.
- Introduction to the Terminal
A terminal is a text-based method of using your computer that is much more powerful than using the restricted point and click interface. To start a terminal, open the Terminal application. It is located in the Applications::Utilities folder.
In a terminal, you can type commands that will let you manipulate files, execute programs, and open documents. Anything you can do in the Finder, you can do in a terminal (the reverse is not true).
When working in a terminal, the current directory is called your working directory. A terminal will usually start in the home directory of your account.
In a terminal we often need to specify a file or directory on the hard drive. The complete description of where a directory or file on a computer is located is called its path.
Directories in a path are separated by a forward slash /. The topmost directory on the filesystem is indicated by a single slash. The directory tree for your computer is all relative to the top level directory. In other words, starting with a /, you can type the complete path to any file on your computer. For example, the following is the path to my home directory.
Most of the time, however, you will be specifying paths relative to your current directory. Any path that starts without a slash is relative to your current working directory. Note that paths are case-sensitive, so be sure to use proper capitalization.
You will use paths, and filenames to change directories, list the contents of directories, and run Python programs.
Some important terminal commands are the following. We'll walk through how to use them together in lab.
- ls - with no argument, ls lists the files in the current directory. If you put a path after the ls command, it will list the contents of the specified path.
cd <directory> - change directory. If you use the command with no arguments, it changes the directory back to your top-level home directory. The current directory is specified as . and the parent directory of the current working directory is specified using ..
Example cd commands cd Set the current directory to your home directory cd ~ Set the current directory to your home directory. (The ~ character is shorthand for your home directory.) cd .. Move up to the parent directory of the current working directory cd ../../ Move up to the parent of the parent of the current working directory. cd blah Move into the sub-directory blah cd /Volumes/Courses/CS151/ Move to the directory give by the absolute path (starts with /). cd - Move to your last working directory
- mv <from> <to> - move a file, including just renaming it in the current directory. The mv command will remove the file in the old location after copying it to the new location.
- cp <from> <to> - copy a file to a new location or name
- rm <filename> - remove the file (will not remove a directory). Not that this is not the same as dragging a file to the trash. Once the filed as been removed, it is gone for good.
- rm -r <directory> - remove the directory and all the files and subdirectories in it
Some useful terminal properties are the following.
Wildcard characters: the star character * is a wildcard
character. If you want to see all the files in a directory that
start with A, you can type:
A possibly bad thing to do is to type rm *
Tab completion: when you have typed part of the name of a file or program, pressing the tab key will complete the filename as far as possible while the choice is unique. For example, if you have only one file that starts with the letter b, then typing b and then the tab key will complete the filename.
If you have two or more files that start with b, tab completion will beep at you. If you press tab again, then the terminal will show you the options (all the files that start with b).
- Hitting return - you don't have to be at the end of the line to press return. If you go back and edit something in a terminal command, youcan press return and execute the whole line no matter where the cursor is located. Try it.
Control keys - holding down the control key and pressing another key lets you execute a number of different useful commands. Learning these will make you faster that using arrow keys or the mouse.
Command Action cntl-a move the cursor to the start of the line cntl-b move the curse back one position cntl-c kill the current process running in the terminal cntl-d delete the character under the cursor. cntl-e move the cursor to the end of the current line cntl-f move the cursor forward one position cntl-k delete everything on the current line to the right of the cursor cntl-l clear the terminal screen. cntl-n move forward in the terminal history cntl-p move backward in the terminal history cntl-z freeze the current process running in a terminal. You can put it in the background by typing bg or put it in the foreground by typing fg.
Some useful (but not as common) terminal commands are the following.
- pwd - tells you the complete pathname of the current directory
- less <filename> - scroll through a file
- cat <filename> - send the file to standard output
- echo "a string" write the text within the string to standard output
- touch <filename> touch either updates the modification date on a file or creates an empty file if the named file does not exist. This can be useful in various situations, like when you are learning to create, rename, and delete files using a terminal.
