Attempt to index field ? (A nil Value)

Getting Started Newbie Questions
1 
local containerGroup = {} local container for i = 1, 5 do container = display.newRect(i \* 75, 100, 50, 50) container.fill = \_WHITE\_ container:setStrokeColor(0, 0, 1) container.strokeWidth = 3 container = containerGroup[i] end containerGroup[i].x = centerX

I am trying to spawn a set of 5 rectangles to show the player their units and then move it to the center to avoid an issue that occurs on different devices. But when I try to set containers to be part of containerGroup table, I get the Attempt to index newField ?( A Nil Value ) error. Why does this occur?

Also, if anyone can, could they post some in-depth tutorials on tables in Corona? I feel like I still have a lot to learn about them. Thanks!

2

You just have it the wrong way round. ContainerGroup[i] = container.

3

You just have it the wrong way round. ContainerGroup[i] = container.

4

  1. I recognize a color variable from SSK in your code and I can say this is wrong:

    container.fill = _WHITE_

This is correct.

 container:setFillColor( unpack(\_WHITE\_) )
  1. ‘container’ is a terrible name for an object not made with display.newContainer() or display.newGroup().

It implies the object is a container object when it isn’t.

  1. Sorry, but ‘containerGroup’ is a bad name for a table too.  If you use the word ‘Group’ in a name, it strongly implies to future readers of the code that the object is a display group.
5

Also, this statement is wrong:

containerGroup[i].x = centerX

It is outside the loop, so ‘i’ is no longer defined:

for i = 1, 5 do container = display.newRect(i \* 75, 100, 50, 50) container.fill = \_WHITE\_ container:setStrokeColor(0, 0, 1) container.strokeWidth = 3 container = containerGroup[i] -- MOVE STATEMENT IN LOOP containerGroup[i].x = centerX end

Or, if you’re doing this elsewhere, use a numeric index:

containerGroup[3].x = centerX

Of course, this code won’t work if the table variable is not in scope at the time.

6

The fully corrected and better named code is as follows:

local objSet = {} -- Don't call this a group, its a table. Again, bad naming choice. for i = 1, 5 do local tmp = display.newRect( i \* 75, 100, 50, 50) objSet[i] = tmp tmp:setFillColor( unpack( \_WHITE\_ ) ) -- Tip: \_W\_ is short name for \_WHITE\_ (See SSK docs) tmp:setStrokeColor( unpack(\_B\_) ) -- \_B\_ is blue in SSK tmp.strokeWidth = 3 end

The later, you can access any object (1…5) as follows:

objSet[3].x = centerX -- 'centerX' Also from SSK
7

Thanks! Would you happen to know about any tutorials on tables? I’ve looked at the documentation and Corona University videos, but they are not really that in-depth. 

8

I have a question. I placed the unitSet[i]'s x coordinate to centerX and I realize the mistake I made. How would I have it so that the third rectangle is centered and the other boxes are next to it? Like in this picture, look at where the arrow is pointing.

I am doing this because of issues regarding the positioning of the boxes and the images inside them. 

9

This is personally my favourite Lua tutorial: http://lua-users.org/wiki/TablesTutorial

10

Thanks, but I was kind of hoping for something more closely related with Corona. This is Lua separate from Corona, but I will still look into it. Still need help with the above question though. :slight_smile:

11

With tables though it doesn’t really vary between Corona SDK and barebones Lua, except for the fact that Corona SDK has a few extra table functions that are useful: https://docs.coronalabs.com/api/library/table/index.html. Not sure you’ll find anything more closely related to Corona SDK in terms of a tutorial on tables than what I posted above

12

Ok, thanks!

13

Never mind, I got it! (the second question earlier)

 

In case anyone has a similar problem: all I did was set the id for each rectangle to be -3 + i (which is their place in the table, in this case a max of 5), and then I set their x-coordinate to be the center of the screen plus the id multiplied by 75.

 

The end result:

A responsive set of 5 rectangles next to each other that will always appear in the center of the screen.

for i = 1, 5 do unitBox = display.newRect(i \* 75, 100, 50, 50) unitBox.id = -3 + i unitSet[i] = unitBox unitSet[i].x = centerX + unitBox.id \* 75 end
14

You just have it the wrong way round. ContainerGroup[i] = container.

15

You just have it the wrong way round. ContainerGroup[i] = container.

16

  1. I recognize a color variable from SSK in your code and I can say this is wrong:

    container.fill = _WHITE_

This is correct.

 container:setFillColor( unpack(\_WHITE\_) )
  1. ‘container’ is a terrible name for an object not made with display.newContainer() or display.newGroup().

It implies the object is a container object when it isn’t.

  1. Sorry, but ‘containerGroup’ is a bad name for a table too.  If you use the word ‘Group’ in a name, it strongly implies to future readers of the code that the object is a display group.
17

Also, this statement is wrong:

containerGroup[i].x = centerX

It is outside the loop, so ‘i’ is no longer defined:

for i = 1, 5 do container = display.newRect(i \* 75, 100, 50, 50) container.fill = \_WHITE\_ container:setStrokeColor(0, 0, 1) container.strokeWidth = 3 container = containerGroup[i] -- MOVE STATEMENT IN LOOP containerGroup[i].x = centerX end

Or, if you’re doing this elsewhere, use a numeric index:

containerGroup[3].x = centerX

Of course, this code won’t work if the table variable is not in scope at the time.

18

The fully corrected and better named code is as follows:

local objSet = {} -- Don't call this a group, its a table. Again, bad naming choice. for i = 1, 5 do local tmp = display.newRect( i \* 75, 100, 50, 50) objSet[i] = tmp tmp:setFillColor( unpack( \_WHITE\_ ) ) -- Tip: \_W\_ is short name for \_WHITE\_ (See SSK docs) tmp:setStrokeColor( unpack(\_B\_) ) -- \_B\_ is blue in SSK tmp.strokeWidth = 3 end

The later, you can access any object (1…5) as follows:

objSet[3].x = centerX -- 'centerX' Also from SSK
19

Thanks! Would you happen to know about any tutorials on tables? I’ve looked at the documentation and Corona University videos, but they are not really that in-depth. 

20

I have a question. I placed the unitSet[i]'s x coordinate to centerX and I realize the mistake I made. How would I have it so that the third rectangle is centered and the other boxes are next to it? Like in this picture, look at where the arrow is pointing.

I am doing this because of issues regarding the positioning of the boxes and the images inside them.