Passing array as function parameter in Lua?

Question

I'm trying to alter the class example I found in this video to make it a bit more streamlined to use. Hopefully my comments explain what I'm trying to accomplish well enough. The problem I'm having is that when I try to use the data table it gives me this error: lua: class example.lua:7: attempt to index field 'data' (a nil value)

I'm assuming that this means that array isn't being properly passed into the function, but I don't know why. I am very much a beginner with Lua.

Here's what I've got:

local enemy = {}; --enemy class table

function enemy:New(data)
  local object = {}; --table to store all of data within class
  local len = # data --get length of passed table
  for i = 1, len, 2 do --loop to input all data from passed table into object table
    object.data[i] = data[i + 1];
  end

  function object:getData(choice) --function that allows us to retrieve data from the class
    return self[choice];
  end

  return object; --return class data table so we can create objects using the class
end

local monsterdata = {"name", "monster", "x", 64, "y", 128, "hp", 4}; --table containing data of monster. keys are odd numbered, values to those keys are even numbered
local monster = enemy:New(monsterdata); --create a object using the class

local test = monster:getData("x"); --set variable to a value with the getData function

print(test);

Kornel Kisielewicz · Accepted Answer · 2013-01-02 08:46:23Z

5

You didn't create the object.data table -- each table in Lua needs to be initialized:

local object = {}
local object.data = {}

or

local object = { data = {} }

However, your example will not work the intended way, unless you fix the getData function:

function object:getData(choice) 
  return self.data[choice]
end

Finally, this is Lua, so you don't need any ; in your code :P.

answered Jan 2, 2013 at 8:46

Kornel Kisielewicz

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2 Comments

Lemony Lime Over a year ago

Haha, I know the ; isn't necessary, I just started programming with C, and my teacher was very strict about these things, so it's comforting to have them. :P

Lemony Lime Over a year ago

What I was doing to begin with works (minus the stupid mistake I made that mkluwe pointed out) with less code than this, and no need to save the data table a second time. Is there any reason I should do it this way over the other?

mkluwe · Accepted Answer · 2013-01-02 15:38:05Z

4

If you want object to hold the data, you probably meant to write

object[data[i]] = data[i + 1];

instead of

object.data[i] = data[i + 1];

Doing this the result printed is 64.

answered Jan 2, 2013 at 15:38

mkluwe

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2 Comments

Lemony Lime Over a year ago

Ah, it was such a simple mistake... though I suppose that's usually the case.

mkluwe Over a year ago

Yes, and not an uncommon one. Typing t.a instead of t[ a ] happens to me from time to time.

SelectricSimian · Accepted Answer · 2013-01-02 20:31:52Z

Like the others said, object.data needs to be initialized, and there's a flaw in the for loop and getData. Also, while it's not a bug exactly, your system of passing keys as odd and values as even is a very good way to do it in a C-based language, with no associative-array/dictionary/table literals, but in Lua, the idiom is

{keyname = value, keyname = value, ...}

and, if the table spans multiple lines

{
    keyname = value;
    keyname = value;
    keyname = value;
    ...
}

So in your case, monsterdata could simply be

{
    name = "monster";
    x = 64;
    y = 128;
    hp = 4;
}

and you could remove the for loop altogether

note: you can only represent string keys this way. For other kinds of keys, like numbers, booleans, or even functions and other tables, surround the key in [square brackets]. For example, if you wanted a mynot table, to map booleans to their opposites, you could use:

{
    [true] = false;
    [false] = true;
}

or, if you wanted to map a set of functions to their libaries

{
    [print] = "standard";
    [os.execute] = "standard os";
    [math.sin] = "standard math";
    [function() print "a user function" end] = "me!";
}

I think the more you learn about Lua the more you'll like it. It's really a great language, with a lot of fun little features. Happy coding!

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