Using the REPL
Last updated on 2024-11-15 | Edit this page
Estimated time: 20 minutes
Overview
Questions
- “How to use the REPL?”
Objectives
- “Explore basic functionality of input.”
- “Learn how to declare variables.”
- “Learn about REPL modes.”
Entering the REPL
Melissa and her classmates open a terminal and launch
julia:
JULIA
_
_ _ _(_)_ | Documentation: https://docs.julialang.org
(_) | (_) (_) |
_ _ _| |_ __ _ | Type "?" for help, "]?" for Pkg help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 1.7.2 (2022-02-06)
_/ |\__'_|_|_|\__'_| | Official https://julialang.org/ release
|__/ |
julia>This is the so-called REPL, which stands for read-evaluate-print-loop. The interactive command-line REPL allows quick and easy execution of Julia statements.
Like the terminal, the Julia REPL has a prompt, where it awaits input:
implicit promt
Most of the code boxes that follow do not show the
julia> prompt, even though it’s there in the REPL.
Why?
It’s important to delineate input (what you type) and output (how the machine responds). The prompt can be confusing, so it is excluded. You may assume that any Julia box prepends the prompt on each line of input.
Visual Studio Code
An alternative to using the REPL through a terminal is to work with
Visual Studio Code or its open source altenative
VSCodium. VSC is a source code editor for which a julia
extension is available. After installing the application, simply click
on the “Extension” symbol on the left side and search for
julia. Once installt julia remains usable and
can be selected as a programming language in new documents.
For further guidance and visual aid, check out the provided video!
Variables
The first thing they try is to perform basic arithmetic operations:
OUTPUT
30.2That works as expected. It is also possible to bind a name to a value
via the assignment operator =, which makes it easier to
refer to the value later on. These names are called
variables.
OUTPUT
60.4Melissa notices that assignment also returns the value. She can also check which variables are defined in the current session by running
OUTPUT
name size summary
–––––––––––––––– ––––––––––– –––––––
Base Module
Core Module
InteractiveUtils 270.164 KiB Module
Main Module
ans 8 bytes Float64
distance 8 bytes Float64
distance_x_2 8 bytes Float64Unicode
In Julia, Unicode characters are also allowed as variables like
α = 2. Unicode characters can be entered by a backslash
followed by their LaTeX
name and then pressing tab (in this case
\alphatab).
REPL-modes
Unfortunately Melissa can’t remember the LaTeX name of ∂ so she copies the character , presses ? for help mode,
pastes the ∂ character, then presses enter:
OUTPUT
"∂" can be typed by \partial<tab>Great! This way she can easily look up the names she needs. She gets back to normal mode by pressing backspace.
Exploring Julia’s Help Mode
Help mode can also be used to look up the documentation for Julia
functions. Use Julia’s help mode to read the documentation for the
varinfo() function.
Another useful mode is the shell mode that can be entered by pressing ;. The prompt has now changed:
Shell mode can be used to issue commands to the underlying shell, but don’t confuse it with an actual shell: special shell syntax like piping won’t work. Like before, hit backspace to get back to the Julia prompt.
Hello, shell>
(pwd and cd) !
Two commonly used shell commands are pwd
(print working
directory) and cd (change
directory).
- Use
pwdto find out what is your current working directory. - Type the command
cdin shell mode, which by default will bring you to your “home directory”. - Use
pwdagain. Did you get a different result from before? Why or why not?
Hello, shell>
(pwd and cd) ! (continued)
The working directory is the location from which you launched Julia.
To navigate to a different directory, you can use the cd
command by entering: cd <directory>. By default, this
command will return you to your home directory if a specific directory
is not given. If you initially launched Julia from your home directory,
the working directory remains unchanged, so the output of the second
pwd command will be identical to the first. Conversely, if
you were in a different directory when you started Julia, the results of
the two pwd commands will differ. You can use
cd - to go back to your previous location.
Hello, shell>
(ls)!
Another useful shell command is ls (list
files). Use it to show the contents of your home directory.
Hello, shell>
(ls)! (continued)
The first cd command will bring you to your home
directory. ls will print a list of the files and directorys
in your current location.
Hello, shell>
(nano and cat)!
Use the shell mode to create a file called hello.jl with
the nano terminal text editor. Inside that file write the simple hello
world program print("Hello World!").
Check the content of the file using cat hello.jl and
then run the program using julia hello.jl.
Finally there is package mode that is entered with ] which is used for package management, which will be covered later on:
To exit shell, help or pkg mode, hit backspace.
Key Points
- “The REPL reads the given input, evaluates the given expression and prints the resulting output to the user.”
- “Pressing ? enters help mode.”
- “Pressing ; enters shell mode.”
- “Pressing ] enters pkg mode.”