Re: table.maxn...

[Dirk Laurie <dirk.laurie@...>]

2015-06-01 16:24 GMT+02:00 Roberto Ierusalimschy <roberto@inf.puc-rio.br>:
>> On Mon, Jun 1, 2015 at 6:26 AM, Coda Highland <chighland@gmail.com> wrote:
>> > On Mon, Jun 1, 2015 at 3:06 AM, Tim Hill <drtimhill@gmail.com> wrote:
>> >> This also takes care
>> >> of the issue of FP errors, since all the math is now done on integers.
>>
>> Also, it doesn't resolve FP errors in Lua 5.1/5.2, because numbers are
>> still double-precision floats and not true integers. The act of adding
>> 1 + 1<<53 is what causes the problem, not dealing with fractional
>> values.
>
> I use the following code (or rather I would use it if ever I had to
> check whether a table is a sequence :-):
>
> function issequence (t)
>   local n = #t
>   local c = 0   -- count keys in [1..n]
>   for k in pairs(t) do
>     if type(k) == 'number' and k > 0 then
>       if k ~= math.floor(k) or k > n then return false end
>       c = c + 1
>     end
>   end
>   return c == n
> end

The right question is not whether a table is a sequence, but whether
the default #tbl is deterministic for the particular table, i.e. whether the
table has no holes. Being a sequence is merely a sufficient condition. The
iterations `for i=1,#tbl` and `for i in ipairs(tbl)` both ignore
non-integer keys.

So I would be tempted to change the function name to "hasnoholes" and
the loop body to

    if type(k) == 'number' and k > 0 and k==math.floor(k) then
       if k > n then return false else c=c+1 end
    end