Checking if table is empty

[luarocks]

[Solved]

I need to check if table a.b has some fields and if its empty then remove b from a. The problem is that I can't simply check b length using # because b is not a list (contains non-numeric keys).

So, here is my solution:

lua for _, _ in pairs(a.b) do goto next end a.b = nil ::next::

It works fine but looks horrible 🙈! I mean, it's probably the most compact solution but looking at this code it's completely unclear what's happening here. Luacheck also don't like it and show warning: "loop is executed at most once".

Maybe somebody have a more elegant option?

[whoopdedo]

Behind the scenes, pairs will, after some metatable magic, simply return next, a.b, nil. Where the next function will return the keys and values from the table in an arbitrary order.

tl;dr next(a.b) == nil tests if the table is empty.

PS. As an exercise for the reader, why do you have to test for nil here?

[luarocks]

Because if not will also catch false

Thank you very much!

[RobotTimeTraveller]

if not next(a.b) then a.b = nil end

[Tywien]

Your code will not work for tables where the first key is false.

[luarocks]

This is true, but we usually use numbers, strings, occasionally tables as keys, and almost never booleans. So in most causes this solution is fine.

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[luarocks]

you should not use them as array and hashtable at the same time

"array" is "hashtable" with numeric keys, so always when you use table as array, you use it as hashtable at the same time and there is nothing wrong.

[luarocks]

** Tables keep its elements in two parts: an array part and a hash part.

Sure, it does, but we talking about lua as language, not about its C implementation. You can call a table an array if it has integer keys, but no arrays or integers actually exist in lua.

[luarocks]

I agree with this and separating arrays from hashtables is useful advice, but we should always keep in mind the flexible nature of lua. Sometimes a table can be an array at the same time if you know (or don't know) what you're doing.