[erlang-questions] Re: List comprehension questions

[Olivier BOUDEVILLE]

Hello Robert,

Thanks for your explanation. I guess that the reversal (to restore the 
original list order after a first "if selected by filter then apply" 
traversal) is done then in the list:map/2 counterpart!

My concern was that, for long lists which do not happen to have to respect 
any particular order in their elements, two traversals would be done with 
list comprehensions whereas just one could be sufficient. Well, a minor 
concern as for such cases we can write our own functions to do so in one 
pass.

Thanks again,
Best regards,

Olivier Boudeville.
---------------------------
Olivier Boudeville

EDF R&D : 1, avenue du Général de Gaulle, 92140 Clamart, France
Département SINETICS, groupe ASICS (I2A), bureau B-226
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robert.virding@REDACTED 
31/03/2011 19:40

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olivier.boudeville@REDACTED
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Re: [erlang-questions] List comprehension questions






List comprehensions work as a combined lists:map and lists:filter. So you 
map over a list filtering out elements you don't want in the output list. 
They have the added functionality that they can handle this for nested 
lists.

The expanded code is in fact quite simple and is close to what you 
probably write yourself.

So order is preserved and their is no call to lists:reverse when 
generating the output list.

Robert

----- "Olivier BOUDEVILLE" <olivier.boudeville@REDACTED> wrote: 
> 
> Hi, 
> 
> Reading 
http://www.erlang.org/doc/reference_manual/expressions.html#id77052 it is 
not obvious that the order induced by a single generator is necessarily 
preserved by the comprehension, as nevertheless suggested by all 
experiments like: 
> 
> 1> [ X || X <- [1,2,3] ]. 
> [1,2,3]
> 
> I suppose this order preservation is true. Then, I imagine that, 
internally, list comprehensions are translated to code very similar to the 
one that would be produced if the developer had written an appropriate 
accumulator-based tail-recursive function? 
> 
> If so, does it imply that using a list comprehension on a semantically 
unordered set of elements will involve a useless reversal (like 
lists:reverse/1) of the resulting list? 
> 
> Thanks in advance for any answer, 
> Best regards, 
> 
> Olivier Boudeville.
> ---------------------------
> Olivier Boudeville
> 
> EDF R&D : 1, avenue du Général de Gaulle, 92140 Clamart, France
> Département SINETICS, groupe ASICS (I2A), bureau B-226
> Office : +33 1 47 65 59 58 / Mobile : +33 6 16 83 37 22 / Fax : +33 1 47 
65 27 13
> 
> 
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