ERROR: attempt to perform arithmetic on local 'beginPos' (a nil value)

Getting Started Newbie Questions
#1

Hi guys,

I’ve been trying to get this little piece of code to work but without any luck, this is the code:

local function turnCircle(event) local beginPos if event.phase == "began" then beginPos = level.rotation print(beginPos) elseif event.phase == "moved" then level.rotation = beginPos -(event.x - event.xStart) end return true end

It stores the begin rotation of the object and stores that in beginPos. But I keep getting the following error: 

ERROR: attempt to perform arithmetic on local ‘beginPos’ (a nil value)

The error points to the variable beginPos used in the “moved” if condition. I don’t get how it could be nil, I clearly declared it in the correct scope and I assign a value to it in the “began” condition.

Any thoughts?

Kind regards

Bram

#2

You need to declare beginPos outside of and somewhere above the function. Currently, each time the listener is triggered, a new local variable called beginPos is set up. It does not persist beyond the scope of turnCircle.

#3

I’d make this change:

local function turnCircle(event) local beginPos if event.phase == "began" then level.beginPos = level.rotation print(level.beginPos) elseif( level.beginPos and event.phase == "moved" ) then level.rotation = level.beginPos - (event.x - event.xStart) elseif( event.phase == "ended" ) then level.beginPos = nil end return true end

Notice:

  1. I store ‘beginPos’ as a field on the the variable on the level object.
  2. I check that it exists before I use it.  Why?  Just in case you get a “moved” without a “began”. Not seeing the rest of the code I can’t be sure whether this can happen in your case or not.
  3. I clear the variable on ended so I only operate on one that was created in ‘began’

PS - This calculation looks backwards on second glance:

level.beginPos - (event.x - event.xStart)

I think it should be:

level.beginPos + (event.x - event.xStart)
#4

Hi Rob,

thanks that did the trick. I’d already tried Nick’s solution but it didn’t check the existence of the variable and that gave me some errors.

Thanks!

#5

You need to declare beginPos outside of and somewhere above the function. Currently, each time the listener is triggered, a new local variable called beginPos is set up. It does not persist beyond the scope of turnCircle.

#6

I’d make this change:

local function turnCircle(event) local beginPos if event.phase == "began" then level.beginPos = level.rotation print(level.beginPos) elseif( level.beginPos and event.phase == "moved" ) then level.rotation = level.beginPos - (event.x - event.xStart) elseif( event.phase == "ended" ) then level.beginPos = nil end return true end

Notice:

  1. I store ‘beginPos’ as a field on the the variable on the level object.
  2. I check that it exists before I use it.  Why?  Just in case you get a “moved” without a “began”. Not seeing the rest of the code I can’t be sure whether this can happen in your case or not.
  3. I clear the variable on ended so I only operate on one that was created in ‘began’

PS - This calculation looks backwards on second glance:

level.beginPos - (event.x - event.xStart)

I think it should be:

level.beginPos + (event.x - event.xStart)
#7

Hi Rob,

thanks that did the trick. I’d already tried Nick’s solution but it didn’t check the existence of the variable and that gave me some errors.

Thanks!