Re: [julia-users] indexing over `zip(collection1, collection2)`

[Scott Jones]

I've found it very useful, even if it is O(n) complexity to get the n-th 
element, to allow indexing.

For a database, if you have a packed record (for example, like SQLite's row 
format), and you only need to access a single field, it's better to be able 
to do just that,
and *not* have to unpack the entire record just to do that (skipping fields 
can be many many times faster than unpacking them, plus the amount of time 
wasted on allocating
objects for all of the fields of records scanned and then GCing them).

I think it would be better to have a trait that you could find out if an 
indexable structure is directly indexable or not, kind of like strings used 
to have)
(or does that exist now, with all of the changes for Arraymageddon?)

On Thursday, June 23, 2016 at 3:35:06 PM UTC-4, Mauro wrote:
>
> My recollection is that part of the indexing interface in Julia (just by 
> convention) is that indexing should be of O(1) (or close to that) 
> complexity.  Iterable things, in general, have O(n) complexity to access 
> the n-th element, because you have to traverse the data to get there 
> (the classic example are linked lists).  Thus, it makes sense that 
> indexing is not supported, instead you have to call collect first if you 
> want to index. 
>
> On Thu, 2016-06-23 at 20:43, Davide Lasagna  > wrote: 
> > Hi Jacob, 
> > 
> > In my view, in principle, all "iterators" should be indexable, (at least 
> > read-only), *unless *the underlying data is not indexable by nature, 
> e.g. 
> > with data that comes from a stream...  Doing `zip([1, 2], [2, 3])[1]` 
> > should probably just work. 
> > 
> > I also think that for `Zip`s if the zipped collections are not 
> indexable, 
> > then the "outer" getindex method should fail internally with a 
> MethodError, 
> > on the "inner" getindex calls not implemented for the non-indexable 
> > collections. 
> > 
> > On Thursday, June 23, 2016 at 7:23:34 PM UTC+1, Jacob Quinn wrote: 
> >> 
> >> Sorry, to clarify a little: 
> >> 
> >> The things you're zipping are not necessarily indexable (i.e. other 
> >> iterators), so it's not safe to assume you can always index a Zip. 
> >> 
> >> On Thu, Jun 23, 2016 at 2:21 PM, Jacob Quinn  >> > wrote: 
> >> 
> >>> Most "iterator" types are not indexable, AFAIK. The typical 
> >>> recommendation/idiom is to just call `collect(itr)` if you need to 
> >>> specifically index. 
> >>> 
> >>> -Jacob 
> >>> 
> >>> On Thu, Jun 23, 2016 at 2:18 PM, Davide Lasagna  >>> > wrote: 
> >>> 
>  Is there any particular reason why `Zip` objects are iterable but not 
>  indexable? Python allows that. 
>  
>  From previous discussion on the topic (2014 topic at 
>  https://groups.google.com/forum/#!topic/julia-dev/5bgMvzJveWA) it 
> seems 
>  that it has not been implemented yet. 
>  
>  Thanks, 
>  
>  
>  
> >>> 
> >> 
>

Re: [julia-users] indexing over `zip(collection1, collection2)`

[Mauro]

On Fri, 2016-06-24 at 05:26, Rafael Fourquet  wrote:
>> My recollection is that part of the indexing interface in Julia (just by
>> convention) is that indexing should be of O(1) (or close to that)
>> complexity.
>
> As the OP suggested, this could still be the case, the zip object
> would simply forward the indexing to the zipped collections, which
> would fail if one of then is not indexable (in O(1)). Alternatively
> there could be a trait for that.
> I've also wanted that for a long time, and even setindex!, to be able
> to sort an array2 according to the order defined in array1:
> sort!(zip(array1, array2)).

Yes, you're right.

Re: [julia-users] indexing over `zip(collection1, collection2)`

[Mauro]

My recollection is that part of the indexing interface in Julia (just by
convention) is that indexing should be of O(1) (or close to that)
complexity.  Iterable things, in general, have O(n) complexity to access
the n-th element, because you have to traverse the data to get there
(the classic example are linked lists).  Thus, it makes sense that
indexing is not supported, instead you have to call collect first if you
want to index.

