2023-08-24
Julia is a programming language designed for scientific and mathematical applications. Its development dates from 2009 at MIT, with the first public release in 2012. The language reached v1.0 in 2018, and in 2019 three of its lead developers were awarded the Wilkinson Prize for Numerical Software.
This lesson introduces two software applications for running and creating Julia programs. By the end of this lesson, you should be able to
To learn the content of this and subsequent lessons, you should try out the examples and exercises while reading.
The lessons aim to provide the minimum necessary information for you to make effective use of Julia in your courses. At the end of each lesson, we provide suggestions for further reading. The Linux version of Julia is available on the PCs in the Maths & Stats computer labs. To run Julia and VSCode on your own computer, you can download the installers for your operating system (Windows, MacOS or Linux) from https://julialang.org and https://code.visualstudio.com/download. Windows users can also install Julia and VSCode from the Windows store. After installing both applications, follow these instructions to install the Julia extension for VSCode.
The term REPL is an acronym for Read-Evaluate-Print-Loop, and describes the simplest kind of text-based method for interacting with a computer via a terminal. The REPL displays a prompt, which, in the case of Julia looks like
julia>and indicates that the system is ready to receive input from the
user.
After the user types a statement and presses the Enter key, the REPL
reads this input, evaluates the expression,
prints the value obtained (or an error message if expression
was invalid in some way), and then prints the prompt ready to repeat
these steps again. Usually, after pressing Enter the prompt will
reappear at once, but if the statement initiated a long-running
computation then a noticeable delay will occur.
If you are using a PC in one of the Maths & Stats computer labs,
then you need to boot into Linux and start the Konsole
application to open a terminal running Bash, the default Unix
shell. At the $ prompt, type julia and press Enter. You
should see something similar to the image below.
On Windows, you should be able to find a graphical launcher that opens the Julia REPL in a command window. On MacOS, you should be able to run Julia from a Bash terminal just as on Linux.
At the Julia prompt, try typing
x = 2
y = x/2 + 1/xThe first statement creates a variable x and assigns it
the value 2. The second statement creates a second variable
y and assigns it the value of the expression
x/2 + 1/x, which in this case equals
2/2 + 1/2, that is, 1.5. Notice that Julia
prints these values in the REPL as you type the commands.
The REPL maintains a command history that you can access using the up
and down arrow keys. Try pressing the up arrow key once to recall
y = x/2 + 1/x and use the backspace key to delete
1/x, then replace it by 3/x and press Enter.
Now y will take the new value 2.5. Using the
command history in this way can save a lot of typing.
Typing a question mark ? in the REPL changes the prompt
from julia> to help?>, after which you
can see a help message about any Julia object by typing its name and
pressing Enter. Try typing ?hypot to see an explanation of
the hypot function. Notice that the julia>
prompt returns after the message is printed, so you can now try
doing
hypot(3, 4)The REPL supports tab completion in many contexts. For example, if you are not sure about the name of a Julia object you can try typing the first few letters followed by Tab key.
Exercise. Type ?sin followed by Tab.
You should see a list of several functions. Look up the help messages
for some of them.
You can quit the REPL by typing Ctrl-D as the first character
following the julia> or help>
prompt.
VSCode is an Integrated Development Environment, or IDE, that supports the use of extensions for a wide variety of programming languages including Julia. You should be able to find a launcher with the following logo.
To use VSCode on your own computer, download the installer. When you start VSCode, either from a terminal or via the launcher, a window should open that looks something like the screenshot below.
On Linux you can start VSCode from the Bash terminal by typing
code.
Open the VSCode command palette by typing Ctrl+Shift+P or by
selecting this option from the View menu. In the text box, type
start REPL.
Assuming that the Jula extension is installed, you should see “Julia
env:v1.9” and “Julia: Starting Language Server” on the bottom bar of the
VSCode window, and soon one of the panes should display the
julia> prompt. VSCode will start downloading some cache
files, but you can start using the REPL immediately.
Download the file fibonacci.jl and save
it to a convenient folder. In VSCode, click on the Explorer icon (the
top one in the left bar) or type Ctrl+Shift+E, and then click on the
blue “Open Folder” button and navigate to the folder containing
fibonacci.jl. Alternatively, select the “Open Folder …”
option from the File menu. A popup will ask “Do you trust the authors of
the files in this folder?”. Click on “Yes, I trust the authors”.
You should see fibonacci.jl listed in the Explorer pane.
Click on the file name to open it in an editor pane, which should look
like the screenshot below.
Hover your mouse over the triangle in the top right of the editor pane (the one shaped like a Play button). The pop-up text should read “Julia: execute active File in REPL”. Click on the triangle. You should see the output below appear in the REPL pane.
When you are finished, close VSCode by selecting “Exit” from the File menu.
In this lesson you have seen how to
julia> prompt;The Julia website provides many resources for learning about the language. In particular, you can find the official documentation, which can be intimidating at first but should become more accessible as you complete these lessons.
Similarly, the VSCode website provides the official documentation about using this IDE.