Author: Gerd Isenberg
Date: 12:41:02 12/05/02
Go up one level in this thread
On December 05, 2002 at 12:48:02, Dieter Buerssner wrote:
>On December 05, 2002 at 11:25:27, Gerd Isenberg wrote:
>
>>Most (All?) C-compilers have no problem with unary minus and constants:
>>
>>unsigned int A = -CONST;
>
>Assume 32 bit integers, and const is -2^31.
>I believe, the above is undefined behaviour. In this case a cast will make it
>defined behaviour:
>
> unsigned int A = -(unsigned)CONST.
>
Hi Dieter,
I guess it's null, because 2^31 is INT_MIN. I had small constants in mind.
So -1 is very common to to set all bits in an unsigned variable, don't care
about the word size.
unsigned int A = -1;
if ( popCount(A) == 64)
{
// ah, nice native 64 bit ints
}
Where i have some problems with sometimes, is the implicite type of a direct
constant expressions, calculated by the compiler, specially with propritary
64 bit integers types.
Eg.: MSC6.0
unsigned __int64 FFFF = -1; // that' fine, all 64 bits set
unsigned __int64 H8BB = 1<<63; // oups zero
produces an internal unsigned int overflow and assigns zero( without any
warning, even with warning level 4). Why isn't the compiler able to cast the 1
implicitly to the 64bit type?
unsigned __int64 H8BB = (unsigned __int64)1<<63; // that's ok
unsigned __int64 H8BB = (-1)<<63; // also wrong
>Totally unrelated to chess. If you want to write a function to convert int to
>ASCII (without using sprintf) the typical way may be:
>
> int toconvert;
> unsigned uval;
> if (toconvert < 0)
> uval = -toconvert; /* Oops, may not work for toconvert = INT_MIN */
> /* and go on to convert the unsigned value */
>
>>Because -CONST is a kind synonym for the compiler which means implicitly
>> (2**wordLengthInBits) - CONST ==> 0 - CONST
>
>All unsigned arithmetics is guaranteed to yield results mod 2 ^ bits.
except imul and idiv ;-)
>
>Cheers,
>Dieter
See you,
Gerd
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