- Introduction to the file servers
A fileserver is a central file system you can access from any computer. It's like having a virtual USB key you can plug into a computer to store your files. Colby maintains two different fileservers we use in this course. One (the Personal server) is for you to store the code you are working on. The other (Courses) is for you to turn in code. We will talk about the Courses directory late in the lab. For now, let's focus on where you should keep code that is work in progress.
There are two ways to mount the file server.
- Option 1: Load the root server directory and navigate to your directory.
You can mount the Colby fileserver root directory by going to the Finder and typing cmd-K, or selecting 'Connect To Server...' from the Go menu. It will bring up a dialog box, into which you want to enter the following.
Macs (CS Computer Lab): smb://filer.colby.edu Windows: \\filer.colby.edu
and, if necessary, click on the appropriate sub-directories until you find your personal directory.
This system is backed up up regularly, and you can access it from any computer on the Colby network. We strongly suggest you store all of your work for this course into your personal directory. You can, in fact, work directly from your personal directory. Let's practice doing that for the first project.
- Mount your personal directory (Cmd-K, then smb://filer.colby.edu/Personal/srtaylor for Stephanie).
- In the terminal, navigate to it. Type cd followed by the path to the directory. E.g. for Stephanie, it may look like this:
Or, use a shortcut. If you type cd in the terminal, then drag the icon for your directory onto the terminal, then the path to it will automatically be pasted.
- Now make a directory (in your personal folder) named project1. You can do this using the Finder, or you can type mkdir project1 into the terminal.
- Option 1: Load the root server directory and navigate to your directory.
- Introduction to text editing
Text editors are the workhorse programs for writing code. You don't want fancy fonts or WYSIWYG layouts, you just want to see lines of text, preferably with syntax highlighting, which means that special words in a language are highlighted to make it easer to read the code. There are many editors to choose from.
In lab, I will be using TextWrangler. It is a free text editor for the Mac and it has all the features I need for writing programs. If you would like to install it on your personal computer, go to the TextWrangler web site and follow their instructions (which will tell you that TextWrangler itself is not what they want you to install - you should install BBedit instead. We are sticking with TextWrangler in lab because we have a working version and like it.).
Creating Your First Python Program:
- Open TextWrangler
- You may want to set some preferences.
For example, it can be useful to display line numbers. This will help later, when you encounter error messages that tell you where an error is by supplying the file's name and the line's number.
- Create a new file (Cmd-N). Save it as smart.py in your project1 directory.
- Put in the line:
print('You are smart')
- Then save the file (Cmd-S).
- Now, you can instruct Python to run the program. In the terminal type
Congratulations, you just ran your first Python program. (Python humor)
Note that it works only if the current working directory is the one that contains the file smart.py. So, one thing you need to do when working on a project is to ensure that your Terminal is in the right directory. Think of it as making sure TextWrangler and Terminal agree about where the programs are. Note that, in general, you run Python programs by typing
For your information
Here are some other text editors you should know about:
- nano/pico - nano (or pico on some systems) is a very simple text editor that works in a terminal. All of the commands use the control key and are listed at the bottom of the screen. To open up a file, just type the name of the editor followed by the name of the file. If the file does not exist nano creates the file.
- BBEdit - BBEdit is an upscale version of TextWrangler designed for writing code and html (web pages). Colby has a set of licenses for it, and it is installed on the lab computers. It has a few more features than TextWrangler, but will only work on the lab computers.
emacs - emacs is a text editor that has been used heavily by computer scientists for almost 40 years. It has many powerful capabilities, and the key commands in emacs are used in many other places (like TextWrangler). emacs is the battery-powered swiss-army knife with optional nuclear reactor of the editor world (emacs humor).
If you open emacs, the opening screen tells you how to access a tutorial. The most important thing to know is how to save and get out. To save, hold the control key down and type x then s (C-x C-s). It will ask you for a filename (and give you a default if you opened a file) and then save it. To exit, hold down the control key and type x then c (C-x C-c).
