Stupid table element removal problem

Getting Started Newbie Questions
#1

Coming from C I’m still not comfortable with how Corona/lua deals with tables, but I’ve settled with this method for removing elements with index i from table elementArray:

elementArray[i]:removeSelf() elementArray[i] = nil table.remove(elementArray, i) 

This standard method has until now worked reasonably well (I don’t know how to hunt for memory leaks etc in Corona yet), but now I’m a bit confused.

Please consider the following code:

gridElemArr = {} -- Create new image local gridElemIdx = #gridElemArr+1 gridElemArr[gridElemIdx] = display.newImage("02\_64x64.png") gridElemArr[gridElemIdx].x = 100 gridElemArr[gridElemIdx].y = 100 gridElemIdx = #gridElemArr+1 gridElemArr[gridElemIdx] = display.newImage("02\_64x64.png") gridElemArr[gridElemIdx].x = 200 gridElemArr[gridElemIdx].y = 100 gridElemIdx = #gridElemArr+1 gridElemArr[gridElemIdx] = display.newImage("02\_64x64.png") gridElemArr[gridElemIdx].x = 300 gridElemArr[gridElemIdx].y = 100 -- Remove grid element with index 2 gridElemArr[2]:removeSelf() print("Num elems A: " .. #gridElemArr) gridElemArr[2] = nil print("Num elems B: " .. #gridElemArr) table.remove(gridElemArr, 2) 

What this code snippet is supposed to do is to create 3 elements (with index 1, 2 & 3) and then remove the one with index 2.

But what happens is that two elements are removed from the table and it seems to happen at the line:

gridElemArr[2] = nil

The output of the code above is:

Num elems A: 3

Num elems B: 1

So with my “standard” way of removing table elements, obviously breaks down in this case. But what am I doing wrong?

#2

Tables in lua an key-value structure. So, you can have as key integer numbers, strings, or even other functions.

The problem that you are seeing is because you are removing the 2nd item twice.

You have 2 ways to remove an entry from a table (and you are using both of them!):

  1. Settings the table index to nil. Example:

    gridElemArr[2] = nil – this removes the 2nd entry and all other entries are not affected.

  2. Using table.remove. This functions also makes a shift down on all other integer indexed values inside the table.

    table.remove(gridElemArr, 2) – this removes the 2nd entry from the table and shifts down all other integer indexes

You can read more about it here:  http://lua-users.org/wiki/TablesTutorial

#3

Tables in lua an key-value structure. So, you can have as key integer numbers, strings, or even other functions.

The problem that you are seeing is because you are removing the 2nd item twice.

You have 2 ways to remove an entry from a table (and you are using both of them!):

  1. Settings the table index to nil. Example:

    gridElemArr[2] = nil – this removes the 2nd entry and all other entries are not affected.

  2. Using table.remove. This functions also makes a shift down on all other integer indexed values inside the table.

    table.remove(gridElemArr, 2) – this removes the 2nd entry from the table and shifts down all other integer indexes

You can read more about it here:  http://lua-users.org/wiki/TablesTutorial