Functions

local function square(n)
  return n * n
end

local function greet(name)
  print("Hello, " .. name .. "!")
end

print(square(7))
greet("Ada")
print(greet("Linus"))

49
Hello, Ada!
Hello, Linus!

• local function name(params) ... end defines a function.
• return sends a value back; a function without return returns nothing (which becomes nil when used as a value).
• Like variables, functions should be local.

Arguments are flexible

local function describe(name, age)
  return tostring(name) .. " / " .. tostring(age)
end

print(describe("Ada", 36))
print(describe("Ada"))
print(describe("Ada", 36, "extra"))

Ada / 36
Ada / nil
Ada / 36

Missing arguments become nil; extra ones are ignored. Lua never complains about the number of arguments — check them yourself when it matters.

Functions are values

local double = function(x) return x * 2 end

local function applyToAll(list, fn)
  local result = {}
  for i, v in ipairs(list) do
    result[i] = fn(v)
  end
  return result
end

print(table.concat(applyToAll({1, 2, 3}, double), " "))
print(table.concat(applyToAll({1, 2, 3}, function(x) return x * x end), " "))

2 4 6
1 4 9

Functions can be stored in variables and tables, passed to other functions, and returned from them — the basis of callbacks, event handlers, and closures (Level 2).

Recursion

local function factorial(n)
  if n <= 1 then return 1 end
  return n * factorial(n - 1)
end
print(factorial(10))

3628800

local function f makes f visible inside its own body, so it can call itself.

Practice

Write isEven(n) and use it to print the even numbers from 1 to 10.

local function isEven(n)
  return n % 2 == 0
end

for n = 1, 10 do
  if isEven(n) then print(n) end
end

2
4
6
8
10