what should the value of an emptied table be?

pie · October 2015

Hi, I am facing a small issue with string indexed arrays.

I need to "intercept" a table (t, loaded from json), remove one or more entries (according to their index), and pass it through the next step of code if there is some entry left (there could be some).

I thought I could just nil the entries (as in example below) and later detect if t is empty to proceed:
the problem is that t is not empty, and I can't use the lenght operator # as a parameter because indexes are strings.

How do I check if t is "empty"? Or how do I remove the entries I need to remove from it, to leave t empty (if there are no other entries left) ?

t = { 
m = {1,3,4,5,"pp"},
n= {"foo"}
}
 
 
t["n"] = nil
t["m"] = nil
 
print("T", t, t == {}) -- t == {} is false, why?
 
for i,v in pairs(t) do
print(i,v)
	for j,k in pairs(v) do
		print(j,k)
	end
end

Thank you

[edit: I noticed that I get nil if I use getmetatable (t) on the empy table, that seems to solve my problem ]

FlimFlamboyant · October 2015

print (#t) works just fine for me. It returns 0 as expected.

pie · October 2015

Unfortunately # won't work on string index (try commenting t["m"] =nil: result of #t is always 0 because there are no num index)

However I found a workaround, I hope it's enough

Thank you

keszegh · November 2015

t == {} would be surpirising to work anyway, as t is one table and {} another (made at that moment). similarly as two tables having the same content are not equal.

pie · November 2015

I see, there was something I couldn't grasp..

thanks
So the "getmetatable workaround" is the only way to understand if a table is empty?

keszegh · November 2015

after a bit of web-search i think this is a good (better?) way:

if not next(myTable) then
    -- Table is empty
end

keszegh · November 2015

check http://docs.giderosmobile.com/reference/lua/global/next
even there it is written that "In particular, you can use next(t) to check whether a table is empty. "

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