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

[Kent Q. <ken...@co...>]

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

L = range(20)
for i in L:
    print i
    if i==4:
        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==4:
        del L[3]       # delete one of the items BEFORE the one you're on

       
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=='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.
>  
> 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:
>  
> DynamicArray<int> list;
> int i;
> for(i=0;i<20;i++){list.Add(i);};
> foreach(i in list){
>     print(i);
>     if(i==8){list.Delete(12);};
> };
>  
> 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#.
>  
> 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).
>  
> 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?
>  
> 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.
>  
>