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 Fred Zwarts 10-08-2009 09:41 AM

Minimum value of floating point types.

In a template function I need the minimum and maximum value of the valid range of values of a type.
For integer types I can use std::numeric_limits<Type>::min () and std::numeric_limits<Type>::max () for this purpose.
But this template function is also used for floating point types.
For floating point types std::numeric_limits<Type>::min () does not return what it suggests, the minimum value,
but it returns the smallest value larger than 0. (I wonder why this confusion was introduced.)
My question is how to find the real minimum value in such cases.
Is -std::numeric_limits<Type>::max () a good choice?
I know that for signed integer types the assumption that
-std::numeric_limits<Type>::max () is the most negative valid value for that type
is often wrong. How is that for floating point types?

Would the following code in a template function result in the best value?

Type RealMinimun = std::numeric_limits<Type>::min () > 0 ? // Test for floating point type
-std::numeric_limits<Type>::max () // Floating point minimum
: std::numeric_limits<Type>::min (); // Integer minimum

This results in a compiler warning when the template is instantiated for unsigned integer types,
which should not be negated.
I can ignore this warning, because the negation is not used in this case.

 Victor Bazarov 10-08-2009 11:56 AM

Re: Minimum value of floating point types.

Fred Zwarts wrote:
> In a template function I need the minimum and maximum value of the valid range of values of a type.
> For integer types I can use std::numeric_limits<Type>::min () and std::numeric_limits<Type>::max () for this purpose.
> But this template function is also used for floating point types.
> For floating point types std::numeric_limits<Type>::min () does not return what it suggests, the minimum value,
> but it returns the smallest value larger than 0. (I wonder why this confusion was introduced.)
> My question is how to find the real minimum value in such cases.
> Is -std::numeric_limits<Type>::max () a good choice?

Yes. The difference between FP reps and integral reps is that the FP
ones are symmetrical AFA the sign goes. Integrals aren't necessarily
such. For example, in 2's complement INT_MIN != -INT_MAX.

> I know that for signed integer types the assumption that
> -std::numeric_limits<Type>::max () is the most negative valid value for that type
> is often wrong. How is that for floating point types?

It is that for floating point types.

> Would the following code in a template function result in the best value?
>
> Type RealMinimun = std::numeric_limits<Type>::min () > 0 ? // Test for floating point type
> -std::numeric_limits<Type>::max () // Floating point minimum
> : std::numeric_limits<Type>::min (); // Integer minimum
>
> This results in a compiler warning when the template is instantiated for unsigned integer types,
> which should not be negated.
> I can ignore this warning, because the negation is not used in this case.

Why don't you write a function per type?

template<class T> T getRealMinimum();

template<> double getRealMinimum<double>() {
return -std::numeric_limits<double>::max();
}

template<> int getRealMinimum<int>() {
return std::numeric_limits<int>::min();
}

... and so on

Then you can simply pass the 'Type' onto that:

Type RealMinimum = getRealMinimum<Type>();

V
--

 Fred Zwarts 10-08-2009 12:47 PM

Re: Minimum value of floating point types.

Victor Bazarov wrote:
> Fred Zwarts wrote:
>> In a template function I need the minimum and maximum value of the
>> valid range of values of a type.
>> For integer types I can use std::numeric_limits<Type>::min () and
>> std::numeric_limits<Type>::max () for this purpose. But this
>> template function is also used for floating point types.
>> For floating point types std::numeric_limits<Type>::min () does not
>> return what it suggests, the minimum value,
>> but it returns the smallest value larger than 0. (I wonder why this
>> confusion was introduced.)
>> My question is how to find the real minimum value in such cases.
>> Is -std::numeric_limits<Type>::max () a good choice?

>
> Yes. The difference between FP reps and integral reps is that the FP
> ones are symmetrical AFA the sign goes. Integrals aren't necessarily
> such. For example, in 2's complement INT_MIN != -INT_MAX.
>
>> I know that for signed integer types the assumption that
>> -std::numeric_limits<Type>::max () is the most negative valid value
>> for that type
>> is often wrong. How is that for floating point types?

