Basics of the Julia language

Author

Marie-Hélène Burle

In this section, we explore the basics of Julia’s syntax.

Let’s try a few commands

x = 10
x
x = 2;
x
y = x;
y
ans
ans + 3

a, b, c = 1, 2, 3
b

3 + 2
+(3, 2)

a = 3
2a
a += 7
a

2\8

a = [1 2; 3 4]
b = a
a[1, 1] = 0
b

[1, 2, 3, 4]
[1 2; 3 4]
[1 2 3 4]
[1 2 3 4]'
collect(1:4)
collect(1:1:4)
1:4
a = 1:4
collect(a)

[1, 2, 3] .* [1, 2, 3]

4//8
8//1
1//2 + 3//4

a = true
b = false
a + b

Your turn:

What does ; at the end of a command do?
What is surprising about 2a?
What does += do?
What does .*do?

a = [3, 1, 2]

sort(a)
println(a)

sort!(a)
println(a)

Your turn:

What does ! at the end of a function name do?

Comments

Comments do not get evaluated by Julia and are for humans only.

# Comments in Julia are identified by hastags

#=
Comments can also spread over multiple lines
if you enclose them with this syntax
=#

x = 2          # Comments can be added at the end of lines

Basic operations

# By default, Julia returns the output
2 + 3

# Trailing semi-colons suppress the output
3 + 7;

# Alternative syntax that can be used with operators
+(2, 5)

# Updating operators
a = 3
a += 8    # this is the same as a = a + 8

# Operator precedence follows standard rules
3 + 2 ^ 3 * 10

More exotic operators

# Usual division
6 / 2

3.0

# Inverse division
2 \ 6

3.0

# Integer division (division truncated to an integer)
7 ÷ 2

# Remainder
7 % 2        # equivalent to rem(7, 2)

# Fraction
4//8

1//2

# Julia supports fraction operations
1//2 + 3//4

5//4

Variables

A variable is a name bound to a value:

a = 3;

It can be called:

Or used in expressions:

a + 2

Assignment

You can re-assign new values to variables:

a = 3;
a = -8.2;
a

-8.2

Even values of a different type:

a = "a is now a string"

"a is now a string"

You can define multiple variables at once:

a, b, c = 1, 2, 3
b

Variable names

These names are extremely flexible and can use Unicode character:

\omega       # press TAB
\sum         # press TAB
\sqrt        # press TAB
\in          # press TAB
\:phone:     # press TAB

δ = 8.5;
🐌 = 3;
δ + 🐌

11.5

Admittedly, using emojis doesn’t seem very useful, but using Greek letters to write equations really makes Julia a great mathematical language:

σ = 3
δ = π
ϕ = 8

(5σ + 3δ) / ϕ

3.0530972450961724

Note how the multiplication operator can be omitted when this does not lead to confusion.
Also note how the mathematical constant π is available in Julia without having to load any module.

If you want to know how to type a symbol, ask Julia: type ? and paste it in the REPL.

The only hard rules for variable names are:

They must begin with a letter or an underscore,
They cannot take the names of built-in keywords such as if, do, try, else,
They cannot take the names of built-in constants (e.g. π) and keywords in use in a session.

We thus get an error here:

false = 3

syntax: invalid assignment location "false" around /home/marie/parvus/prog/mint/julia/intro_basics.qmd:276
Stacktrace:
 [1] top-level scope
   @ ~/parvus/prog/mint/julia/intro_basics.qmd:276

In addition, the Julia Style Guide recommends to follow these conventions:

Use lower case,
Word separation can be indicated by underscores, but better not to use them if the names can be read easily enough without them.

The ans variable

The keyword ans is a variable which, in the REPL, takes the value of the last computation:

a = 3 ^ 2;
ans + 1

Printing

To print the value of a variable in an interactive session, you only need to call it:

a = 3;
a

In non interactive sessions, you have to use the println function:

println(a)

Quotes

Note the difference between single and double quotes:

typeof("a")

String

typeof('a')

Char

"This is a string"

"This is a string"

'This is not a sring'

ParseError:
# Error @ ]8;;file:///home/marie/parvus/prog/mint/julia/intro_basics.qmd#329:2\/home/marie/parvus/prog/mint/julia/intro_basics.qmd:329:2]8;;\
'This is not a sring'
#└─────────────────┘ ── character literal contains multiple characters
Stacktrace:
 [1] top-level scope
   @ ~/parvus/prog/mint/julia/intro_basics.qmd:329

We got an error here since ' is used for the character type and can thus only contain a single character.

'a'

'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)