Re: Question about arrays...

[Choonster TheMage <choonster.2010@...>]

On 10 June 2013 04:10, Andrew Starks <andrew.starks@trms.com> wrote:
>
>
> On Sunday, June 9, 2013, Dirk Laurie wrote:
>>
>> 2013/6/9 Tim Hill <drtimhill@gmail.com>:
>> > As the Lua docs state, an array is a table with a sequence
>> > of elements from 1..N.
>> >
>> > But my question is: is it ok to build the array out of order?
>> > For example:
>> >
>> > a = { 1, [3]=3 }
>> > a[2]=2
>> >
>> > Is "a" an array?
>>
>> The actual term is "sequence" but I know what you mean. Yes, it is.
>>
>> > Lua *seems* to think it is (#a returns 3), but the Lua docs seems
>> > to be a bit vague on this (it seems to be implied by the term
>> > "sequence" in the ref manual).
>>
>> Not vague, but not labouring the point either. Let's say the Manual
>> is mathematically precise on the subject.
>>
>>     We use the term sequence to denote a table where the set of all
>>     positive numeric keys is equal to {1..n} for some integer n,
>>     which is called the length of the sequence.
>>
>> Actually, the routines that depend on this property are not confused
>> by positive numeric keys that can't be taken for integers, but the
>> above is what it says.
>
>
> I've been confused on a related matter:
>
> I know that in the current implementation, a sequence  that also contains
> hashes will still work as a sequence. That is, in my experience:
>
> T = {"at one", "at two", property = "some value"}
>
> ...works in ipairs and with the length operator. Is this behavior that is
> dependent on the implementation or is it something that is a formal part of
> Lua?
>
> -Andrew

Lua 5.1 defines the length of a table to be "any integer index n such
that t[n] is not nil and t[n+1] is nil" [1]. This definition seems to
ignore any non-integer keys.

Lua 5.2 says this about the length of tables without a __len metamethod:
    "... the length of a table t is only defined if the table is a
sequence, that is, the set of its positive numeric keys is equal to
{1..n for some integer n. In that case, n is its length.
    ...
    Note, however, that non-numeric keys do not interfere with whether
a table is a sequence." [2]

Lua 5.1 and 5.2 both describe the behaviour of ipairs (when the table
doesn't have an __ipairs metamethod) as the following:
    "Returns three values: an iterator function, the table t, and 0,
so that the construction

         for i,v in ipairs(t) do body end
    will iterate over the pairs (1,t[1]), (2,t[2]), ..., up to the
first integer key absent from the table." [3][4]

So ipairs also ignores non-integer keys.

I think it's safe to assume that any Lua implementation following the
official manual's definitions of table length and ipairs will ignore
non-integer keys when it's performing array-like operations on a
table.


[1] http://www.lua.org/manual/5.1/manual.html#2.5.5
[2] http://www.lua.org/manual/5.2/manual.html#3.4.6
[3] http://www.lua.org/manual/5.1/manual.html#pdf-ipairs
[4] http://www.lua.org/manual/5.2/manual.html#pdf-ipairs