Re: Question about arrays...

[Andrew Starks <andrew.starks@...>]

On Sunday, June 9, 2013, Choonster TheMage  wrote:
> 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

Thanks for that. I've gone ahead and assumed as much, after reading the same passages. I just didn't see this use of tables explicitly called out / endorsed and so I've always wondered a bit.