Chapter 1: Let's get printing!
In this exercise, we will familiarise ourselves with the Julia REPL and print a few things.
Get to know and love the REPL
The great thing about REPLs is that they make learning a language a lot easier - especially a language that is at least partly intended to allow you to quickly prototype complex ideas in a few lines of code, manipulate your code, iterate until you get the desired results, then flesh it out or tidy it up.
Using the REPL isn't complicated, but it might be unusual at first if you have not used much of a similarly constructed REPL (such as Prelude, the Haskell REPL). It helps to remember a few commands and tricks for the future.
Modes
Julia's REPL has four 'modes', each indicated in the prompt.
Julia
Prompt: julia>
In this mode, you interact with the Julia engine directly. Pressing Return/Enter executes the command in the current line and prints the result. You can also access the result of the last operation in the ans variable. If you don't wish for Julia to print the result, conclude your line with a ; (semicolon).
Help
Prompt: help>
By typing ? at the beginning of a line, you can enter the help mode. Entering anything searches the Julia documentation for that word:
help?> besselj
INFO: Loading help data...
Base.besselj(nu, x)
Bessel function of the first kind of order "nu", J_\nu(x).
To leave help mode, use the Backspace key.
Shell
Prompt: shell>
To execute shell commands, enter ;. The REPL prompt will change to shell>, and anything you enter will be executed as a shell command.
To leave shell mode, use the Backspace key.
Search
Prompt: (reverse-i-search)
By pressing ^R (Ctrl+R), you can initiate a reverse search of your history, including from previous sessions. It will show you any commands that match the pattern you have entered.
Key bindings
The Julia REPL uses a few key bindings that might be very familiar to those who have used *nix based systems frequently in the past. Most importantly, to exit the REPL, you can use ^D (Ctrl+D), which will also close your shell, and you can abort a current operation using ^C (Ctrl+C).
There are many more key bindings that the Julia REPL recognises, but these should be enough to get you off the ground.
Autocompletion and Unicode entry
Julia's autocomplete feature recognizes the Tab key trigger, your new best friend. Julia also knows Unicode math, so this is valid Julia:
julia> 2*2.5*π
15.707963267948966
Pressing the Tab key on a LaTeX symbol name autocompletes to Unicode symbols:
julia> \sqrt[Tab]2
julia> √2
1.4142135623730951
For all Unicode completions, check out the Unicode conversion table in the Julia documentation. Remember, you can also use Unicode symbols in saved code.
Let's say something!
In the following, we'll be exploring a few ways to say hello to Julia. Each of these has their place in the coder's arsenal, and while you will eventually use the REPL a little less and your text editor a little more, you will probably use the REPL quite a bit to test out new ideas. Think of the REPL as your lab and the text editor as your drawing board – scientists who spend all their time in the lab eventually go mad, while those who are always at the drawing board rarely discover much!
Using the REPL
A REPL interface repeats everything you enter. This makes saying hello to the world rather simple – simply declare a variable containing the string literal "Hello, Julia!" or, even simpler, just declare the string literal. Let's see both of these in action in the REPL.
julia> "Hello, Julia!"
"Hello, Julia!"
julia> v = "Hello, Julia!"
"Hello, Julia!"
julia> v
"Hello, Julia!"
Using println()
Now for some actual coding. Time to invoke our first real function. The println function prints the string representation of an object.
julia> println("Hello, Julia!")
Hello, Julia!
You might notice that the "Hello, Julia!" string is not printed in bold type. This is to indicate that rather than part of the REPL's print cycle, it is a system output.
Using string concatenation
String concatenation is just a fancy name for putting strings together. In this case, we create two variables that represent strings, then use the string() function to put them together. The string() function concatenates each of its positional arguments.
julia> what = "Hello"
"Hello"
julia> whom = "Julia"
"Julia"
julia> string(what, ", ", whom, "!")
"Hello, Julia!"
Julia also allows for variables to be called within string literals. So the above is equivalent to:
julia> "$what, $whom\!"
"Hello, Julia!"
And this is true even for maths (or any function!)
julia> "2 plus 2 is $(2+2)."
"2 plus 2 is 4."
Running Julia programs
To run a Julia program, you have two options – either include it in another program or the REPL, or specify it as a positional argument in the command line.
Let's open a file in our favourite text editor, call it hello.jl (the commonly accepted file name for a Julia program), and enter
println("hello world")
We now have two ways to launch it.
By opening the REPL, we can include the file. This will evaluate everything in the file, then return the result of the last valid expression.
julia> include("hello.jl")
"Hello, Julia!"
Alternatively, you can merely grace your command line with your greeting to Julia by launching Julia with the appropriate argument:
$ julia hello.jl
"Hello, Julia!"
Finally, you can use the shebang comment at the beginning of a file to make it shell executable. For instance, you could write:
#!/usr/bin/env julia
println("Hello, Julia\!")
This then allows you to execute your file from the command line by simply typing ./hello.jl (provided that the file is executable - you may need to use chmod to make it so!).
Chapter Conclusion
The journey of a thousand miles begins with a single step. If REPLs are something new to you (they are rarely used with a number of OOP languages, and most people using it in Python are using functional(ish) paradigms), then this chapter was two steps at the very least. Give yourself a pat on the back and a cookie, and play around with Julia. See you soon with the next chapter!