Authors CurseForge

Hi, quick question!

Instead of writing

if class == NN or class == FF or class == PP then

can I write it like this?

if class == NN == FF == PP then

All answers appreciated.

Quote from silkfire

Hi, quick question!

Instead of writing

if class == NN or class == FF or class == PP then

can I write it like this?

if class == NN == FF == PP then

All answers appreciated.

If that were going to work, I'd expect it to be equivalent to "if ... and ... and ..." and not "if ... or ... or ...". But no, it doesn't work in Lua anyway.

I believe your second code snippet is actually equivalent to

if (class == NN) then
  if (FF == true) then
    if (PP == true) then
      -- then
    end
  elseif (PP == false) then
    -- then
  end
else
  if (FF == false) then
    if (PP == true) then
      -- then
    end
  elseif (PP == false) then
    -- then
  end
end

because Lua would interpret each equality operator left-associatively, and the result of equality operators is always 'true' or 'false'.

A better shorthand for your multiple-or would be a switch statement, like

switch (class)
  case NN
  case FF
  case PP
    // then...

but Lua doesn't have switch statements anyway.

So, you're stuck with the verbose version. If you have to access the var 'class' many times, though, you can at least make sure it's local for faster access; that is, if it wasn't defined inside or an argument to your function, then do "local class = class" to local-ize the upvalue.


Edit: another way you could accomplish that, if your list of options is relatively static, would be something like:

local values = {}
values[NN] = true
values[FF] = true
values[PP] = true
...
if (values[class]) then

But that requires you to update "values" any time the pointers "NN", "FF" or "PP" change.

if you need to do that multiple times, or have some other reason to do it quickly, you could always set up another value wherever class gets assigned, like this:

local value

value = class == NN or class == FF or class == PP

then you can access it later by just checking:

if value then

If you really want a shorter syntax (at least different, anyway) you can use Lua's metatable functionality to do a comparison like:

if [a, b, c] == [1, 2, 3] then return end

But this would be a bad idea.

Edit: Against my better judgment, I have implemented it for you here:

Usage

if Set.new({"Rogue"}) == Set.new({"Rogue", "Shaman", "Warrior"}) then
  print("true")
else
  print("false")
end

The Code

Set = {}
Set.mt = {}

function Set.new(t)
  local set = {}
  setmetatable(set, Set.mt)
  for i,v in ipairs(t) do set[i] = v end
  return set
end

Set.mt.__eq = function(a, b)
  local l = (table.maxn(a) >= table.maxn(b)) and a or b

  for _,v in ipairs(l) do
    if a[1] == v then return true end
  end

  return false
end

I sure hope no one takes me out back and shoots me.

Quote from silkfire

if class == NN == FF == PP then

All answers appreciated.

No programming language in the world will let you write a conditional like that. Even if one did, this would not do what you expect. Logically, it would return true only if NN, FF, and PP are all equal to class (e.g. class == NN and class == FF and class == PP).

Quote from amoeba

if Set.new({"Rogue"}) == Set.new({"Rogue", "Shaman", "Warrior"}) then

Gotta nip this one in the bud right now. Do *NOT* get in the habit of using tables like this, it is very wasteful and only creates needless garbage to be collected. Use the and/or chaining instead, or if you have a very large set of possible values...

<script src="http://gist.github.com/48303.js"></script>

Thank you guys!

This wouldn't work either?

if class == NN || FF || PP then

|| is the 'or' operator, right?

Lua doesn't have C-style logic operations by default. It does have and/or/not but they have other behavior which is both elegant for some purposes and useless for what you want to do. In essence and/or mimick what C does with the "conditional ? return a : return b" behavior, which isn't a clean logic operation, but conditional mixed with return behavior.

There is nothing wrong with doing

if class == "A" or class =="B" or class=="C" then

and if you want to go fancy, the lua way of solving this is basically was tek posted with table indexing.

If you want to twist your spine for this you can use "bit" logic masked as integer operations.

e.g. A=1, B=2, C=4, D=8, E=16

set A..E if class =="A" then...

then do

if nClass = A+B then -- exactly A and B are present
if nClass < A+B+C then -- any of A,B,C, but nothing else is present
etc

You can also use luas bit logic extension which will give you faithful bit logic handling. (bit.bor, bit.band etc)

Both of these only make sense if you frequently look for very specific combinations of things. Else just go with the if or the table.

Quote from Tekkub

Gotta nip this one in the bud right now. Do *NOT* get in the habit of using tables like this, it is very wasteful and only creates needless garbage to be collected. Use the and/or chaining instead, or if you have a very large set of possible values...

Yeah. I should have thrown a disclaimer in my code. My only goal was to get the syntax as I could to his and what I wrote ends up being much closer than properly defining the tables as locals. There are too many things wrong with my proposed code snippet to count.