Re: Why no for table-iterator like e. g. "hashpairs"?

[Javier Guerra Giraldez <javier@...>]

On Tue, 5 Nov 2019 at 01:58, bil til <flyer31@googlemail.com> wrote:
> would be VERY nice, that currently it seems to be possilbe to iterate
> through the hash part of some larger ipairs-table with a lua for loop in
> some acceptable efficiency?

one little issue that isn't always evident:  the array part of Lua
tables is an implementation detail, a performance trick that is
supposed to be invisible to Lua code.

in principle, a table is a set of (key, value) pairs, unordered and
undistinguished.

Of course, small, dense integer keys are very useful, so several
functions help on dealing with the special case of sequences.

To make them faster and smaller, tables can hold some (but not
necessarily all) of those keys in an internal array.   Even so, all
these functions must handle the frequent case where integer keys are
stored in the "main" hash table.

Check the source code of ipairs or the # operator, you'll see they
don't go directly to the array.

So, what you're asking is not to "simply iterate the hash part".  It's
"iterate everything except sequence-keys".   consider:

for k, v in hpairs({'a', 'b', [1000]='c', y='d') do
   print (k, v)
end

It would obviously skip the (1, 'a') and (2, 'b') pairs, but what
about the (1000, 'c') pair?  it would probably appear in the hash.
ipairs() doesn't return it, since it's not part of a sequence.  So,
should it print?

what about the "nasty" tables, like that one shown by Robeto?
{x=1,y=2,[1]=3,[2]=4,[3]=5}  ipairs does show all three integer keys,
but they're likely stored on the hash part.  should hpairs() just
ignore any integer key?   now it seems it would have to first check if
they form a contiguous sequence or not....

the key point is that the array optimization is just a performance
optimization, not an explicit behaviour.  to a Lua program it should
always behave as a homogeneous set of (key, value) pairs.

-- 
Javier