operation on ‘numsels’ may be undefined

I don't see what's so surprising here.

That statement is identical to writing:

     if (arrMask[i] == 1) {
         numsels = ++numsels;
     } else {
         numsels = numsels;
     }

and

     numsels = ++numsels;

has two statements modifying the value of numsels (= and prefix-++) in
a single sequence point. (Do we increment then assign, or assign then
increment? The C / C++ standards leave this undefined.)

Imagine writing the operations out as functions: we have the `=`
function, and the `prefix-++` function -- both of these 'mo dify' (one
of) their arguments. Do we evaluate it as `=`(a, `prefix-++`(a)) or
`prefix-++`(`=`(a, a))?

The C standard leaves this undefined, so
compilers are free to do what they wish (and the nice ones warn you
when there is such an ambiguity). I guess the net result of the
operation is the same in each case _here_, but this is of course not
the case for other functions that modify the value of their
operand(s). And, in truth, this is _undefined behaviour_ and so the
compiler could still rightly make demons fly out of your nose if it
wanted to upon program execution.

I highly recommend reading the slides at
http://www.slideshare.net/olvemaudal/deep-c, especially the bit on
sequence points.

Cheers,
Kevin

On Mon, Jun 23, 2014 at 9:22 PM, Kasper Daniel Hansen
<kasperdanielhansen at gmail.com> wrote:

I am not an expert on this, but I note that the section on -Wsequence-point
at
   http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
specifically mentions ? and :.  Perhaps some more work on tracking down
their definitions and precedence might lead to insights.

Best,
Kasper


On Mon, Jun 23, 2014 at 6:42 PM, Herv? Pag?s <hpages at fhcrc.org> wrote:

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
_._._._._._._._._._._._._._._._._._

--
Herv? Pag?s

Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N, M1-B514
P.O. Box 19024
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