Can I not use dt in local calculations? What is a table value?

[mejonat]

Hi,

I'm new to coding, and this is my first project, so forgive me if there's something really simple I'm not understanding yet, but I ran into a problem trying to use dt for my character's movement. I created a new file for the character knight, and tried to include all of his movement in his file so I didn't have to write it in main. Here's knight.lua. The problem is in self.move().

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knight = {}

knight.new = function()
  local self = self or {}
  self.x = 200
  self.y = 350
  self.xvel = 0
  self.yvel = 0
  self.gdecay = 0
  self.adecay = 0
  self.grav = 80
  self.weight = nil
  self.onground = true
  
  self.move = function(dt)
    self.x = self.x + self.xvel*dt
    self.y = self.y + self.yvel*dt
    if love.keyboard.isDown("d") then
      self.x = self.x + 10
    elseif love.keyboard.isDown("a") then
      self.x = self.x - 10
    end
    if self.onground == true then
      self.y = 350
      self.yvel = 0
      if love.keyboard.isDown("l") then
        self.yvel = -1600
        self.onground = false
      end
    end
  if self.onground == false then
    self.yvel = self.yvel + self.grav
  end
  if self.y > 350 then
    self.onground = true
  end
  end
  
  return self
end

It was a crude attempt but I basically tried to just grab all the code for movement that worked in love.update(dt) in main and stick it in a local function knight.move(). I clearly have no idea what I'm doing lolll
Here's my entire main file.

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require("knight")

function love.load()
  knight = knight.new()
end

function love.update(dt)
  knight:move(dt)
end

function love.draw()
  love.graphics.rectangle("fill", knight.x, knight.y, 40, 50)
  love.graphics.rectangle("line", 0, 400, 900, 300)
end

The error message I get upon running it is knight.lua:16: attempt to perform arithmetic on local 'dt' (a table value)

I guess my big questions are as follows:
What exactly is dt? What is its classification? I want to know exactly what I can or cannot do with it/where I can or cannot use it and why.
If I can actually code my character's movement in his file, what did I do wrong? I need to use dt for calculations, so is there another way of doing that?

Sorry for the long post. Thanks for reading if you did.

[pgimeno]

Hello, welcome to the forums.

The problem here is that you're using colon syntax to call knight:move(dt), which is OK, but you're not using it nor adding the implicit 'self' argument when declaring the function. Take into account that this syntax:

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table:method(params)

is equivalent to this:

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table.method(table, params)

It said it was a table, because in place of 'dt' it sent the table. 'dt' was passed as the second argument, which you didn't even declare.


Now, to fix it, you can replace this line:

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  self.move = function(dt)

with this:

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  function self:move(dt)

or even with this:

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  self.move = function(self, dt)

(the last two are equivalent).

[mejonat]

Thank you! This fixed my problem.