|Re: [eigen] Matrix2i mean|
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- To: eigen@xxxxxxxxxxxxxxxxxxx
- Subject: Re: [eigen] Matrix2i mean
- From: Petr Kubánek <pkubanek@xxxxxxxxx>
- Date: Tue, 05 Nov 2019 08:44:10 -0700
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I know what's the problem. I am just looking either to document it or
for a solution.
template <typename dt> dt sum()
break API/ABI? You will be able to call either sum() or sum<int32_t>(),
shouldn't you? I will try that and submit a patch.
Having sum<dt>() working, one can hopefully create mean<dt_sum,
dt_mean>(), so one can code:
On Tue, 2019-11-05 at 16:36 +0100, Christoph Hertzberg wrote:
> On 05/11/2019 16.11, Peter wrote:
> > [...]
> > actually, this would also be interesting for the scalar products
> > in
> > general, namely a different
> > type for accumulating the sums within a scalar product, e.g. as
> > yet
> > another template parameter for the matrices.
> Adding another template parameter to basic types is not an option.
> would break ABI and API compatibility (even if the parameter has a
> You could create your own custom type `my_int16` for which `my_int16
> my_int16` results in a `my_int32` (this needs to be told to Eigen,
> similar to how real*complex products are handled).
> > > [...]
> > I think it's more subtle than that.
> > Even
> > int16_t Two = 2;
> > int16_t Max = INT16_MAX;
> > int16_t mean = ( Max/2 + Max + Two + Two ) / int16_t(4);
> > doesn't produce an overflow.
> Yes, because `Max/2` gets implicitly converted to `int`. Actually,
> adding two `int16_t` get implicitly converted to `int` (search for
> integer promotion rules -- I don't know them entirely either). And
> dividing an `int` by an `int16_t` results in an `int` which is only
> afterwards converted to `int16_t`.
> In contrast, Eigen::DenseBase::sum() does something (more or less)
> equivalent to:
> T sum = T(0);
> for(Index i=0; i< size(); ++i)
> return sum;
> i.e., after each addition the result gets reduced to the scalar type
> the matrix, thus it will immediately overflow.
> And mean() essentially just takes the return value of `sum()` and
> divides it by the size.