Re: A very basic thing I don't get

[Lorenzo Donati <lorenzodonatibz@...>]

On 01/10/2011 21.52, Dirk Laurie wrote:
> On Sat, Oct 01, 2011 at 08:43:02PM +0200, Stefan Reich wrote:
> > Well... I would generally say that I am not totally happy with the way
> > arrays and lists in Lua work. It's probably the only thing I would
> > change right now if I was to change something in the language.
> ...
> > I find the relation between nil and lists also confusing.
> ...
> > I have to say: Arrays in Lua are indeed a little confusing.
>
> Only while you still think in some other language.
>
> Think of it this way:

Very good summary of an often recurring topic!

Some additions for the archive:

[...]
>    8. The function `pairs` iterates over those (key,value) pairs for
>       which the value is not nil, in some undefined order.
Which is a sort of wrapper for the true iterator function "next"
>    9. Since the particular set of keys (1,2,3,...,n) occurs very often in
>       applications, Lua offers extra support for those.
>       - the length operator `#`
>       - the function `ipairs` which iterates over the (key,value) pairs
>         in the well-defined order 1,2,3,...
>       - the table library
>       - the abbreviated table constructor with implicit keys
>       It is convenient to call such tables "lists" or "arrays".
In Lua 5.2 the new terminology for those ones are "sequences" (see 
section 3.4.6 of refman)

>   10. Those functions do not actually fail on tables that are not lists,
>       but their results are not as useful in those cases.
In Lua 5.1 `#' was well defined also for non-sequences, albeit it didn't 
return a generally useful value. In 5.2 `#' is defined only for 
sequences. Although the old behaviour is still there for non-sequences, 
now is to be considered an implementation detail.

Note: as the aforementioned section 3.4.6 states, non-integer keys 
doesn't interfere with a table being a sequence, thus the following *is* 
a sequence:

{ 10, 20, 30, 'a', 'b', foo = 1, bar = 'hello!'}

(this may come up as unexpected for newbies or for people not reading 
the manual carefully, since the meaning of the term "sequence" in Lua is 
slightly different from the usual math meaning).

[...]

> Dirk

Cheers.
-- Lorenzo