在 Lua 中，如何将 nil 参数正确设置为某个默认值？

Question

For Lua code below:

local function foo(x, y, z)
    local x = x or true
    local y = y or 1234
    z = z or "default"
end

I always thought the meaning of these three lines inside the function is:

If x/y/z is nil, x/y/z is set to true/1234/"default". Otherwise, it remains whatever it is. Therefore, I have such a line in many places to set a parameter to some default value in case it might be passed into a function as nil.

However, it seems not totally correct in my experiments. I am not sure where I learnt this Lua coding concept. How to do it correctly?

Answer 1

The method will work as long as your boolean(?) variable x was not initialised as false . If you only want to use defaults against nil values, the a or b method is correct.

If your variable can be false , you'd have to use a strict if-then block:

if x == nil then x = true end

You can see a few more methods/examples about ternary operators on lua wiki .

Answer 2

There exists one more strict method if you want to be able pass nil as argument(eg you want distinguish foo(1,2,nil) and foo(1,2) )

function foo(...)
  local n,x,y,z = select('#', ...), ...
  if n < 3 then z = default end
  if n < 2 then y = default end
  if n < 1 then x = default end
  -- here x,y or z  can be nil
  ...
end

But you have to be shure you know what you doing because it may be not expected behavior for users of your function.

Answer 3

This should work as expected:

function foo(x, y, z)
    x = (x == nil) or x
    y = (y == nil and 1234) or y
    z = (z == nil and "default") or z
    print(x, y, z)
end

> foo()
true    1234    default
> foo(false, false, "me")
false   false   me
> foo(nil, 50, "me")
true    50  me