Problem with indexs of table

redbol · August 31, 2020, 10:44pm

Hello!
Function:

function table:rebuild()
	if type(self) == "table" then
		local Rebuilded = {}
		for i=1, #self do
			if self[i] then
				Rebuilded[#Rebuilded+1] = self[i]
			else
				print(i,"--")
			end
		end
		print(#Rebuilded, "#objs")
		return Rebuilded
	else
		error("Cant rebuild table")
	end
end

Video:

bandicam 2020-09-01 00-34-56-120

Question: Why is the function not working correctly? Why doesn’t it write “num --”(print(i,"–") in the code) after each deletion of an object?

PS:
Function where I am using table:rebuild()

	function SpriteImgs:UpdatePositionOfSpells()
		if ((#self["metadata"]["stats"]["spells"]["list"]) == 0) then do return end end
		local IsHero = false
		if self.metadata["character"]["prefix"] == "hero" then 
			IsHero = true
		end
		self.metadata["stats"]["spells"]["list"] = table.rebuild(self.metadata["stats"]["spells"]["list"])
		local TableOfSpells = self.metadata["stats"]["spells"]["list"]
		local GetX
		local GetY = - 25
		if IsHero then
			GetX = - (self.width*0.30)
		else
			GetX = (self.width*0.30)
		end
		for i=1, #TableOfSpells do
			local GetObj = TableOfSpells[i]["img"]
			GetObj.x = GetX - (i*(GetObj.width*1.15))
			GetObj.y = GetY
		end
	end

Markus_Ranner · September 1, 2020, 7:32am

I don’t really understand what you are trying to do and why?

A few notes on your code from what I can see though:

Try print(i .. "--"), instead of print(i, "--")

You can make the code easier to read by indexing with the dot notation instead of strings when the indexes you are looking for are static values.

For example:

self.metadata.stats.spells.list

Is the same as this:

self["metadata"]["stats"]["spells"]["list"]

The naming of your variables is a little strange to me, and that also makes the code harder to read and understand. Why do you name variables GetX, GetY and GetObj instead of just x, y and obj? Something that is named with a ‘get’ prefix is by convention a function, but in your case they seem to be just simple value variables.

redbol · September 1, 2020, 10:36am

Let me give you another example:

function table:rebuild()
	if type(self) == "table" then
		local Rebuilded = {}
		print(#self, "#Old amount of objs in the table")
		for i=1, #self do
			if self[i] then
				Rebuilded[#Rebuilded+1] = self[i]
			else
				print("Index "..i.. " is nil.")
			end
		end
		print(#Rebuilded, "#Now amount of objs in the table")
		return Rebuilded
	else
		error("Cant rebuild table")
	end
end

local Table = {}

local function AddRect()
	local NewObj = display.newRect(50,50,25,25)
	NewObj.anchorX = 0
	NewObj.x = ((#Table)*30)
	Table[#Table+1] = NewObj
end

for i=1, 10 do
	AddRect()
end


local BtnToRemoveObj = display.newRect(50,100,25,25)
BtnToRemoveObj:setFillColor(1,0,0)
BtnToRemoveObj:addEventListener("tap", function() 
	if #Table == 0 then do return end end
	local GetRandomIndex = math.random(1, #Table)
	display.remove(Table[GetRandomIndex])
	Table[GetRandomIndex] = nil
	print("------------")
	print("GetRandomIndex is " .. GetRandomIndex)
	-- Now rebuild table to avoid Table[GetRandomIndex] as nil and get the table without index "nil"
	Table = table.rebuild(Table)
end)

Logically, all objects should be removed. But for some reason, some object will still remain, even though #Table will be equal to 0…
Try to execute this code 3 times and you will get completely different results.

redbol · September 1, 2020, 10:53am

bandicam 2020-09-01 12-49-59-847

For example, then there may be one square, then two squares, although #Table == 0
Or the code can be executed correctly and then all white squares are removed…

Markus_Ranner · September 1, 2020, 12:14pm

I can’t spot the error you’re describing.

But do you know that you can use table.remove(aTable, index) instead of rebuilding your entire table to get rid of nil values?

jedihenryk · September 1, 2020, 2:02pm

The length operator applied on a table returns a border in that table. A border in a table t is any natural number that satisfies the following condition:
 (border == 0 or t[border] ~= nil) and t[border + 1] == nil
When t is a sequence, #t returns its only border, which corresponds to the intuitive notion of the length of the sequence. When t is not a sequence, #t can return any of its borders.

(I’m just quoting the manual, by the way )
https://www.lua.org/manual/5.4/manual.html#3.4.7

In other words you cannot count on table being empty even when #Table is 0.

I think your experience with tables will improve immensely if you iterate over them with either
pairs or ipairs.

You can also find a lot of tips and tricks with tables on lua-users wiki, starting with tables tutorial

redbol · September 1, 2020, 2:29pm

@Markus_Ranner Thank! I knew about the table.remove(), but did not know that it can also “rebuild” the table… Your advice helped me, but it’s a pity that my code didn’t work as it should
@jedihenryk Thanks, I read more articles on tables. Anyway, I’m glad that my experience with code has become much better than it was at the very beginning of my arrival on the forum

adam120 · September 1, 2020, 4:26pm

you cannot iterate over table as array, read more about tables.
table is mainly a dictionary DS, it’s not array that mean not all members stored consequentially (by index)
and #table doesn’t return number of valid entries in table,

to iterate over table entries use:
for k,v in pairs(table_name) do

instead of
for i,v in ipairs(table_name) do
or iterate by index

anyway why you need to rebuild table?

roaminggamer · September 1, 2020, 4:35pm

There are two basic ways to index objects in table, which affect the way you iterate over the table.

(Note: I am not talking about table functions like insert, etc here. I am talking about the indexing method you use.)

Numeric Indexes:

local some_objects = {}

for i = 1, 10 do
   local obj = display.newCircle( ... )
   some_objects[#some_objects+1] = obj  -- LOOK HERE
end

Now, you can iterate over the table using numbers:

for j = 1, #some_objects do
   print( some_objects[j].x, some_objects[j].y )end
end

(You can also use ipairs() and pairs() iteration techniques.)

Object indexes:

local some_objects = {}

for i = 1, 10 do
   local obj = display.newCircle( ... )
   some_objects[obj] = obj -- LOOK HERE
end

Now, you can iterate over the table using pairs():

for k,v in pairs(some_objects) do
   print( v.x, v.y )
end

You CANNOT use number iteration (including ipairs()) on this table.

print( #some_objects) -- prints 0

You can simultaneously use numeric indexing and have key-value pairs if you like, but that is generally a messy proposition.

local some_objects = {}

for i = 1, 10 do
   local obj = display.newCircle( ... )
   some_objects[#some_objects+1] = obj -- LOOK HERE
end

some_objects.purpose = "game objects"

Now, some_objects has 10 numerically indexed objects in it and a field ‘purpose’ with the value ‘game objects’ in it.

Gotcha: If you do what I just did in 3 and attempt to JSON encode a table with both numeric entries and key-value pairs, then decode the JSON, all numeric indexes will be strings and not indexeable by numeric iterator.