Part 3 - Running julia code

For a lot of this course, we will be using the julia programming language rather than the command line.

There are a few different ways to run julia code, and this section will get you acquaninted with a couple of them.

The julia REPL

Open julia, which you should have installed in the first part of this lesson.

Your terminal application should open, running julia:

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.4.0 (2020-03-21)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia>

Technically speaking, this is the julia "Read, evaluate, print, loop", or "REPL".

When you enter text at the julia> prompt, the REPL reads it, evaluates it as julia code, prints^[1] the result, and then loops back to the prompt.

Let's try it! Type println("Hello, World!") at the prompt and hit enter.

julia> println("Hello, World!")
Hello, World!

Don't worry if you don't understand all of the components of this command - we'll get there.

Tip

As much as possible, try typing out the commands in these lessons, rather than copy-pasting. It's important to build the muscle memory, and to see the errors that appear when you have typos!

For example, what did I miss here:

println(Hello, World!)

Running julia from the command line

You can also execute short snippets of code from the command line.

To Do

Open your terminal and enter the following:

$ julia -e 'println("Hello, World!")'

Hello, World!

The -e is a command-line flag that tells julia to just execute the next command as julia code. Note the use of single quotes (') surrounding the command.

Checking Questions

What happens if you just enter julia at the command line without additional arguments?
What happens if you use double quotes instead of single quotes? Why do you think that is?

Running julia scripts

Our code is often going to be much more complicated than what we've done so far. In those cases, and in order to keep a record of what we're doing, it's useful to put our julia code in a file.

To Do

Open up VS Code, and create a new file called hello.jl.
Type println("Hello, World!") into the file and save it. Note the path to the directory where you saved the file!
run:
sh $ julia <path_to_directory>/hello.jl Hello, World!

When code is saved into a file that can be run from the commandline, it's called a "script." All of your assigments will be julia code written into files and commited to code repositories using git.

But it's important to realize that all of this code is the same; it's just text. That text has specific requirements in order to be parsed by the julia interpreter, but whether you run code in the REPL, from the command line, or in a script, it has the same behavior.

1print - In the days before monitors, results would literally be printed on a piece of paper. These days, "printing" just means displaying the results.