[Rcpp-devel] Datetime substraction bug ?

Romain Francois romain at r-enthusiasts.com
Tue Apr 2 17:42:28 CEST 2013


What. In c++, when we define an operator-(a, b), we are supposed to use 
it like this in code:

diff = a - b ;

What you agree with suggest that diff should be the result of b - a. 
This is non sense.

If we write a function, like "diff" in R, then why not give it whatever 
behavior suits the habits of R programmers.

But seriously, you are defending that a-b gives the result of b-a.

Le 02/04/13 17:34, Patrick Burns a écrit :
> In case Dirk is feeling lonely, I'll side with him.
>
> I think the ultimate issue is abstraction versus
> application.  In a purely abstract setting, then I
> agree that `-`(1, 3) should be negative.  But we
> don't have a purely abstract setting.  I agree with
> Dirk's New Year's/Easter example -- if the dates are
> in order, then the diff should be positive to my
> mind.
>
> Note also:
>
>  > diff(c(1, 3))
> [1] 2
>
> Pat
>
>
> On 02/04/2013 15:33, Romain Francois wrote:
>> Le 30/03/13 00:44, Dirk Eddelbuettel a écrit :
>>>
>>> On 29 March 2013 at 23:37, Robin Girard wrote:
>>> | Hi List
>>> |
>>> | why is it that we have in the Datetime definition
>>> | (here
>>> http://dirk.eddelbuettel.com/code/rcpp/html/Date_8cpp_source.html#l00271
>>> )
>>> |
>>> | double operator-(const Datetime& d1, const Datetime& d2) { return
>>> d2.m_dt - d1.m_dt; }
>>> | bool operator<(const Datetime &d1, const Datetime& d2) { return
>>> d1.m_dt < d2.m_dt; }
>>> |
>>> | I feel there is a contradiction
>>> | substraction shoulb be d1.m_dt - d2.m_dt;  no ?
>>>
>>> It's been a while since I wrote that and I think I followed the
>>> difftime()
>>> convention in R. Or maybe not.  To me d1 comes before d2 in the way the
>>> arguments are given, so d2 is larger and I want to subtract d1 from d2.
>>
>> Well. operator-(a, b) is supposed to implement a-b. See e.g. wikipedia
>> http://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B
>>
>> The current implementation does not make sense to me. So I second Robin
>> here.
>>
>>> In any event, if it bugs you greatly you are always free to write your
>>> own
>>> function. ;-)
>>>
>>> Dirk
>>
>> wtf. This is the same as saying 3-1 = -2
>>
>> And if i follow your logic, why is the operator< implemented this way
>> (correctly this time).
>>
>>> | Robin
>>> |
>>> | Dr. Girard Robin
>>> | Chargé de Recherche
>>> |
>>> | MINES-ParisTech / Département Energétique et Procédés / PERSEE /
>>> Groupe ERSEI
>>> | Centre Procédés, Energies Renouvelables et Systèmes Energétiques
>>> (PERSEE)
>>> | Center for Processes, Renewables Energies and Energy Systems
>>> | Renewable Energies & Smartgrids (ERSEI)
>>> |
>>> | 1 Rue Claude Daunesse - CS 10207 - F-06904 Sophia Antipolis Cedex
>>> | Tel: +33.4.93.67.89.64 (~99), Fax: +33.4.93.95.75.35
>>> | e-mail : robin.girard at mines-paristech.fr
>>> |
>>> | web page perso
>>> http://www.mines-paristech.fr/Services/Annuaire/&?id=8828
>>> | statoverflow : http://stats.stackexchange.com/users/223/robin-girard
>>> | web page centre PERSEE : http://www.cep.mines-paristech.fr/
>>> | linkedin :
>>> http://www.linkedin.com/profile/view?id=14907272&trk=tab_pro
>>> |
>>> | Le lien vaut mieux que le bien.
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>>
>


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Romain Francois
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