Experimental Physics and
Industrial Control System

Eric Norum <[email protected]> · Fri, 17 Jun 2005 16:32:14 -0500

On Jun 17, 2005, at 4:21 PM, Benjamin Franksen wrote:

Regarding Jeff's argument as for the advantages of programming with
numbers that are garanteed to be non-negative. I would argue that this
is also mostly valid in C/C++, where arrays are always indexed starting
with zero. Many languages allow upper /and/ lower index bounds to be
arbitrary (signed) integers, or even any other data type, provided the
programmer can specify a one-to-one mapping onto a bounded interval of
integers. Thus, non-negativity seems to be a somewhat arbitrary
guarantee (why not, for instance, strict positivity?). Furthermore,
with regard to the efficiency question (only one range-check for upper
bound instead of two for upper and lower), in  C/C++ you are always
free to apply the zero-cost type cast from signed to unsigned, thereby
mapping negative numbers to large positive ones, and then range-test
only for the upper bound. This will fail for /exactly/ the cases where
the original check failed, as long as you don't rely on the upper half
of the possible range, something you (Jeff) suggested is to be avoided
anyway.

The inefficiency is only in typing a few extra characters on your

keyboard. Any modern optimizing compiler is going to optimize

	if ((i < 0) || (i >= j))

(where i and j are signed integer values) into a single test without

you needing to help it by performing the cast.

--
Eric Norum <[email protected]>
Advanced Photon Source
Argonne National Laboratory
(630) 252-4793

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System