[Dune] Fwd: Re: another little c++ question that bugs me

[Aleksejs Fomins]

Oh dear, thanks Markus.

I have a C++ book, this was just a huge blunder on my side.

Of course I always try to initialize variables at the start.

I usually pass vectors by reference, but this was a test program with
vectors of length 100 at most, so I got a little sloppy, sorry.

I was not aware of BlockVector, but I will make use of this not to copy
code.

Thank you,
Aleksejs


On 08/13/2014 11:17 AM, Markus Blatt wrote:
> Hi,
> 
> On Wed, Aug 13, 2014 at 11:02:48AM +0200, Aleksejs Fomins wrote:
>> I have a little c++ question that bugs me.
>>
> 
> You really should grep yourself a C++ book and study that.
>  
>> [...]
>>
>> rez += norm2sq<dim>(xVec1[i], xVec2[i]);
>> norm2sq<dim>(xVec1[i], xVec2[i]);
>>
>> by simply calling the function norm2sq twice for no reason, the function
>> norm2sqVector returns the correct result.
>>
>> I have no idea why is this happening, but it must be sth trivial that I
>> can't see.
>>
>> Could someone please take a look?
>>
>> Thanks,
>> Aleksejs
>>
>>
>>
>> --------------------------------------------------------------
>>
>>
>>
>> // Finds the 2-norm distance between two coordinates
>> template<int dim>
>> double norm2sq(
>>   FieldVector< double, dim > x1,
>>   FieldVector< double, dim > x2
> 
> I would use constant references for the function arguments, here. Like
> this the vectors are copied.
>> ) {
>> 	double rez;
>> 	for (int i = 0; i < x1.size(); i++)
>> 	{
>> 		rez += pow(x1[i] - x2[i],2);
>> 	}
>> 	return rez;
>> }
> 
> 
> pods (Plain old datatypes like int, float, double, std::array, etc.)
> are never default initialized. That means that
> 
> double rez;
> 
> leaves rez undefined. It basically contains what was written to the
> memory that rez owns before. That might be any value.
> 
> 
> Are you aware that what you are trying to do is already present in
> dune-istl, if you use a BlockVector instead of std::vector?
> 
> typedef Dune::BlockVector<Dune::FieldVector<double, dim> > V;
> V tmp=xVec1;
> tmp-=xVec2;
> double rez=tmp.two_norm();
> 
> 
> 
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