Lua Functions

Functions group reusable operations. Functions such as print() and io.read() are already part of the standard library.

Defining Functions

function name(arguments)
  statements
end

function sum(x, y)
  return x + y
end

print(sum(10, 20))

Arguments are passed as values. Reassigning a parameter does not reassign the caller’s variable.

Returning Values

Lua functions can return multiple values.

function values()
  return 10, 20
end

hoge, piyo = values()
print(hoge, piyo)

Extra return values are discarded. Missing values are assigned nil. A function without a return statement returns no values.

Functions as Values

Functions can be assigned to variables.

function sum(x, y) return x + y end
function mul(x, y) return x * y end

operation = sum
print(operation(10, 20))
operation = mul
print(operation(10, 20))

Functions can also be defined inside functions and returned as closures.

function createCounter(value)
  return function(amount)
    value = value + amount
    return value
  end
end

counter = createCounter(10)
print(counter(5))

Common Library Functions

assert() raises an error when its first argument is false.

assert(value, "value must be true")

dofile() executes another Lua file.

dofile("call.lua")

type() returns a value’s type. tonumber() converts a value to a number when possible, and tostring() converts a value to a string.

print(type(10))
print(tonumber("25"))
print(tostring(25))