Re: pairs(t, skey) and ipairs(t, skey)

[Leo Razoumov <slonik.az@...>]

On 10/3/13, Andrew Starks <andrew.starks@trms.com> wrote:
> On Thursday, October 3, 2013, Leo Razoumov wrote:
>
>> On 10/3/13, Roberto Ierusalimschy <roberto@inf.puc-rio.br <javascript:;>>
>> wrote:
>> >> On 10/2/13, Luiz Henrique de Figueiredo
>> >> <lhf@tecgraf.puc-rio.br<javascript:;>>
>> wrote:
>> >> >> Another habit we should not be practicing is thinking of Lua tables
>> as
>> >> >> having
>> >> >> array and hash parts.
>> >> >
>> >> > Exactly. This is an implementation detail for getting performance
>> >> > and
>> >> > reduced
>> >> > memory usage but Lua tables remain what they are: associative
>> >> > arrays.
>> >> >
>> >>
>> >> Unfortunately, Lua itself encourages such a separation and provides
>> >> both
>> >> pairs and ipairs to reinforce the distinction.
>> >
>> > pairs traverses all keys, ipairs traverses integer keys starting from
>> > 1. They have NOTHING TO DO with whether those keys live in the array
>> > part or the hash part of a table. (How many times we will have to
>> > repeat this?)
>>
>> Roberto,
>> I am not talking about implementation (hash part or array part) but about
>> table semantics. Lua table distinguishes sequential keys (from 1 through
>> N)
>> that can be iterated in order using ipairs from the other keys iterated
>> via pairs that visit all the keys in a non-specified order.
>> Moreover, sequential keys lack length operator, for #tbl does not
>> necessarily return N.
>>
>> Quick question: I know how to iterate sequential part forwards from 1 to
>> N
>> using ipairs. How can I iterate it backwards from N down to 1?
>> I am afraid Lua does not provide a convenient way to find out the value
>> of
>> N.
>>
>> Another related question: How can I iterate sequential part of a table
>> from 1 through N visiting only even (or odd) elements?
>>
>> --Leo--
>>
>>
> Leo,
>
> Yes it does. This is always true:
>
> local i
>
> for idx, v in ipairs(t) do
>   i = idx
> end
>
> assert (i == #t)
>
> -andrew
>

No it is not!  For tables with holes your assertion can fail.
>From the Lua-5.1.5 manual:
"If the array has "holes" (that is, nil values between other non-nil values),
 then #t can be any of the indices that directly precedes a nil value
 (that is, it may consider any such nil value as the end of the array)."

--Leo--