player={} table initiation, sockmonkee dev

[effortlessartist]

So I'm watching sockmonkee's video's on youtube and he keeps using this:

player = {}
player.x = 5
player.y = 200
player.xvel = 5


Why does he initiate a table and then create variables/methods based on the table? I don't know what this is called in programming or what to search google for to get a why for this. I'm not entirely new to programming, but this is something I haven't seen before (at least in this form) and I don't understand exactly what's going on here. The only thing that makes sense is that he's making it kind of like:

player = {["x"]=5, ["y"]=200, ["xvel"] = 5} <-- key value pairs.

is this essentially what he's doing or is there a better explanation?

Thanks!

[ejmr]

is this essentially what he's doing or is there a better explanation?

No, you pretty much nailed it. It is because of this:

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-- For the sake of example let's say we never bother to
-- write 'player = {}' anywhere in scope.  So Lua knows nothing about it.

player.x = 10   -- And so this is a fatal error.

It is common to create an empty table before adding properties to it for that reason, and because people oftem find it more readable than to do it all at once entirely in the table constructor (i.e. your second example).

[raidho36]

Performance-wise though, the second example is better, because Lua knows in beforehand what the table needs to be and allocates space for it properly, only once. The first example though will trigger table re-hashing several times, which is a slow operation, and will most likely end it up with extra unused yet occupied memory. You should only resort to dynamic table extension if you can't know in advance just how much you gonna do that, e.g. if you do it in a loop that depends on some volatile variable.

You can still use multiline notation in static table creation:

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player = { x = 10,
  y = 20,
  z = 30
}

[Plu]

As an additional potential argument, when you have properties relying on existing arguments in the table, you cannot add them in one go.

This doesn't work:

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enemy = {
  x = math.random( 1, 100 ),
  y = math.random( 1, 100 ),
  goalX = x + 50
  goalY = y + 50
}

And this looks a little weird (to me, at least):

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enemy = {
  x = math.random( 1, 100 ),
  y = math.random( 1, 100 ),
}
enemy.goalX =  enemy.x + 50
enemy.goalY = enemy.y + 50

So most people either resort to this:

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enemy = {}
enemy.x = math.random( 1, 100 )
enemy.y = math.random( 1, 100 )
enemy.goalX = enemy.x + 50
enemy.goalY = enemy.y + 50

Or this:

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local x = math.random( 1, 100 )
local y = math.random( 1, 100 )
enemy = {
  x = x,
  y = y,
  goalX = x + 50,
  goalY = y + 50
}

But that's mostly personal preference. (Except the first one, which doesn't work)

[effortlessartist]

I think the reason the first one doesn't work is because (or at least you do this in javascript):

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enemy = {
  x = math.random( 1, 100 ),
  y = math.random( 1, 100 ),
  goalX = x + 50
  goalY = y + 50
}

Should be:
(Not sure if this.x is the right thing, but maybe theres something with a similar use in lua)

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enemy = {
  x = math.random( 1, 100 ),
  y = math.random( 1, 100 ),
  goalX = this.x + 50,
  goalY = this.y + 50
}

Could be wrong though. I only read one article about learning lua in 30 minutes. I pretty much know nothing about lua, but I work with perl, python, bash, and mysql all day.

[bartbes]

Could be wrong though.

You are, 'this' does not exist in lua. But you do point out a flaw, you can't refer to other members in the (at that point anonymous) table while defining it.

[raidho36]

Is having a couple of more variables is so much of a trouble?

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local x, y = math.random ( 1, 100 ), math.random ( 1, 100 )
player = { x = x, y = y, goalx = x + 50, goaly = y + 50 }

[Robin]

you can't refer to other members in the (at that point anonymous) table while defining it.

Is having a couple of more variables is so much of a trouble?

That's... the gist of Plu's post.

[raidho36]

Oh, yeah. I skipped through most of the Plu's post. TL;DR.

[bartbes]

That's... the gist of Plu's post.

Exactly! (Nice save, me!)