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

From: Michael Hennebry
Subject: RE: [Help-glpk] bigmul in glblib05.c
Date: Wed, 6 Jan 2010 14:31:50 -0600 (CST)
User-agent: Alpine 1.00 (DEB 882 2007-12-20)

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) +

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 . 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."

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