Mastering the Julia REPL

By: Steven Whitaker

Re-posted from: https://glcs.hashnode.dev/julia-repl

Julia is a relatively new,free, and open-source programming language.It has a syntaxsimilar to that of other popular programming languagessuch as MATLAB and Python,but it boasts being able to achieve C-like speeds.

Similarly to other dynamic languages,Julia provides a REPL(read-eval-print loop)for interactive development.

Here are some reasons whythe Julia REPL is a useful tool:

  • It speeds up prototypingbecause not all your codeneeds to compile each timeyou tweak a function.
  • It enables interactivelyworking with and inspectingJulia objects(for example,to see what data is stored,or to discover object fields).
  • It provides different prompt modes,enabling interactive accessto documentationand providing a clean interfacefor package management.

In this post,we will learn how to use the Julia REPL,including the different prompt modesand some useful keybindings.

This post assumes you already have Julia installed.If you haven’t yet,check out our earlierpost on how to install Julia.

Starting the Julia REPL

The Julia REPL starts immediatelywhen the Julia executable runs,either by double-clicking the executableor by calling julia from the terminal(with no arguments).

$ julia               _   _       _ _(_)_     |  Documentation: https://docs.julialang.org  (_)     | (_) (_)    |   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.  | | | | | | |/ _` |  |  | | |_| | | | (_| |  |  Version 1.X.X (20XX-XX-XX) _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release|__/                   |julia>

Once the REPL starts,you will be at the Julia prompt.

REPL Prompt Modes

The Julia REPL can operatein different prompt modes:

  • Julia mode (the default),
  • help mode,
  • Pkg mode, and
  • shell mode.

To enter help, Pkg, or shell mode,place the cursorat the beginning of the Julia mode promptandtype a question mark (?),a closing bracket (]),or a semicolon (;),respectively.To return to Julia mode,place the cursorat the beginning of the promptand press Backspace.

Toggling REPL modes

Julia Mode

The default prompt modeis the Julia prompt,where Julia code can be evaluated.We can type any valid Julia codeand press Enter,and then the code will be evaluatedand the result displayed.

julia> 1 + 12

We can also type invalid Julia code,and the REPL will tell us what went wrong.

julia> 1 + 1)ERROR: ParseError:# Error @ REPL[1]:1:61 + 1)#      extra tokens after end of expressionStacktrace: [1] top-level scope   @ none:1

We can assign variablesand define functionsto use in subsequent expressions.

julia> a = "hello""hello"julia> function say(x)           "say " * x       endsay (generic function with 1 method)julia> say(a)"say hello"

Note that the most recently evaluated statementis stored in a variable called ans.

julia> ans"say hello"

If desired,we can suppress outputwith a semicolon ;.

julia> "no output";

Note, however,that a semicolon does not suppressthe output of commands like print.

julia> print("this still prints");this still prints

Including Code from a File

We can also evaluate codestored in a file.For example,suppose we have a file, code.jl,with the following contents:

# code.jlfunction plus1(x)    return x + 1enda = 3b = plus1(a)

We can run this code in the REPLwith the include function:

julia> include("code.jl")4

Note that 4 is the resultof the last evaluated lineof the file,so that is what is displayedat the REPL.

Now that code.jl has been included,we can use what was definedin the file.

julia> c = a + b7julia plus1(c)8

Numbered Prompt

As noted earlier,ans stores the valueof the most recently evaluated statement.This means that ans is overwrittenevery time code is evaluated.Sometimes, however,it can be useful to storemore than just the most recent result.Julia provides a numbered promptfor this purpose.It can be activated as follows:

julia> using REPLjulia> REPL.numbered_prompt!()

Then the prompt will changefrom julia> to In [x]:,and results that would have been written to answill now be written to Out[x].

In [3]: 2^7128In [4]: Out[3]128

Help Mode

Help mode is useful if,as the name suggests,you need help.Help mode displays the documentation(if available)for what is typed in the prompt.

With your cursor at the beginningof the Julia prompt,type ? to enter help mode.Doing so will change the promptfrom julia> to help?>.From there,just type what you want help with.

We can pull up the documentation for a function:

help?> printsearch: print println printstyled sprint isprint prevind parentindices  print([io::IO], xs...)  Write to io (or to the default output stream stdout if io is not given)  a canonical (un-decorated) text representation.

We can find helpfor a variable we have defined:

julia> a = 4;help?> asearch: a any all abs ans Any axes atan asin asec any! all! acsc acot acos  No documentation found.  a is of type Int64.

We can also figure outhow to type unicode characters:

help?> "" can be typed by \equiv<tab>

(Just copy-paste the unicode characteryou want help typing.)

Pkg Mode

Pkg mode provides a convenient interfacefor package management.

With your cursor at the beginningof the Julia prompt,type ] to enter Pkg mode.Doing so will change the promptfrom julia> to (environment) pkg>.Here,environment refers tothe currently active package environment,which by default is @v1.X(where 1.X is the Julia version currently running).

