Re: table.maxn...

[Tim Hill <drtimhill@...>]

On Jun 1, 2015, at 8:05 AM, Dirk Laurie <dirk.laurie@gmail.com> wrote:

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

No, Roberto is using “pairs()” to scan all keys (including non-integral). So it does work as intended and will exclude a table with a positive non-integral key. Tables that “have no holes” are a sufficient condition for being a sequence, which is a more strict definition that determines predictable behavior for # as well as many table library functions.

—Tim