Functions

Functions package code you can reuse. There are two ways to define one:

function area(width, height)
    return width * height
end

perimeter(w, h) = 2 * (w + h)

println(area(3, 4))
println(perimeter(3, 4))

12
14

• The function ... end form is for longer functions.
• The one-line assignment form f(x) = ... is for short ones — it looks just like math.
• A function returns the value of its last expression, so return is optional. Use it for clarity or to return early.

Default and keyword arguments

function greet(name, greeting = "Hello"; punctuation = "!", shout = false)
    msg = "$greeting, $name$punctuation"
    return shout ? uppercase(msg) : msg
end

println(greet("Ada"))
println(greet("Ada", "Hi"))
println(greet("Ada", punctuation = "?"))
println(greet("Ada"; shout = true))

Hello, Ada!
Hi, Ada!
Hello, Ada?
HELLO, ADA!

• Arguments with = value are optional.
• Arguments after the ; are keyword arguments: callers pass them by name, in any order. Most options in Julia libraries are keyword arguments (like sort(xs, rev=true)).

Variable numbers of arguments

function total(xs...)
    s = 0
    for x in xs
        s += x
    end
    return s
end

println(total())
println(total(1, 2, 3))
nums = [10, 20, 30]
println(total(nums...))
println(max(nums...))

0
6
60
30

xs... in a definition collects extra arguments into a tuple ("slurp"); nums... in a call spreads a collection into separate arguments ("splat").

Arguments are passed by sharing

function add_bonus!(scores)
    for i in eachindex(scores)
        scores[i] += 5
    end
    return scores
end

s = [70, 80]
add_bonus!(s)
println(s)

[75, 85]

A function receives the same array, not a copy, so it can modify it. By convention, name such functions with a !. eachindex(a) gives the valid indexes of a.

Practice

Write bmi(weight_kg, height_m) using the one-line form, and a function describe_bmi(w, h; digits = 1) that returns a string like BMI 22.9, rounding to digits decimals.

bmi(weight_kg, height_m) = weight_kg / height_m^2

function describe_bmi(w, h; digits = 1)
    return "BMI $(round(bmi(w, h), digits = digits))"
end

println(describe_bmi(70, 1.75))
println(describe_bmi(70, 1.75, digits = 3))

BMI 22.9
BMI 22.857