Thread: Re: [GD-Windows] Array foreach in scripting languages

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The reason I'd like to have this work is that it is a
recurring pattern in lots of code I see (iterating a
list and deleting/inserting entries).
=20
If I go the route with tracked iterators, any insert
or delete operation will work as expected since then
I'm dealing with these cases per iterator:
=20
- delete item before or at current position
  -> decrement position and size
- delete item after current position
  -> decrement size
- insert item before or at current position
  -> increment position and size
- insert item after current position
  -> increment size
=20
The case "delete item at current position" potentially
has to deal with the actual element being deleted.
In C++ this would be a problem but in my scripting
language the list entry would go away but the actual
value referenced will stay around due the iterator
still holding onto it.
=20
I'm quite surprised that the C# docs don't even mention
this situation. I'm even more surprised to find that MS
added a "for each" to C++. It's supposed to be only for
managed code but it worked fine in my standard STL
unmanaged test.
=20
Simply banning this scenario would mean that users have
to write such code:
=20
int i,iC=3DList.GetSize();
for(i=3D0;i<iC;i++)
{
    if(Condition(List[i]))
    {
        List.Delete(i);
        i--;iC--;
    }
};
=20
manually. Which is goes against the whole idea of pro-
viding a foreach and IMHO even more error-prone.
I'm not sure if I more dislike the C#/STL behaviour of
throwing an exception or the Python behaviour of simply
ignoring the problem.
=20
<rant>
In my case, an exception is not an option since I'm not
going to clutter my cute little scripting language with
a concept (exceptions) that I consider totally broken
in the first place. There's no place in a game for such
stuff. It has to run, period. So you have to handle all
error conditions anyway.
</rant>
=20
Best regards,
=20
Carsten Orthbandt
Founder + Development Director
SEK SpieleEntwicklungsKombinat GmbH
http://www.sek-ost.de

Wenn ich Visionen habe, gehe ich zum Arzt. - Helmut Schmidt
 =20
=20

________________________________

Von: gam...@li... im Auftrag von =
Kent Quirk
Gesendet: So 13.08.2006 04:39
An: Game Development for MS Windows
Betreff: Re: [GD-Windows] Array foreach in scripting languages

In Python, I was shocked to find that this succeeds:

L =3D range(20)
for i in L:
    print i
    if i=3D=3D4:
        del L[6]

Result:
-------
0
1
2
3
4
5
7
8
9

I fully expected it to fail. I consider messing with the list you're
traversing with an iterator to be on a par with sawing off the branch
you're standing on, unless, as you imply, the list is known to be
implemented as a linked list. In python, you can surprise yourself.
Consider this:

for i in L:
    print i
    if i=3D=3D4:
        del L[3]       # delete one of the items BEFORE the one you're =
on

     =20
0
1
2
3
4
6
7
8
9

So what happened is that we ended up skipping element #5, even though we
deleted element 3.

This is really dependent on the implementation details of the particular
interpreter, and probably isn't guaranteed by the average language spec.

If you feel strongly about being able to do this, you have to deal with
the case of deleting the item you're on right now, and also about
deleting the last element while you're at the last element. Check this =
out:

 >>> L
['h', 'e', 'l', 'l', 'o']
 >>> for c in L:
            if c=3D=3D'l':
                L.remove(c)

 >>> L
['h', 'e', 'l', 'o']

In other words, even though I intended to remove BOTH 'l' characters,
only one of them was removed because deleting one caused me to skip the
next element.

I think this stuff is so fraught with potential errors by the user, even
if YOU get it right, that I'd prefer to just ban the practice and make
people use a "filter" concept like in python, or a remove_if as in STL.

    Kent

Carsten Orthbandt wrote:
> I'm currently building a small scripting language for interactive =
stuff. Now I
> know there's plenty out there, I'm just doing this for fun.
>=20
> A tricky question came up and I'd like to collect opinions and =
experiences on
> this. It's about foreach for dynamic arrays. Consider this C++-like =
pseudo code:
>=20
> DynamicArray<int> list;
> int i;
> for(i=3D0;i<20;i++){list.Add(i);};
> foreach(i in list){
>     print(i);
>     if(i=3D=3D8){list.Delete(12);};
> };
>=20
> The obvious question is how should an active iterator react if the =
traversed list
> changes? To answer this, I simply tried it out in two languages I know =
that do
> provide both dynamic arrays and foreach: STL, Tcl and C#.
>=20
> In STL C++ code, I get an exception because of iterator =
incompatibility.
> In Tcl this problem isn't one because the iterator takes a snapshot of =
the list
> object on start and there's no language construct to actually change =
this instance.
> In C# with the List<int> generic, I get an exception because the =
iterated object
> changes (interestingly, this happens on ANY change to the list, even =
appending).
>=20
> Now what I'd like to know:
> - how do other languages cope with this? (Tcl doesn't need to, C++/C# =
simply fails)
> - what do YOU think would be sensible behaviour?
>=20
> Currently I'm considering having all iterators be elements of a =
double-linked list
> and let the array object be the dl-list head. So if the number of =
array elements
> changes, it can run through all attached iterators and fix their =
indices or stop them.
>=20
> =20

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