- JEdit - JEdit is very simlar to TextWrangler, and you can download it for free and run it on any operating system. Many students use it since it runs on Windows. It is slightly more complex than TextWrangler.
- XCode - XCode is a powerful integrated development environment for Mac OSX, but you can also use it just for its editor, which is pretty decent.
- Introduction to Python
Python is a simple yet powerful interpreted language for making computers do stuff. Today we're going to look at some basic arithmetic operations.
We generally write code using a text editor and then run the code using the command line, just as in the prior example. Now we're going to write a program that takes in three numbers and calculates their sum and average.
Doing Some Arithmetic
- In your text editor, create a new file. Save the file as addthree.py
- Type your name, a date, and the course and project in a comment at the top of the file. Every Python file you ever write should start with these three lines.
- Type the following three lines of code.
print( 'version 1' )
print( 'sum', 42 + 21 + 5 )
print( 'avg', (42 + 21 + 5) / 3 )
- Run the program from the terminal.
- What does the program do? What happens if you change the 5 to a 6 on the last two lines of code and run it again?
Remember, you have to save the file before you run it again on the Terminal.
Defining a Function
- A function is a named block of code. You can execute the code by using the function's name along with any arguments the function requires.
- To define a function, start with the keyword
deffollowed by the name of the function. A pair of parentheses after the function's name can include parameters to the function. Finally, the function definition ends with a colon, which is how Python defines the start of a block of code. Type the following example.
def myfunction(a, b, c):
The name of this function is
myfunctionand it has three parameters
a, b, c. The parameters hold the values given to the function when it is called. After the function definition, we can write the code that belongs to the function. Any code inside the function must be tabbed in relative to the function definition. If we want to print the sum of the three values in
a, b, cwe can use the following code.
print('sum', a + b + c)
- If you run your addthree.py file, it still won't do anything different. Defining a function does not execute it. In order to execute, or call the function, we need to use the function's name, followed by parentheses with the function's arguments. After the function definition, and not tabbed in, write the following code.
myfunction( 4, 5, 6 )
Now if you save your file and run it in the terminal, it should print out the sum as 15.
One handy thing about a function is you can easily call it many times with different arguments. Add another call to the function, with different arguments, and see what happens.
- Writing and Submitting Your Project Reports and Rubrics
Each week, in addition to your code, you will submit a short written report describing the task, your solution, and your results. For this course, we would like you to use Google Docs to write and submit your reports.
For each project we will also provide a rubric that identifies all of the expectations for the project. The rubrics will be attached to your project in the Google classroom. Leave the rubric attached to your project and attach your Google doce report. You will receive your feedback on each project in comments given on the rubric and your written report. In order for the graders to access your rubric form, you need to hit submit when you are done with your project. Once you hit submit, you can no longer edit your report and we can edit and provide feedback.
- How to turn in code
You will turn in your code by putting it in a directory in the Courses server. On the Courses server, you should have access to a directory called CS151, and within that, a directory with your user name. Files that you put into that directory you can edit, read, and write, and the professor can edit, read, and write, but no one else. To hand in your code and other materials, you will create a new directory, such as project1, and then copy your code into the project directory for that week.
You can mount the Colby Courses fileserver by going to the Finder and typing cmd-K, or selecting 'Connect To Server...' from the Go menu. It will bring up a dialog box, into which you want to enter the following.
Macs (CS Computer Lab): smb://filer.colby.edu Windows: \\filer.colby.edu
Select Courses from the set of choices. Then, in the resulting finder window, click on the CS151 directory, and then your hand-in directory (it will have your username as its name).
Practice turning in your code by copying your entire project1 directory from your Personal server to the Courses server. The easiest way to do this is to drag and drop the folder from one Finder (one open to Personal) to another (one open to Courses).
For more information about Python, see the Python 3.7 documentation pages for reference materials and tutorials.
When you are done with the lab exercises, you may start on the rest of the project.