[Rd] operation on ‘numsels’ may be undefined

[Romain François]

Le 23 juin 2014 à 15:20, cstrato <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/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://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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