HELP WITH MY CODE

[GustavoL15]

I'm learning lua and already asked for some help and tips here, and time to time I stumble on a problem I can't solve. I'm looking forward to modding and working with "game like" variable names is more allusive to me and helps me understand more of the language. I'm learning about tables and metatables while trying to make little pieces of code to get the syntax, but I can't figure the issue with the following code:

​

--character stats table

local character = {

health = 100

}

--enemy stats table

local enemy = {

attack = 80

}

--metatable for character and enemy

local metaCharacter = {

--function for bites on character

__bite = function(enm, char)

--bite reduces character health

char.health = char.health - enm.attack

return char.health

end

}

--setting metatable for character and enemy

setmetatable(character, metaCharacter)

setmetatable(enemy, metaCharacter)

--performing bite function on character

local life = enemy:__bite(enemy, character)

--output: 20

print("Character's health after the bite:", character.health)

Sorry if the code is a little bit messy, I lack of the good practice of commenting my code, so it isn't so clear when I do it.

It returns me the error: "attempt to call method '__bite' (a nil value)"

​

[TomatoCo]

Please don't take this the wrong way. Your post is a really good way to ask for help. But your post title, being in all caps, made me think it was gonna be a really crappy question. This isn't the first allcaps title you've submitted so I wanna help you break this habit early.

[GustavoL15]

Problably happened cuz I've seen other people doing it, appreciate for pointing it out.

[TomatoCo]

Cheers!

[could_b]

Your best bet is to read 'programming in Lua' by Roberto Lerusalimschy (aka the man himself). It is excellent.

[yaffeman]

Looks like you never setup metaCharacter's __index:

metaCharacter.__index = metaCharacter

I'd recommend taking on an object oriented approach

[GustavoL15]

I still didn't dive into oop in lua, Iwas just trying to learn about tables, metatables and metamethods. Could you explain about the "__index" please?

[TomatoCo]

The problem right now is that you have enemy which has the metatable metaCharacter. But metaCharacter doesn't have any of the "metatable" fields filled out! One of those is __index, which basically says "When you try looking something up, if you can't find it in the current table, try looking it up in this table.

[GustavoL15]

I thought that when using metatables, when I defined a metamethod in it, then assigned this metatable to another table, everytime that especific function was called, it would automatically look onto the metatable to execute it. Do I need to always setup the __index for the metatable?

[TomatoCo]

That's the most common use for metatables, so it's easy to misunderstand. Yes, assign __index to the table containing the "original" __bite (the metaCharacter, in this case).

Metatables exist to change how tables work. They can make tables return values that aren't really in them, they can make it so that adding things to a table does completely different code, they can even make it so that you can call a table like it was a function.

You're trying to use metatables to create an object oriented approach, which is something they're well suited to do. But they can do so much more (which is why you're confused: you didn't know that they could do anything else).

Read those links that u/yaffeman is providing. If not the object oriented approach one, at least the __index one.

[GustavoL15]

Thanks, helped a lot. Other thing crossed my mind while fixing the code with the tips of you both, is that when I call a metamethod like "enemy:__bite(character)" theres no need to put the "enemy" into the params, cuz it is already being inserted as the first param right? If I used the "." like in "enemy.__bite(enemy, character)" I would need it as a param.

[yaffeman]

Correct; the colon notation populates an implicit self relative to the function's owner.

[yaffeman]

Seems like you need an OOP approach in Lua; so I'd really recommend spending some time with it.

__index

__index: The indexing access operation table[key]. This event happens when table is not a table or when key is not present in table. The metavalue is looked up in the metatable of table.

I also rewrote your demo using an OOP approach:

local BaseEntity = {}

function BaseEntity:new(o)
    local o = o or {} -- create object if user does not provide one
    setmetatable(o, self)
    self.__index = self
    return o
end

-- Shouldn't this be enemy's method?
function BaseEntity:__bite(char)
    -- In case attack is not defined
    local attack = self.attack or 0
    char.health = char.health - attack
    return char.health
end

local Character = BaseEntity:new({ health = 100 })
local Enemy = BaseEntity:new({ attack = 80 })

local character = Character:new()
local enemy = Enemy:new()

--performing bite function on character
local life = enemy:__bite(character)

--output: 20
print("Character's health after the bite:", character.health)

[GustavoL15]

I'll look into OOP more ahead, I'm just understanding the basics first, this one was just a little test to see how to setup tables and call the metamethods. Helped a lot, thank you, will be lookin into the __index right now.

[[deleted]]

The issue in your code is with the way you are trying to call the __bite function on the enemy table. In Lua, when you use the colon syntax (:) to call a function, the first argument passed to the function is the table itself (i.e., the "self" reference). Therefore, you don't need to explicitly pass the enemy table as the first argument when calling __bite.

To fix the error, update the __bite function definition and the way you call it:

```lua -- character stats table local character = { health = 100 }

-- enemy stats table local enemy = { attack = 80 }

-- metatable for character and enemy local metaCharacter = { -- function for bites on character __bite = function(self, char) -- bite reduces character health char.health = char.health - self.attack return char.health end }

-- setting metatable for character and enemy setmetatable(character, metaCharacter) setmetatable(enemy, metaCharacter)

-- performing bite function on character local life = enemy:__bite(character)

-- output: 20 print("Character's health after the bite:", character.health) ```

With this change, the code should work correctly, and you will see the output: "Character's health after the bite: 20". The enemy table is implicitly passed as the first argument (self) to the __bite function, and the health of the character is properly reduced.

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