[Rd] operation on ‘numsels’ may be undefined
cstrato
cstrato at aon.at
Mon Jun 23 23:54:03 CEST 2014
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 François wrote:
>
>
> Le 23 juin 2014 à 18:28, cstrato <cstrato at aon.at
> <mailto:cstrato at aon.at>> a écrit :
>
> 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 François wrote:
>
>
> Le 23 juin 2014 à 15:20, cstrato <cstrato at aon.at
> <mailto:cstrato at aon.at>> a écrit :
>
> 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
> _._._._._._._._._._._._._._._.___._._
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