Why is indexed foreach restricted to build in array ?

[John Colvin via Digitalmars-d-learn]

On Saturday, 11 April 2015 at 12:04:06 UTC, matovitch wrote:
> 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.

This is roughly as expected. DMD is not good at optimising 
range-based code.

What OS are you on?