pico8 lua: unexpeted change of size of an array

Question

I am seeing some unexpected behaviour when using a table as an array in pico8 lua when compared to regular PUC-Rio lua

If I run the following code using PUC-Rio lua5.4.4 (ubuntu)

local t={}
for i=1,10 do t[i] = i*10 end
t[2]=nil
t[4]=nil
t[6]=nil
t[8]=nil
print()
for i=1,#t do print(t[i]) end

I get the expected output

10
nil
30
nil
50
nil
70
nil
90
100

However if i run the same code with pico-8 I get:

10

This appears triggered only when I delete (ie set to nil) the t[8] element. if I comment out that line then I get the expected on pico8

10
nil
30
nil
50
nil
70
80
90
100

It appears, in pico8 lua, that the #t size of the array changes to 1 when the t[8] element is set to nil.

Answer 1

Both are expected results, the length operator # in lua returns a number n where t[n] ~= nil and t[n+1] == nil, if there are holes (nil value) inside, the result is undefined.

To find the maximum numeric index, in lua 5.1 you can use table.maxn, in other versions you have to write one.

table.maxn = function(t)
    local n = 0
    for k, v in pairs(t) do
        if type(k) == 'number' and k > n then
            n = k
        end
    end
    return n
end

Answer 2

It seem that the size operator #t is just not well defined in lua in the presence of nil values.

https://www.lua.org/pil/19.1.html

"undefined behaviour" in a scripting language.. Nice.