Why is indexed foreach restricted to build in array ?

[matovitch via Digitalmars-d-learn]

On Saturday, 11 April 2015 at 11:24:32 UTC, John Colvin wrote:
> On Saturday, 11 April 2015 at 11:03:28 UTC, matovitch wrote:
>> On Saturday, 11 April 2015 at 10:53:46 UTC, Jakob Ovrum wrote:
>>> On Saturday, 11 April 2015 at 10:50:17 UTC, matovitch wrote:
>>>> Hello,
>>>>
>>>> The question is in the title. It should be possible for a 
>>>> finite random access ranges to perform an indexed foreach no 
>>>> ? I mean like :
>>>>
>>>> foreach(size_t i = 0, auto ref x; R)
>>>> {
>>>>  /*...*/
>>>> }
>>>>
>>>> Why are other foreach statements overloadable but this one ?
>>>>
>>>> Thanks in advance.
>>>
>>> As of 2.067, you can use std.range.enumerate[1]. See the PR 
>>> that added it[2] and the enhancement request that proposed 
>>> it[3] for more information about why it's a library function.
>>>
>>> [1] http://dlang.org/phobos/std_range#enumerate
>>> [2] https://github.com/D-Programming-Language/phobos/pull/1866
>>> [3] https://issues.dlang.org/show_bug.cgi?id=5550
>>
>> Thanks for the tip...I just tried it on the generic kmeans 
>> algorithm I coded, there are huge performance issue with dmd 
>> 2.0.67 :
>>
>> //With foreach ennumerate
>> cbrugel at eleanor ~/w/D/kmeans++> time ./kmeans_example
>> Point(0.742677, 0.749284, 0.746855)
>> Point(0.246975, 0.247246, 0.251123)
>> Point(0.751372, 0.754126, 0.247526)
>> Point(0.250743, 0.754682, 0.250682)
>> Point(0.755332, 0.249898, 0.749533)
>> Point(0.254945, 0.25063, 0.750403)
>> Point(0.746505, 0.258751, 0.249303)
>> Point(0.244185, 0.748149, 0.750536)
>> 4.72user 0.00system 0:04.73elapsed 99%CPU (0avgtext+0avgdata 
>> 4796maxresident)k
>> 0inputs+0outputs (0major+697minor)pagefaults 0swaps
>>
>> //with a classic for loop
>> cbrugel at eleanor ~/w/D/kmeans++> dmd kmeans_example.d kmeans.d
>> cbrugel at eleanor ~/w/D/kmeans++> time ./kmeans_example
>> Point(0.744609, 0.251452, 0.252298)
>> Point(0.750793, 0.754791, 0.248945)
>> Point(0.752109, 0.245865, 0.754593)
>> Point(0.752743, 0.746093, 0.748006)
>> Point(0.250339, 0.749277, 0.746064)
>> Point(0.249227, 0.24674, 0.751623)
>> Point(0.250478, 0.745153, 0.245349)
>> Point(0.243387, 0.249, 0.24996)
>> 1.30user 0.00system 0:01.30elapsed 99%CPU (0avgtext+0avgdata 
>> 6048maxresident)k
>> 0inputs+0outputs (0major+560minor)pagefaults 0swaps
>
> enumerate will kill unoptimised performance. Try with -O 
> -release -inline and see what times you get.
>
> Even better, get ldc or gdc and try them.

well ldc doesn't compile :

kmeans.d(40): Error: no property 'enumerate' for type 'Range'

With -O -release -inline  I get around 2s with foreach and 0.5s 
with a simple for.