Removing display objects from a table

Getting Started Newbie Questions
#1

The code below illustrates my problem. I can spawn a moving bullet in the centre of the screen with a tap. When it goes outside the screen, it gets removed. This works fine until I fire off a couple of bullets in quick succession. Then I get an error that reads:    ‘main.lua:26: attempt to index field ‘?’ (a nil value)’.

Any enlightenment would be appreciated.

BULLETSPEED = 15 local bullets = {} local function addBullet(event) --spawns circle at screen centre, travelling in direction of tap, adds it to "bullets" table local index = #bullets + 1 bullets[index] = display.newCircle(display.contentCenterX, display.contentCenterY, 10) local dY = event.y - display.contentCenterY local dX = event.x - display.contentCenterX local bulletAngle = math.atan2(dY, dX) bullets[index].velocity = {math.cos(bulletAngle) \* BULLETSPEED, math.sin(bulletAngle) \* BULLETSPEED} end local function frameUpdate(event) -- destroy out-of-bounds bullets for i = #bullets, 1, -1 do if bullets[i].x \< 0 or bullets[i].y \< 0 or bullets[i].y \> display.contentHeight or bullets[i].x \> display.contentWidth then bullets[i]:removeSelf() bullets[i] = nil end end -- move bullets for i = #bullets, 1, -1 do bullets[i].x = bullets[i].x + bullets[i].velocity[1] -- line 26 bullets[i].y = bullets[i].y + bullets[i].velocity[2] end end Runtime:addEventListener("tap", addBullet) Runtime:addEventListener("enterFrame", frameUpdate)
#2

The problem here is that you are setting an entry in the table to nil:

for i = #bullets, 1, -1 do if bullets[i].x \< 0 or bullets[i].y \< 0 or bullets[i].y \> display.contentHeight or bullets[i].x \> display.contentWidth then bullets[i]:removeSelf() bullets[i] = nil --setting index i to nil here end end

 and then trying to access that table entry afterward:

for i = #bullets, 1, -1 do --buttons[i] (where i == the value removed above) no longer exists, so there is no bullets[i].x value to perform arithmetic on bullets[i].x = bullets[i].x + bullets[i].velocity[1] -- line 26 bullets[i].y = bullets[i].y + bullets[i].velocity[2] end

The same principle is easier to demonstrate when the data in the table is not a display object e.g:

local myTable = {1, 2, 3, 4, 5} myTable[3] = nil -- myTable is now {1, 2, nil, 4, 5}

What you need to do is remove the entry entirely from the table, and not just set it to nil:

for i = #bullets, 1, -1 do if bullets[i].x \< 0 or bullets[i].y \< 0 or bullets[i].y \> display.contentHeight or bullets[i].x \> display.contentWidth then bullets[i]:removeSelf() table.remove(bullets, i) --remove entry from table entirely end end

This way, when the next loop runs it will be looping over a table which has all valid entries. This function also moves any entries with a higher index down to close the “hole” left be removing an entry, so your bullet creation function should be unaffected by the change.

#3

Nicely explained, thanks! 

#4

The problem here is that you are setting an entry in the table to nil:

for i = #bullets, 1, -1 do if bullets[i].x \< 0 or bullets[i].y \< 0 or bullets[i].y \> display.contentHeight or bullets[i].x \> display.contentWidth then bullets[i]:removeSelf() bullets[i] = nil --setting index i to nil here end end

 and then trying to access that table entry afterward:

for i = #bullets, 1, -1 do --buttons[i] (where i == the value removed above) no longer exists, so there is no bullets[i].x value to perform arithmetic on bullets[i].x = bullets[i].x + bullets[i].velocity[1] -- line 26 bullets[i].y = bullets[i].y + bullets[i].velocity[2] end

The same principle is easier to demonstrate when the data in the table is not a display object e.g:

local myTable = {1, 2, 3, 4, 5} myTable[3] = nil -- myTable is now {1, 2, nil, 4, 5}

What you need to do is remove the entry entirely from the table, and not just set it to nil:

for i = #bullets, 1, -1 do if bullets[i].x \< 0 or bullets[i].y \< 0 or bullets[i].y \> display.contentHeight or bullets[i].x \> display.contentWidth then bullets[i]:removeSelf() table.remove(bullets, i) --remove entry from table entirely end end

This way, when the next loop runs it will be looping over a table which has all valid entries. This function also moves any entries with a higher index down to close the “hole” left be removing an entry, so your bullet creation function should be unaffected by the change.

#5

Nicely explained, thanks!