>
> It is that for floating point types.
>
>> Would the following code in a template function result in the best
>> value?
>>
>> Type RealMinimun = std::numeric_limits<Type>::min () > 0 ? //
>> Test for floating point type -std::numeric_limits<Type>::max
>> () // Floating point minimum :
>> std::numeric_limits<Type>::min (); // Integer minimum
>>
>> This results in a compiler warning when the template is instantiated
>> for unsigned integer types,
>> which should not be negated.
>> I can ignore this warning, because the negation is not used in this
>> case.

>
> Why don't you write a function per type?

The first reason is that I don't like to copy almost identical code repeatedly.

The second reason is that I write this code for several platforms,
which do not all have the same set of types.
A generic approach solves the problem with less code and with a
less complicated coverage of the different platforms.

I have thought about specializations for the floating point types to
remove the compiler warnings, but even the set of floating point
types is different on these platforms.

 Fred Zwarts 10-08-2009 12:50 PM

Re: Minimum value of floating point types.

Victor Bazarov wrote:
> Fred Zwarts wrote:
>> In a template function I need the minimum and maximum value of the
>> valid range of values of a type.
>> For integer types I can use std::numeric_limits<Type>::min () and
>> std::numeric_limits<Type>::max () for this purpose. But this
>> template function is also used for floating point types.
>> For floating point types std::numeric_limits<Type>::min () does not
>> return what it suggests, the minimum value,
>> but it returns the smallest value larger than 0. (I wonder why this
>> confusion was introduced.)
>> My question is how to find the real minimum value in such cases.
>> Is -std::numeric_limits<Type>::max () a good choice?

>
> Yes. The difference between FP reps and integral reps is that the FP
> ones are symmetrical AFA the sign goes.

Do you know where in the C++ standard this is specified?

 Victor Bazarov 10-08-2009 01:12 PM

Re: Minimum value of floating point types.

Fred Zwarts wrote:
> Victor Bazarov wrote:
>> Fred Zwarts wrote:
>>> In a template function I need the minimum and maximum value of the
>>> valid range of values of a type.
>>> For integer types I can use std::numeric_limits<Type>::min () and
>>> std::numeric_limits<Type>::max () for this purpose. But this
>>> template function is also used for floating point types.
>>> For floating point types std::numeric_limits<Type>::min () does not
>>> return what it suggests, the minimum value,
>>> but it returns the smallest value larger than 0. (I wonder why this
>>> confusion was introduced.)
>>> My question is how to find the real minimum value in such cases.
>>> Is -std::numeric_limits<Type>::max () a good choice?

>> Yes. The difference between FP reps and integral reps is that the FP
>> ones are symmetrical AFA the sign goes.

>
> Do you know where in the C++ standard this is specified?

No, and I'm too lazy to look it up for you.

V
--

 puzzlecracker 10-08-2009 03:46 PM

Re: Minimum value of floating point types.

On Oct 8, 9:25*am, Pete Becker <p...@versatilecoding.com> wrote:
> Fred Zwarts wrote:
> > Victor Bazarov wrote:
> >> Fred Zwarts wrote:
> >>> In a template function I need the minimum and maximum value of the
> >>> valid range of values of a type.
> >>> For integer types I can use std::numeric_limits<Type>::min () and
> >>> std::numeric_limits<Type>::max () for this purpose. But this
> >>> template function is also used for floating point types.
> >>> For floating point types std::numeric_limits<Type>::min () does not
> >>> return what it suggests, the minimum value,
> >>> but it returns the smallest value larger than 0. (I wonder why this
> >>> confusion was introduced.)
> >>> My question is how to find the real minimum value in such cases.
> >>> Is -std::numeric_limits<Type>::max () a good choice?
> >> Yes. *The difference between FP reps and integral reps is that the FP
> >> ones are symmetrical AFA the sign goes. *

>
> > Do you know where in the C++ standard this is specified?

>
> It's not. But it's true, nonetheless. <g>
>
> --
> * *Pete
> Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of
> "The Standard C++ Library Extensions: a Tutorial and Reference"
> (www.petebecker.com/tr1book)

Agh, sorry it is off-topic
Pate and Victor -- it has been a long time -- why don't you guys join
stackoverfow group: http://stackoverflow.com/questions/tagged/c%2b%2b?
just curious. I am there and I love it!

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