From cstrato at aon.at  Mon Jun 23 23:54:03 2014
From: cstrato at aon.at (cstrato)
Date: Mon, 23 Jun 2014 23:54:03 +0200
Subject: [Rd]
 =?utf-8?q?operation_on_=E2=80=98numsels=E2=80=99_may_be_unde?=
 =?utf-8?q?fined?=
In-Reply-To: <CAC2h7usxy2-X-Nq4NYwExyuDZuVW4D7kqRALdhBhmGZVnVJKpA@mail.gmail.com>
References: <53A8298C.9010605@aon.at>
	<244EC819-4336-4A49-BAEB-86B57E8A2970@r-enthusiasts.com>
	<53A855C6.1000301@aon.at>
	<BE6B83B4-2A60-4F84-8B5F-C0754997E5C5@r-enthusiasts.com>
	<53A877EE.1080201@aon.at>
	<CAC2h7usxy2-X-Nq4NYwExyuDZuVW4D7kqRALdhBhmGZVnVJKpA@mail.gmail.com>
Message-ID: <53A8A1FB.3040803@aon.at>

Dear Kasper,

What do you mean with 'undefined aspect of ++'?

Every compiler has to evaluate first the expression on the right side 
and then apply the result to the variable on the left side, as in:
    i = i + 1;
I understand that the expression:
    i = i++;
may be confusing, but the expression:
    i = ++i;
should work. What about:
    i = (++i); or i = {++i}
Would this also result in a warning message?

