I want to check if a part is not within the workspace using the “not” operator.
ROBLOX throws an error when the part is not found within the workspace.
Why is this a thing? Simply go into studio, write a short line if not workspace.somepart then print("not part") else print("part") end and run a test. It will error saying that somepart is not a valid member of workspace, but that’s literally what you’re checking for, if it’s not in the workspace?? It’s so counter intuitive and the thing that makes even less sense is that it works if you actually have something named somepart, it prints true in that scenario.
How can you check if a part is NOT in the workspace using the “not” operator if it just errors when it isn’t?
The whole point is, roblox throws an exception when you try indexing for something that doesn’t exist. It’s not like this behaviour magically changes by applying a not.
Use instead :FindFirstChild. So not workspace:FindFirstChild("somepart")
https://gyazo.com/62f2ebed4acdd139948585d4c3b6e593 the regular “not” in lua seems to work like that, actually printing FALSE if something does not exist.
Besides, “FindFirstChild” throws an infinite yield warning. I’ve tried it, should have mentioned that in the thread.
Whenever you try to access a property of some object, the normal behavior is to throw an error if that field doesn’t exist. Roblox does this with all objects, for something like a table though you’d have to implement that behavior yourself with metatables. For tables, the default behavior is just to return nil; so you can just do if not someTable.prop then
Oh yeah, I actually wrote WaitForChild instead of FindFirstChild. Sorry, and thanks. Still, it’s pretty confusing to see normal lua work like that and it being different on ROBLOX with no indication.
@incapaxx is correct, the infinite yield warning is for WaitForChild.
Basically, and this may be confusing, what’s happening is Roblox is setting a metatable on the table to error.
So what we’d get here is something like this:
local fakeWorkspace = setmetatable({}, {
__index = function(self, index) -- operation on indexing when nothing is there.
error("Bad index")
end;
end
fakeWorkspace.blah --> Error: Bad index
So basically order of operation is
Check index
Roblox managed code throws error
Not/if branch is never executed
But if we didn’t have this, we’d have something like this:
Check index
Return nil
not nil = true
If statement evaluated if true then
Your code executes
Basically, because Roblox adds an error automatically on indexing when nothing exists, instead of returning nil, you have to use FindFirstChild.
Look here I’ll show this example so you see the issue.
local t = setmetatable({ a = 0 }, { __index = function(_, key)
error(key .. " is not a valid member of t", 2)
end })
print(t.a) --> 0
print(t.b) --> b is not a valid member of t
You can implement roblox’s behaviour more or so like that. Roblox did NOT change the behaviour of the not operator. The issue is, you’re still indexing. The reason it works fine on just tables is because their metatable (if they have one) __index function (if it points to one) is probably not written to throw exception on nonexistent keys.
