Lua. Attempt to index global “a” (a nil value)

Question

I'm getting the following error:

Attempt to index global "a" (a nil value)

   local gfx = love.graphics
    return{
        new = function( Image, Animation, Time)        
        return{
        current_frame = 1,
        current_anim = 1,
        image = Image,
        a = Animation,
        play = false,
        time = Time or 0,2,
        counter = 0,
    
        update = function(dt)
        if play then
                    counter = counter + dt
                    if counter >= time then
                        counter = 0
                        current_frame = current_frame + 1
                    end
    
                    if current_frame > #a[current_anim] then
                        current_frame = 1
                    end
                else
    
                end
        end,
    
        play = function()
        play = true
        end,
    
        stop = function()
        play = false    
        end,
    
        set_animation = function(anim)
        if anim > #a then error("there is no animation ", anim) return end
        current_anim = anim
    
        end,
    
        draw = function(data)
        gfx.draw(image, a[current_anim][current_frame], data[1], data[2])
        end
    
        }
        end
    }
    

it seems to me, that I do give "a" a value - a table, containing a set of images, which I assign through second parameter.

    -----
    
    local quad = love.graphics.newQuad
    local anim_data = {
      quad(0,0, 32, 48, 192, 256),
      quad(32,0, 32, 48, 192, 256),
      quad(64,0, 32, 48, 192, 256),
      quad(96,0, 32, 48, 192, 256),
      quad(129,0, 32, 48, 192, 256),
      quad(162,0, 32, 48, 192, 256)
    
    }
    local image = love.graphics.newImage("images/player_animation.png")
    image:setFilter("nearest","nearest")

Here I supposedly give 'a' a value

    animation = require("animations"):new(
            image,
    
            {
              { -- idle
                anim_data[1]
              },
              { -- walk
                anim_data[2],
                anim_data[3],
                anim_data[4],
                anim_data[5],
                anim_data[6]
              },
            },
            
            0.2
          )
    
    animation.play()

Answer 1

The a you are looking for is part of the table you are returning from the new function. It is not visible from the scope of those functions.

The easiest solution is to move the values you've placed in the table up in the scope of the new function, encapsulating them. However, this removes them from the table, so they won't be visible outside of the new , .play , .pause , .draw , .update , or .set_animation functions.

A better solution is to employ a more typical OOP solution, so I would suggest reading chapter 16 of Programming in Lua: Object-oriented Programming . Once you've read that chapter, go read every other just to be safe (particularly 5, 6, and 11).

local gfx = love.graphics

return {
    new = function (Image, Animation, Time)        
        local current_frame = 1
        local current_anim = 1
        local image = Image
        local a = Animation
        local play = false
        local time = Time or 0,2
        local counter = 0

        return {
            update = function (dt)
                if play then
                    counter = counter + dt
                    if counter >= time then
                        counter = 0
                        current_frame = current_frame + 1
                    end

                    if current_frame > #a[current_anim] then
                        current_frame = 1
                    end
                end
            end,

            play = function ()
                play = true
            end,

            stop = function ()
                play = false
            end,

            set_animation = function (anim)
                if anim > #a then
                    error("there is no animation ", anim)
                    return
                end

                current_anim = anim

            end,

            draw = function (data)
                gfx.draw(image, a[current_anim][current_frame], data[1], data[2])
            end
        }
    end
}