Calendar buttons aren't being removed correctly - Lua - mobile

Question

I'm trying to make a calendar to be able to switch from month to month. My problem is in removing each day button when the next or previous buttons are touched.
Here is my code for the Previous Button to switch from the current month to the previous month. My Next button code is almost exactly the same. It works perfectly fine when I tap the button for the first time, but when I tap it again, I get an error at the child:removeSelf() line, and the print message tells me there are 61 elements in the table. It seems to add extra buttons to the table every time I go to a month that hasn't been seen yet.
This is really frustrating to me because I don't see any reason why the code is making extra buttons for every month, and even if it does, each one should still get removed when the button is tapped. Can someone please help me?
local prev_function = function(event)
if event.phase == "release" then
today.year = 2012
today.month = 3
today.day = 29
today.wday = 5
if monthNum == 1 then
monthNum = 12
yearNum = yearNum - 1
elseif monthNum ~= 1 then
monthNum = monthNum - 1
end
local month = ""
if monthNum == 1 then month = "January"
elseif monthNum == 2 then month = "February"
elseif monthNum == 3 then month = "March"
elseif monthNum == 4 then month = "April"
elseif monthNum == 5 then month = "May"
elseif monthNum == 6 then month = "June"
elseif monthNum == 7 then month = "July"
elseif monthNum == 8 then month = "August"
elseif monthNum == 9 then month = "September"
elseif monthNum == 10 then month = "October"
elseif monthNum == 11 then month = "November"
elseif monthNum == 12 then month = "December"
end
monthText.text = month .. " " .. yearNum
print("Table elements before button deletion: " .. #buttonTable)
for i = #buttonTable, 1, -1 do
--[[if button[i] ~= nil then
table.remove(buttonTable)
button[i]:removeSelf()
button[i] = nil
end--]]
local child = table.remove(buttonTable)
if child ~= nil then
child:removeSelf()
child = nil
end
end
print("Table elements after button deletion: " .. #buttonTable)
next_button.alpha = 1
for i = 1, math.floor(numYears * 365.25) do
dateTable[i] = calendar.getInfo(today) --calculate the next day's date
if dateTable[i].year == yearNum and dateTable[i].month == monthNum then -- create a button if the date's year and month match the desired month
button[i] = ui.newButton{
default = "images/day.png",
over = "images/dayover.png",
text = dateTable[i].day,
size = 30,
font = native.systemFontBold,
textColor = {0, 0, 0, 255},
onEvent = addExpense_function,
offset = -35 }
if dateTable[i].wday == 1 then button[i].x = math.floor(col/2)
elseif dateTable[i].wday == 2 then button[i].x = (col * 1) + math.floor(col/2)
elseif dateTable[i].wday == 3 then button[i].x = (col * 2) + math.floor(col/2)
elseif dateTable[i].wday == 4 then button[i].x = (col * 3) + math.floor(col/2)
elseif dateTable[i].wday == 5 then button[i].x = (col * 4) + math.floor(col/2)
elseif dateTable[i].wday == 6 then button[i].x = (col * 5) + math.floor(col/2)
elseif dateTable[i].wday == 7 then button[i].x = (col * 6) + math.floor(col/2)
end
if dateTable[i].day == 1 then button[i].y = wDayBar.y + wDayBar.height/2 + math.floor(row/2)
elseif dateTable[i].wday == 1 then button[i].y = button[i-1].y + row
else button[i].y = button[i-1].y
end
end
today = dateTable[i]
table.insert(buttonTable, button[i])
--button[i].id = "button_" .. i
end
print("Table elements after button creation: " .. #buttonTable)
end
return true
end

Answer 1

The reason why the code in the question doesn't work (and putting the table.insert inside the if...then does work) is due to the way you're using the button table.
The loop that starts for i = 1, math.floor(numYears * 365.25) do is going to create indices from 1 to a few hundred/thousand (depending on numYears).
However, as you're using button[i]= for filling the table and you're only filling 30 or so at a time, what you're creating is a sparse table with a few non-nil entries somewhere in the middle of the table.
Now with table.insert(buttonTable, button[i]) outside the if..then what actually happens is that most of the "inserts" are inserting nil. First time around this actually works ok for 2 reasons:
inserting nil on an empty table does nothing nor does inserting nil at the end of a table
there are only a month's worth of non-nil entries in button so only a month's worth will be inserted into buttonTable
With button and buttonTable setup like this, the first part of the call to the previous or next functions works as expected and removeSelf will be called on the month's worth of buttons. However this doesn't actually remove the button from the button table.
So when you come to the numYears loop again, not only will you add your freshly created buttons to both button and buttonTable, you'll also still have the buttons that you've just called removeSelf on in button so these will be added to buttonTable again. Crucially though, they are just tables now, so when you call removeSelf on them the second time it barfs. I presume this is the cause of the error you saw.
Moving table.insert(buttonTable, button[i]) to the if...then will resolve this problem as it will only ever add freshly minted buttons to buttonTable that are then removed each time next or previous are called.
A word of caution though. The button table is a bit of a mess. After a bit of navigating around, it'll have a bunch of dead buttons (that can't be garbage collected unless you've declared the values as weak) plus a month of working buttons. If you were to try to do anything with it, the chances are that you'll get problems.
It's not clear from the code if you even need the button table. If it's not required elsewhere it would probably be safer to just hold the button references in buttonTable.

Answer 2

Aside from making a new table for months as well as column coordinates like Nicol suggested, I found that I needed to put the table.insert() command inside the if ... then structure. I'm not sure exactly why putting it immediately after the if ... then structure caused it to not work properly, but I'm guessing it's because the buttons are being created inside the structure.
local prev_function = function(event)
if event.phase == "release" then
--set date to start from
today.year = 2012
today.month = 2
today.day = 29
today.wday = 4
--determine new month number
if monthNum == 1 then
monthNum = 12
yearNum = yearNum - 1
elseif monthNum ~= 1 then
monthNum = monthNum - 1
end
--set month string
local month = monthTable[monthNum]
monthText.text = month .. " " .. yearNum
print("Table elements before button deletion: " .. #buttonTable)
--remove all elements in buttonTable
for i = #buttonTable, 1, -1 do
local child = table.remove(buttonTable)
if child ~= nil then
child:removeSelf()
child = nil
end
end
print("Table elements after button deletion: " .. #buttonTable)
next_button.alpha = 1
--hide prev_button if we get to the month after the month we started from
if monthNum == today.month + 1 and yearNum == today.year then
prev_button.alpha = 0
end
--generate dates for specified number of years
for i = 1, math.floor(numYears * 365.25) do
dateTable[i] = calendar.getInfo(today)
--create a button for each date inside desired month
if dateTable[i].year == yearNum and dateTable[i].month == monthNum then
button[i] = ui.newButton{
default = "images/day.png",
over = "images/dayover.png",
text = dateTable[i].day,
size = 30,
font = native.systemFontBold,
textColor = {0, 0, 0, 255},
onEvent = addExpense_function,
offset = -35 }
--set x and y
button[i].x = colTable[dateTable[i].wday]
if dateTable[i].day == 1 then button[i].y = wDayBar.y + wDayBar.height/2 + math.floor(row/2)
elseif dateTable[i].wday == 1 then button[i].y = button[i-1].y + row
else button[i].y = button[i-1].y
end
table.insert(buttonTable, button[i])
end
--change 'today' for next iteration in the for loop
today = dateTable[i]
end
print("Table elements after button creation: " .. #buttonTable)
end
return true
end