Best regards,
Christian


On 6/23/14 10:05 PM, Kasper Daniel Hansen wrote:
> You're getting this message because you are using an undefined aspect of
> ++.  Depending on compiler convention re. the interpretation of ++, your
> code may be interpreted differently; ie. different compilers will
> interpret the code differently.  This is a bad thing.
>
> You're presumably getting the warning now, because the compiler flag has
> been added on the Bioc build servers.
>
> There was a recent thread on this specific aspect of ++ recently, but I
> forgot which email list.
>
> Kasper
>
>
> On Mon, Jun 23, 2014 at 2:54 PM, cstrato <cstrato at aon.at
> <mailto:cstrato at aon.at>> wrote:
>
>     Dear Romain,
>
>     I do not know enough about compilers, but as far as I remember, they
>     'work' from right to left, so numsels = ++numsels should not confuse
>     the compiler. Anyhow I will change my code to your first suggestion
>     since it is more elegant.
>
>     Best regards,
>     Christian
>
>
>
>     On 6/23/14 7:13 PM, Romain Fran?ois wrote:
>
>
>         Le 23 juin 2014 ? 18:28, cstrato <cstrato at aon.at
>         <mailto:cstrato at aon.at>> a ?crit :
>
>             Dear Romain,
>
>             Thank you for your suggestions, I like especially the first one.
>
>             However, you did not explain why I have never got this
>             warning message on any compiler, and why only one of the two
>             identical Ubuntu compilers did give this warning message?
>
>             Best regards,
>             Christian
>
>
>         I don?t know, but this:
>
>         numsels = ++numsels ;
>
>         seems fishy to me, and so it keeps feeling weird with the
>         addition of the ternary operator.
>
>         There is obviously a difference of setup between these two
>         machines, but I don?t have time to sherlock that for you. One of
>         the compilers is getting more careful than the other. Getting
>         warnings you did not get before is a good thing, as it helps you
>         update the code with that new insight.
>
>         Welcome to my world, I?m sometimes thrown all kinds of new
>         warnings from esoteric compilers, all of them have value .
>
>         Romain
>
>             On 6/23/14 3:45 PM, Romain Fran?ois wrote:
>
>
>                 Le 23 juin 2014 ? 15:20, cstrato <cstrato at aon.at
>                 <mailto:cstrato at aon.at>> a ?crit :
>
>                     Dear all,
>
>                     Since many years the following C++ code does compile
>                     on ALL Bioconductor servers (Linux, Windows, Mac)
>                     without any warnings:
>
>                         Int_t numsels = 0;  //number of selected entries
>                         ...
>                         for (Int_t i=0; i<size; i++) {
>                            numsels = (arrMask[i] == 1) ? ++numsels :
>                     numsels;
>                         }//for_i
>
>
>                 This is confusing. I would write the loop body like this:
>
>                 numsels += (arrMask[i] == 1) ;
>
>
>                 or preferably using the STL:
>
>                 Int_t numsels = std::count( begin(arrMask),
>                 end(arrMask), 1 ) ;
>
>                 or some other variation of this, i.e. perhaps you don?t
>                 have a C++11 compiler, so perhaps one of these depending
>                 on what is arrMask:
>
>                 Int_t numsels = std::count( arrMask.begin(),
>                 arrMask.end(), 1 ) ;
>                 Int_t numsels = std::count( arrMask, arrMask + size, 1 ) ;
>
>                 Romain
>
>                     Even on the recently added release server 'zin2'
>                     Linux (Ubuntu 12.04.4 LTS) the above code compiles
>                     w/o warnings.
>
>                     However, on the new development server 'zin1' Linux
>                     (Ubuntu 12.04.4 LTS) I get suddenly the following
>                     warning message:
>
>                     Found the following significant warnings:
>                        XPSPreProcessing.cxx:3026:56: warning: operation
>                     on ?numsels? may be undefined [-Wsequence-point]
>
>                     Interestingly, both servers do not only run the same
>                     version of Ubuntu, but also the same version of the
>                     C++ compiler, i.e. g++ (Ubuntu/Linaro
>                     4.6.3-1ubuntu5) 4.6.3, and use the same flags, see:
>                     http://bioconductor.org/__checkResults/2.14/bioc-LATEST/__zin2-NodeInfo.html
>                     <http://bioconductor.org/checkResults/2.14/bioc-LATEST/zin2-NodeInfo.html>
>                     http://bioconductor.org/__checkResults/devel/bioc-__LATEST/zin1-NodeInfo.html
>                     <http://bioconductor.org/checkResults/devel/bioc-LATEST/zin1-NodeInfo.html>
>
>                     My question is now, why do I suddenly get the
>                     compiler warning?
>
>                     The reason why I ask at R-devel and not Bioc-devel
>                     is that it may not only be a Bioc question, since I
>                     found the following links:
>                     http://c-faq.com/expr/__seqpoints.html
>                     <http://c-faq.com/expr/seqpoints.html>
>                     http://stackoverflow.com/__questions/16838884/why-i-got-__operation-may-be-undefined-in-__statement-expression-in-c
>                     <http://stackoverflow.com/questions/16838884/why-i-got-operation-may-be-undefined-in-statement-expression-in-c>
>
>                     I am not sure if I understand the meaning, but until
>                     now I have never got any warning from any compiler
>                     the I have used (including MS Visual C++).
>
>                     Do I really have to replace '++numsels' with
>                     'numsels+1'?
>
>                     Best regards,
>                     Christian
>                     _._._._._._._._._._._._._._._.___._._
>                     C.h.r.i.s.t.i.a.n   S.t.r.a.t.o.w.a
>                     V.i.e.n.n.a           A.u.s.t.r.i.a
>                     e.m.a.i.l:        cstrato at aon.at <http://aon.at>
>                     _._._._._._._._._._._._._._._.___._._
>
>                     ________________________________________________
>                     R-devel at r-project.org <mailto:R-devel at r-project.org>
>                     mailing list
>                     https://stat.ethz.ch/mailman/__listinfo/r-devel
>                     <https://stat.ethz.ch/mailman/listinfo/r-devel>
>
>
>
>
>
>
>     ________________________________________________
>     R-devel at r-project.org <mailto:R-devel at r-project.org> mailing list
>     https://stat.ethz.ch/mailman/__listinfo/r-devel
>     <https://stat.ethz.ch/mailman/listinfo/r-devel>
>
>



[Rd] =?utf-8?q?operation_on_=E2=80=98numsels=E2=80=99_may_be_unde?= =?utf-8?q?fined?=
cstrato