REPL

Syntax#

julia>
help?>
shell>
\[latex]

Remarks#

Other packages may define their own REPL modes in addition to the default modes. For instance, the Cxx package defines the cxx> shell mode for a C++ REPL. These modes are usually accessible with their own special keys; see package documentation for more details.

Launch the REPL

After installing Julia, to launch the read-eval-print-loop (REPL):

On Unix Systems

Open a terminal window, then type julia at the prompt, then hit Return. You should see something like this come up:

On Windows

Find the Julia program in your start menu, and click it. The REPL should be launched.

Using the REPL as a Calculator

The Julia REPL is an excellent calculator. We can start with some simple operations:

julia> 1 + 1
2

julia> 8 * 8
64

julia> 9 ^ 2
81

The ans variable contains the result of the last calculation:

julia> 4 + 9
13

julia> ans + 9
22

We can define our own variables using the assignment = operator:

julia> x = 10
10

julia> y = 20
20

julia> x + y
30

Julia has implicit multiplication for numeric literals, which makes some calculations quicker to write:

julia> 10x
100

julia> 2(x + y)
60

If we make a mistake and do something that is not allowed, the Julia REPL will throw an error, often with a helpful tip on how to fix the problem:

julia> 1 ^ -1
ERROR: DomainError:
Cannot raise an integer x to a negative power -n. 
Make x a float by adding a zero decimal (e.g. 2.0^-n instead of 2^-n), or write
1/x^n, float(x)^-n, or (x//1)^-n.
 in power_by_squaring at ./intfuncs.jl:82
 in ^ at ./intfuncs.jl:106

julia> 1.0 ^ -1
1.0

To access or edit previous commands, use the ↑ (Up) key, which moves to the last item in history. The ↓ moves to the next item in history. The ← and → keys can be used to move and make edits to a line.

Julia has some built-in mathematical constants, including e and pi (or π).

julia> e
e = 2.7182818284590...

julia> pi
π = 3.1415926535897...

julia> 3π
9.42477796076938

We can type characters like π quickly by using their LaTeX codes: press \, then p and i, then hit the Tab key to substitute the \pi just typed with π. This works for other Greek letters and additional unicode symbols.

We can use any of Julia’s built-in math functions, which range from simple to fairly powerful:

julia> cos(π)
-1.0

julia> besselh(1, 1, 1)
0.44005058574493355 - 0.7812128213002889im

Complex numbers are supported using im as an imaginary unit:

julia> abs(3 + 4im)
5.0

Some functions will not return a complex result unless you give it a complex input, even if the input is real:

julia> sqrt(-1)
ERROR: DomainError:
sqrt will only return a complex result if called with a complex argument. Try
sqrt(complex(x)).
 in sqrt at math.jl:146

julia> sqrt(-1+0im)
0.0 + 1.0im

julia> sqrt(complex(-1))
0.0 + 1.0im

Exact operations on rational numbers are possible using the // rational division operator:

julia> 1//3 + 1//3
2//3

See the https://stackoverflow.com/documentation/julia-lang/3848/arithmetic#t=201608312001481144658 topic for more about what sorts of arithmetic operators are supported by Julia.

Dealing with Machine Precision

Note that machine integers are constrained in size, and will overflow if the result is too big to be stored:

julia> 2^62
4611686018427387904

julia> 2^63
-9223372036854775808

This can be prevented by using arbitrary-precision integers in the computation:

julia> big"2"^62
4611686018427387904

julia> big"2"^63
9223372036854775808

Machine floating points are also limited in precision:

julia> 0.1 + 0.2
0.30000000000000004

More (but still limited) precision is possible by again using big:

julia> big"0.1" + big"0.2"
3.000000000000000000000000000000000000000000000000000000000000000000000000000017e-01

Exact arithmetic can be done in some cases using Rationals:

julia> 1//10 + 2//10
3//10

Using REPL Modes

There are three built-in REPL modes in Julia: the Julia mode, the help mode, and the shell mode.

The Help Mode

The Julia REPL comes with a built-in help system. Press ? at the julia> prompt to access the help?> prompt.

At the help prompt, type the name of some function or type to get help for:

Even if you do not spell the function correctly, Julia can suggest some functions that are possibly what you meant:

help?> printline
search:

Couldn't find printline
Perhaps you meant println, pipeline, @inline or print
  No documentation found.

  Binding printline does not exist.

This documentation works for other modules too, as long as they use the Julia documentation system.

julia> using Currencies

help?> @usingcurrencies
  Export each given currency symbol into the current namespace. The individual unit
  exported will be a full unit of the currency specified, not the smallest possible
  unit. For instance, @usingcurrencies EUR will export EUR, a currency unit worth
  1€, not a currency unit worth 0.01€.

  @usingcurrencies EUR, GBP, AUD
  7AUD  # 7.00 AUD

  There is no sane unit for certain currencies like XAU or XAG, so this macro does
  not work for those. Instead, define them manually:

  const XAU = Monetary(:XAU; precision=4)

The Shell Mode

See https://stackoverflow.com/documentation/julia-lang/5420/shell-scripting-and-piping/19299/using-shell-from-inside-the-repl#t=201608312119344259037 for more details about how to use Julia’s shell mode, which is accessible by hitting ; at the prompt. This shell mode supports interpolating data from the Julia REPL session, which makes it easy to call Julia functions and make their results into shell commands: