Functions in

About Functions

The Basics Concept introduced two ways to define a function.

Most generally, the multiline form:

function muladd(x, y, z)
    x * y + z
end

The "assignment", or "single-line" form for short definitions:

muladd(x, y, z) = x * y + z

In a third and even shorter form, a short, single-use function can be created without a name:

julia> map(x -> 2x, 1:3)
4-element Vector{Int64}:
 2
 4
 6

julia> map((x, y) -> x * y, 1:3, 4:6)
3-element Vector{Int64}:
  4
 10
 18

In this case, x -> 2x is an "anonymous function". This is equivalent to what some other languages call a "lambda function".

Note that multiple arguments need parentheses, as in (x, y) -> x * y.

Anonymous functions are common in Julia code, especially when combined with higher-order functions such as map() and filter().

Function arguments

So far in the syllabus, we have only looked at functions which have a precise number of arguments, and require function calls to supply all of them, in the correct order. This would be limiting and inconvenient, so there are several other options.

Optional arguments

Like many languages, Julia allows function definitions to supply default values for individual arguments.

Function call can then either supply a value for that argument, or omit it and rely on the default.

julia> f(x, y=10) = x * y
f (generic function with 2 methods)

julia> f(2, 3)
6

julia> f(2)
20

All arguments without defaults must come before any arguments with defaults, meaning that f(x=2, y) would be invalid.

Keyword arguments

All the examples so far use positional arguments, where values supplied in a function call must match the order of the corresponding arguments in the function definition.

Like many languages, Julia also allows keyword arguments. Function calls must specify the argument name, but multiple keyword arguments can then be specified in any order.

A distinctive feature of Julia is that the keyword arguments (if any) in the function definition must be preceded by a semicolon ; to separate them from any positional arguments. A function call can use either ; or , between the last positional argument and the first keyword argument.

julia> b(x; y) = x + y
b (generic function with 1 method)

julia> b(2, y=3)
5

# keyword is required when calling
julia> b(2, 3)
ERROR: MethodError: no method matching b(::Int64, ::Int64)
The function `b` exists, but no method is defined for this combination of argument types.

Default values can optionally be specified, exactly as for positional arguments.

It is common to end up with syntax like myarg=myarg within a function call, when a variable with the same name as the parameter was pre-calculated. A shorthand syntax is allowed in this situation:

julia> width = 4.0
4.0

julia> height = √ width
2.0

julia> area(; width, height) = width * height
area (generic function with 1 method)

# repetition
julia> area(; width=width, height=height)
8.0

# shorthand form
julia> area(; width, height)
8.0

Splat and slurp

These are the standard names for a useful aspect of Julia syntax, in case you wondered. Both refer to the ... operator.

Splat

Splatting is used in function calls, to expand collections into individual values required by the function.

This may be easier to demonstrate than to explain:

julia> fxyz(x, y, z) = x * y * z
fxyz (generic function with 1 method)

julia> xyz = [2, 3, 4]
3-element Vector{Int64}:
 2
 3
 4

# Using the vector directly in a function call is invalid
julia> fxyz(xyz)
ERROR: MethodError: no method matching fxyz(::Vector{Int64})
The function `fxyz` exists, but no method is defined for this combination of argument types.

# splatting converts the vector to 3 numbers, used as positional arguments
julia> fxyz(xyz...)
24

Some "function calls" are hidden by syntactic sugar, so splatting can also be used in less obvious ways.

For example, multiple assignment uses a tuple constructor function internally:

julia> first, rest... = [1, 2, 3, 4]
4-element Vector{Int64}:
 1
 2
 3
 4

julia> first
1

julia> rest
3-element Vector{Int64}:
 2
 3
 4

Keyword arguments can also be supplied by splatting, typically using a named tuple. A Dict will also work, but the keys must be symbols (strings will not work here).

# function with 3 keyword arguments
julia> fabc(; a, b, c) = a + b + c
fabc (generic function with 1 method)

# named tuple
julia> abc_nt = (a=2, b=3, c=4)
(a = 2, b = 3, c = 4)

# there are no positional arguments, so need to use ; before kw argument
julia> fabc(;abc_nt...)
9

# Dict
julia> abc_dict = Dict(:a=>2, :b=>3, :c=>4)
Dict{Symbol, Int64} with 3 entries:
  :a => 2
  :b => 3
  :c => 4

julia> fabc(;abc_dict...)
9

Slurp

Slurping is used in the function definition, to pack an arbitrary number of individual values into a collection.

julia> f_more(i, j, more...) = i + j + sum(more)
f_more (generic function with 1 method)

julia> f_more(1, 3, 5, 7, 9, 11)
36

The name of the slurped argument (in this case more) is not significant. The type of this variable is chosen by the compiler, but for positional arguments is likely to be tuple or something similar.

Keyword arguments can also be slurped, giving a Dict (or similar).

julia> f_kwslurp(x, y; switches...) = :mult in keys(switches) ? x * y : x + y
f_kwslurp (generic function with 1 method)

julia> f_kwslurp(5, 6; mult=true)
30

julia> f_kwslurp(5, 6)
11

Any keyword arguments can be used in the call. It is for the function definition to decide which keywords to respond to and which to ignore.