Variadic Functions and table.pack/unpack

Lua functions can accept a variable number of arguments using ... (the vararg expression), and tables can be converted to and from argument lists with table.pack/table.unpack.

Accepting any number of arguments

local function sum(...)
  local total = 0
  for _, n in ipairs({...}) do
    total = total + n
  end
  return total
end

print(sum(1, 2, 3))        -- 6
print(sum(10, 20, 30, 40)) -- 100

{...} packs all the varargs into a plain table, from there ipairs works as usual. This breaks down if any argument is nil, though, since ipairs stops at the first nil.

select: inspecting varargs directly

local function describe(...)
  print("got " .. select("#", ...) .. " arguments")
  print(select(2, ...)) -- every argument from position 2 onward
end

describe("a", "b", "c") -- got 3 arguments  /  b  c

select("#", ...) returns the exact argument count, including trailing nils, unlike #{...} which can undercount if a nil is in the middle or at the end. select(n, ...) returns every argument from position n onward.

table.pack and table.unpack

local function packed(...)
  return table.pack(...) -- { n = 3, 1, 2, 3 } for example
end

local args = packed(1, 2, 3)
print(args.n, args[1], args[2], args[3]) -- 3  1  2  3

print(table.unpack({10, 20, 30})) -- 10  20  30, spreads a table back into multiple values

table.pack(...) is exactly {...} plus a reliable n field for the true count (nil-safe, unlike #). table.unpack(t) does the reverse, spreading a table's elements out as individual values, handy for forwarding a captured argument list to another function: someFn(table.unpack(args)).