• Subject: RE: Bug report in length of simple table
• From: <Tomas.Lavicka@...>
• Date: Wed, 14 Sep 2016 06:32:04 +0000

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-----Original Message-----
From: lua-l-bounces@lists.lua.org [mailto:lua-l-bounces@lists.lua.org] On Behalf Of Oliver Kroth
Sent: Tuesday, September 13, 2016 12:06
To: lua-l@lists.lua.org
Subject: Re: Bug report in length of simple table

Am 13.09.2016 um 11:37 schrieb Malma:
>
> I think this explanation must be in official doc if it's a source of
> confusion.
>
> print(# {[1] = 10, [2] = 20, [3] = nil, [4] = nil, [5] = 40} ) -- is
> not sequence ?
> print(# {10, 20, nil, nil, 40} ) -- is sequence ?  From doc: Note that
> a table like {10, 20, nil, 40}  is not a sequence...
>
> output:
>
> 2
> 5
>
The # operator computes the number of elements of a sequence in a Lua table # returns a valid result only for sequences.

A Lua table contains a sequence if and only if all integer keys greater zero are contiguous; i.e. there must be no missing numbers.

If there are gaps, the table contains no sequence.

Th table may contains values for other keys, including 0, negative, or non-integer numbers, this does not affect the sequence.

Note that this definition is completely independent of the implementation detail has part / array part; one does not need to care about that.

nil can't be stored in a table. Assigning nil to a table key removes the key. Use another value to tag special (e.g.missing) entries, like false

{10, 20, nil, nil, 40} contains NOT a sequence {10, 20, false false, 40] contains a sequence { [0]="null", 10, 20, 30, a="alpha",  [math.pi] = "pi" } contains a sequence

--
Oliver

-----------------------------------------------------------------------------------

I am new to lua and I want to understand the language. According book (Programming in Lua, 3rd edition):
"More precisely, a sequence is a table where the numeric keys comprise a set 1, . . . , n for some n." (page 24)
So table {10, nil, 30, 40} contains sequence {10} (n=1), which length is 1. Is there any other definition?
It is very confusing and operator # can be unusable for tables, because in many cases I need to check sequences and I cannot trust this operator. It should return or 1 (book definition) or if holes in sequence break the sequence, it should return 0. Otherwise I will be pushed for every sequence check to check all array fields manually in some kind of loop.

Tomas Lavicka
(please excuse my bad english - I am not english native - but I hope that you understand, what I mean)
```