Factorio Forums

Hi,

Can you help me understanding something?
If i run the code it outputs {1, 3}, as i expected.
But if i run the code then i expected result == {{1,2},{1,2,3}}
but i got result == {{1,2,3},{1,2,3}}
(I know comparing two tables and the values of two tables are two different things in lua, it's just the shortest way to write it down here. I checked by printing the values and not by comparing.)
Where is my thinking mistake?

This does not perform deepcopy, it only puts reference to another table. Since there is only one instance of "tab", both elements inside result will be same.

Ah, i see. Thank you <3

There's a little "gotcha" with tables I'd like to point out for OP: if you pass a table into a function, it makes sense you're editing that table... unless you change the table by doing myvar = {} or something like that. You re-assign the local variable's table, and are no longer accessing the table that was passed in. It makes sense, but I ran into that and was confused why my original table wasn't being edited inside my function, and worse, all my actions were succeeding on the local table

boskid wrote: ↑

Mon Jan 06, 2020 8:51 am

Code: Select all

result[#result+1] = tab

This does not perform deepcopy, it only puts reference to another table. Since there is only one instance of "tab", both elements inside result will be same.

Code: Select all

local x = 3
local result = {}
for k = 2, x do
    local tab = {} -- moved into loop
    for v = 1, k do
        tab[v] = v
    end
    result[#result+1] = tab
end

This is more of a Lua question than anything else, and it's about fine details - I came from programming very explicit/strict languages, and I'd probably do the "initialization" of local tab = nil outside the loop, and do a tab = {} inside the loop. Lua has some laziness to the memory operations and whatnot, so there's probably not a performance/memory difference, comparing a "local <foo>" inside and outside a loop?