(A package environment is essentiallythe set of packages currently available to use.We will talk more about package environmentsin a future post.)

Once in Pkg mode,we can (among other things)

  • list the currently available packages: 
    pkg> st
  • add packages: 
    pkg> add Plots
  • remove packages: 
    pkg> rm Plots
  • and update packages: 
    pkg> up

We can also ask for help:

pkg> ?  Welcome to the Pkg REPL-mode. To return to the julia> prompt, either press  backspace when the input line is empty or press Ctrl+C.  pkg> ? st  [st|status] [-d|--diff] [-o|--outdated] [pkgs...]  [st|status] [-d|--diff] [-o|--outdated] [-p|--project] [pkgs...]  [st|status] [-d|--diff] [-o|--outdated] [-m|--manifest] [pkgs...]  [st|status] [-d|--diff] [-e|--extensions] [-p|--project] [pkgs...]  [st|status] [-d|--diff] [-e|--extensions] [-m|--manifest] [pkgs...]  [st|status] [-c|--compat] [pkgs...]  Show the status of the current environment.

Visit the full Pkg documentationfor more details.

Shell Mode

Shell mode enables quick accessto the system shell.

With your cursor at the beginningof the Julia prompt,type ; to enter help mode.Doing so will change the promptfrom julia> to shell>.

Once in shell mode,we can type system commands.For example:

shell> echo shell modeshell mode

Note that on Windows,windows shell commands are not exposed.However,PowerShell or the command promptcan respectively be accessed via

shell> powershell

or

shell> cmd

Useful Keybindings

There are several useful keybindings,or keyboard shortcuts,that help improve and streamlinework at the REPL.

Accessing Previous Commands

When working at the REPL,it is often the casewhere previous commands need to be repeated,possibly with some modification.There are two keybindingsfor accessing previous REPL commands.

  • Up (i.e., the up arrow key):Press Upto cycle through previous commandsin reverse chronological order.If there is text before the cursor,only commands that matchup to the cursorwill be found.For example,pressing Up on a blank REPL linewill bring up the previous command,while pressing Upafter typing inwill bring up the previous commandthat starts with in(e.g., include("code.jl"),but not 1 + 1).
  • ctrl-r and ctrl-s:Press Control and the r key simultaneouslyto enable reverse search mode,and then type part of the command to find.The most recent commandthat matches the search querywill be found.Press ctrl-r again to findthe next most recent, matching command,and so on.If you accidentally skip the commandyou were looking for,press ctrl-s to searchin the opposite direction(forward search mode).Press Enter to select the command found,or press ctrl-c to exit search mode.

Tab Completion

Tab completion is another useful featureof the Julia REPL.After typing the first character(s)of a word,press Tab to complete the wordif there is an existing Julia objectthat matches what was typed.For example:

julia> my_very_long_variable = 1;julia> my<tab>julia> my_very_long_variable

If there are multiple matches,Tab will complete up to the ambiguity.Press Tab again to display the matches.

julia> prin<tab>julia> print<tab>print        println      printstyled

Dictionary keys and object fieldscan also be discoveredwith tab completion.

julia> d = Dict("key1" => 1, "key2" => 2, "another_key" => 3);julia> d["k<tab>julia> d["key<tab>"key1"  "key2"
julia> t = (foo = 1, foobar = 2);julia> t.f<tab>julia> t.foo<tab>foo     foobar

Finally,tab completion can also displayavailable function methods.

julia> split(<tab>split(str::T, splitter; limit, keepempty) where T<:AbstractString @ Base strings/util.jl:601split(str::AbstractString; limit, keepempty) @ Base strings/util.jl:608

Splitting a Command into Multiple Lines (alt-Enter)

Normally, when typing at the REPL,pressing Enterwill evaluate the typed expression(if it is a complete expression).Sometimes, however,we just want to insert a new line.Pressing alt-Enter(or alt-Shift-Enter on Windows)will do just that.

Using a Text Editor (alt-e)

Instead of typing commandsat the REPL directly,we can type commandsin a text editor of our choice(e.g., specified by the EDITORenvironment variable on Linux)by pressing alt-e.After saving and exiting the text editor,the contents will be pastedinto the REPL.

Halting Execution (ctrl-c)

If we want to stop a command,we can press ctrl-cto send an interrupt signaland return to the Julia prompt.

julia> sleep(100)^CERROR: InterruptException:julia>

Note that ctrl-c displays as ^Cin the REPL.

Pressing ctrl-c can also bean effective wayto get an empty prompt(even if no code is running).

julia> I've typed a lot and don't feel like pressing Backspace^Cjulia>

Terminating the REPL (ctrl-d)

Finally,pressing ctrl-dis a quick wayto terminate the REPL.

Summary

In this post,we learned about the Julia REPL,including the different prompt modesand some useful keybindings.

How has your experience beenwith the Julia REPL?Let us know in the comments below!

Comfortable with the Julia REPL?Move on to thenext post to learn about vectorizing, or broadcasting, functions!Or,feel free to take a lookat our other Julia tutorial posts!

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