Inserts at wrong position - why does lua table behave this way?

Question

I needed to add elements to a lua table in a certain order and tried to insert each element to its final position (but in a random order). I'm new to lua and hadn't worked with table.insert before (only read that tables support both an associative and an array form), but I was pretty sure it couldn't work this way, so I made a little test:

local test = {}
table.insert(test, 5, "5")
table.insert(test, 1, "1")
table.insert(test, 4, "4")
table.insert(test, 3, "3")
table.insert(test, 2, "2")

Test output after each insert delivered this interesting behavior:

["5"]
["5","1"]
["1","4","5"]
["1","3","4","5"]
["1","2","3","4","5"]

Actually it worked better than expected (I thought that inserting to a table with two elements at position 4 would probably append), but the lines 2 and 3 got me absolutely confused. Inserting at position 1 appends, and the next insert reorders the other elements?!?

Next try was to avoid table.insert and instead use test[5] = "5" etc. The result was exactly the same...

Only way to fix it was to initialize elements 1 through 5 with an empty string first, and then inserting the actual values in random order.

Does anybody have an idea why the tables behave this way?

Answer 1

Your operations do not apply sensibly a table without a sequence. The first statement creates a table without a sequence. Then it all goes south.

6.6 – Table Manipulation

Remember that, whenever an operation needs the length of a table, all caveats about the length operator apply (see §3.4.7).

One generally chooses to maintain the sequence of a table or not. If not, avoid functions and the # operator (it's built-in implementation) that are designed for sequences.

You could build up the table as @lhf describes:

local test = {}
test[5] = "5"
test[1] = "1"
test[4] = "4"
test[3] = "3"
test[2] = "2"

and then at a point when you are convinced that you have created a table with a sequence, begin treating the table as such.