On Thu, 2016-06-23 at 20:43, Davide Lasagna  wrote:
> Hi Jacob,
>
> In my view, in principle, all "iterators" should be indexable, (at least
> read-only), *unless *the underlying data is not indexable by nature, e.g.
> with data that comes from a stream...  Doing `zip([1, 2], [2, 3])[1]`
> should probably just work.
>
> I also think that for `Zip`s if the zipped collections are not indexable,
> then the "outer" getindex method should fail internally with a MethodError,
> on the "inner" getindex calls not implemented for the non-indexable
> collections.
>
> On Thursday, June 23, 2016 at 7:23:34 PM UTC+1, Jacob Quinn wrote:
>>
>> Sorry, to clarify a little:
>>
>> The things you're zipping are not necessarily indexable (i.e. other
>> iterators), so it's not safe to assume you can always index a Zip.
>>
>> On Thu, Jun 23, 2016 at 2:21 PM, Jacob Quinn > > wrote:
>>
>>> Most "iterator" types are not indexable, AFAIK. The typical
>>> recommendation/idiom is to just call `collect(itr)` if you need to
>>> specifically index.
>>>
>>> -Jacob
>>>
>>> On Thu, Jun 23, 2016 at 2:18 PM, Davide Lasagna >> > wrote:
>>>
 Is there any particular reason why `Zip` objects are iterable but not
 indexable? Python allows that.

 From previous discussion on the topic (2014 topic at
 https://groups.google.com/forum/#!topic/julia-dev/5bgMvzJveWA) it seems
 that it has not been implemented yet.

 Thanks,



>>>
>>

Re: [julia-users] indexing over `zip(collection1, collection2)`

[Davide Lasagna]

Hi Jacob, 

In my view, in principle, all "iterators" should be indexable, (at least 
read-only), *unless *the underlying data is not indexable by nature, e.g. 
with data that comes from a stream...  Doing `zip([1, 2], [2, 3])[1]` 
should probably just work.

I also think that for `Zip`s if the zipped collections are not indexable, 
then the "outer" getindex method should fail internally with a MethodError, 
on the "inner" getindex calls not implemented for the non-indexable 
collections.

On Thursday, June 23, 2016 at 7:23:34 PM UTC+1, Jacob Quinn wrote:
>
> Sorry, to clarify a little:
>
> The things you're zipping are not necessarily indexable (i.e. other 
> iterators), so it's not safe to assume you can always index a Zip.
>
> On Thu, Jun 23, 2016 at 2:21 PM, Jacob Quinn  > wrote:
>
>> Most "iterator" types are not indexable, AFAIK. The typical 
>> recommendation/idiom is to just call `collect(itr)` if you need to 
>> specifically index.
>>
>> -Jacob
>>
>> On Thu, Jun 23, 2016 at 2:18 PM, Davide Lasagna > > wrote:
>>
>>> Is there any particular reason why `Zip` objects are iterable but not 
>>> indexable? Python allows that.
>>>
>>> From previous discussion on the topic (2014 topic at 
>>> https://groups.google.com/forum/#!topic/julia-dev/5bgMvzJveWA) it seems 
>>> that it has not been implemented yet. 
>>>
>>> Thanks,
>>>
>>>
>>>
>>
>

Re: [julia-users] indexing over `zip(collection1, collection2)`

[Jacob Quinn]

Sorry, to clarify a little:

The things you're zipping are not necessarily indexable (i.e. other
iterators), so it's not safe to assume you can always index a Zip.

On Thu, Jun 23, 2016 at 2:21 PM, Jacob Quinn  wrote:

> Most "iterator" types are not indexable, AFAIK. The typical
> recommendation/idiom is to just call `collect(itr)` if you need to
> specifically index.
>
> -Jacob
>
> On Thu, Jun 23, 2016 at 2:18 PM, Davide Lasagna 
> wrote:
>
>> Is there any particular reason why `Zip` objects are iterable but not
>> indexable? Python allows that.
>>
>> From previous discussion on the topic (2014 topic at
>> https://groups.google.com/forum/#!topic/julia-dev/5bgMvzJveWA) it seems
>> that it has not been implemented yet.
>>
>> Thanks,
>>
>>
>>
>

Re: [julia-users] indexing over `zip(collection1, collection2)`

[Jacob Quinn]

Most "iterator" types are not indexable, AFAIK. The typical
recommendation/idiom is to just call `collect(itr)` if you need to
specifically index.

-Jacob

On Thu, Jun 23, 2016 at 2:18 PM, Davide Lasagna 
wrote:

> Is there any particular reason why `Zip` objects are iterable but not
> indexable? Python allows that.
>
> From previous discussion on the topic (2014 topic at
> https://groups.google.com/forum/#!topic/julia-dev/5bgMvzJveWA) it seems
> that it has not been implemented yet.
>
> Thanks,
>
>
>

[julia-users] indexing over `zip(collection1, collection2)`

[Davide Lasagna]

Is there any particular reason why `Zip` objects are iterable but not 
indexable? Python allows that.

>From previous discussion on the topic (2014 topic 
at https://groups.google.com/forum/#!topic/julia-dev/5bgMvzJveWA) it seems 
that it has not been implemented yet. 

Thanks,