RE: [Help-glpk] bigmul in glblib05.c

[Michael Hennebry]

On Wed, 6 Jan 2010, Cheng,Jen-Min wrote:

> In the code, I is declared as unsigned int.

In that case, the two statements are equivalent.
The second is still tidier.
The first might have a hapless reader scouring the New Testament,
trying and failing to find the reason for the extra complexity.
At least put in a comment.

> -----Original Message-----
> From: Andrew Makhorin [mailto:address@hidden
> Sent: Wednesday, January 06, 2010 9:47 AM
> To: Michael Hennebry
> Cc: Cheng,Jen-Min; address@hidden
> Subject: Re: [Help-glpk] bigmul in glblib05.c
>
> > > The statement:
> > >
> > >   unsigned short c = (unsigned short)( i & USHRT_MAX);
> > >
> > > is absolutely equivalent to:
> > >
> > >   unsigned short c = (unsigned short)i;
>
> > Actually it's not.
> > The result of the latter statement can be determined
> > from the value of i and the value of USHRT_MAX.
> > If i is negative, the result of the former also
> > depends on the representation of negative numbers.
> > I'd go with the latter.
> > Failing that, I'd put in a test for twos complement:
> > unsigned short c = (unsigned short)( (i & USHRT_MAX) +
> (1-1/(-1==1+~1)));
>
> In a general sense you are right. And the ISO C standard also gives
> a freedom to particular implementations (though in most cases it
> just say that the result is implementation-defined). However, to-day
> there are few programming models that are really used on practice;
> see http://www.unix.org/version2/whatsnew/lp64_wp.html . So we can

In this case, it doesn't really matter: all the types are unsigned.

> assume that byte/char has 8 bits, short has 16 bits, negative numbers
> are represented by two's complement, etc. Even floating-point numbers
> are now standardized due to IEEE, and I don't think that all these

All the world is a pentium.

> things will change in the future (at least for next 30 years) just
> because there is no reason to change them. Computers with 47-bit words
> have sunk into Lethe.

I've heard of 60-bit words, but not 47.

--
Michael   address@hidden
"Pessimist: The glass is half empty.
Optimist:   The glass is half full.
Engineer:   The glass is twice as big as it needs to be."