From hpages at fhcrc.org  Tue Jun 24 00:42:07 2014
From: hpages at fhcrc.org (=?windows-1252?Q?Herv=E9_Pag=E8s?=)
Date: Mon, 23 Jun 2014 15:42:07 -0700
Subject: [Rd]
 =?windows-1252?q?operation_on_=91numsels=92_may_be_undefined?=
In-Reply-To: <53A8A79A.9070301@fhcrc.org>
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>
	<53A8A79A.9070301@fhcrc.org>
Message-ID: <53A8AD3F.2030901@fhcrc.org>

On 06/23/2014 03:18 PM, Herv? Pag?s wrote:
> Hi Christian,
>
> On 06/23/2014 11:54 AM, cstrato wrote:
>> Dear Romain,
>>
>> I do not know enough about compilers, but as far as I remember, they
>> 'work' from right to left,
>
> Not necessarily. So you should not rely on that. This is what the
> (somewhat obscure) warning you see on zin1 is trying to tell you.

Actually, I don't see an ambiguity in your code:

   numsels = (arrMask[i] == 1) ? ++numsels : numsels;

Yes it's confusing and unnecessarily complicated but I don't see that
it relies on some undefined behavior. It's not like in the thread on
Bioc-devel where the expression was:

   *p++ = tolower(*p);

In that case the left-value of the assignment is itself an expression
that needs to be evaluated and the outcome of the assignment depends
on whether the left-value is evaluated before the right expression or
not. But in your case the left-value is a variable name so there is
nothing to evaluate.

So I don't know. Looks like a false positive from the gcc compiler to
me. Someone on the list might have a better insight.

Cheers,
H.


> Personally I would just do:
>
>      if (arrMask[i] == 1) numsels++;
>
> which is the standard way to implement the "if (some condition)
> then increment counter" idiom.
>
> As Kasper mentioned, a similar issue was recently discussed here:
>
>    https://stat.ethz.ch/pipermail/bioc-devel/2014-June/005858.html
>
> Cheers,
> H.
>
>
>> 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> 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> 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/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
>>>>>> _._._._._._._._._._._._._._._._._._
>>>>>> 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
>>>>>> _._._._._._._._._._._._._._._._._._
>>>>>>
>>>>>> ______________________________________________
>>>>>> R-devel at r-project.org mailing list
>>>>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>>>
>>>>>
>>>
>>>
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
Herv? Pag?s

Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
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P.O. Box 19024
Seattle, WA 98109-1024

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