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Date: 15 Nov 91 18:59:59 GMT
From: milton!mfeldman@beaver.cs.washington.edu  (Michael Feldman)
Subject: Red-faced professor gets bitten in search for portability
Message-ID: <1991Nov15.185959.5002@milton.u.washington.edu>
List-Id: <comp.lang.ada>

Asked by students whether Ada has an equivalent of the Pascal "Trunc" 
operation, which just returns the integer part of its argument, 
I put together a function based on the old trick we used
in the Fortran days.

  FUNCTION Trunc (X: Float) RETURN Integer IS
  BEGIN
    RETURN Integer(X - 0.5);
  END Trunc;

Since conversion of Float to Integer is, in Ada, a rounding operation,
this looks like a good solution, right? WRONG! The trouble is that,
according to the LRM, the result of the conversion is implementation-
dependent if the fractional part of the float quantity lies just 
between the two integers (that is, = 0.5).  (LRM sect. 4.6)

To see the hidden nastiness here, suppose Y has an integral value.
If Y = 10.0 (say), then Trunc(Y) returns 10 on compilers where the 0.5 
case rounds _up_, and returns 9 on compilers where the 0.5 case rounds 
_down_. Oops! A simple bit of code falls right into a portability trap.

The only solution seems to lie in forcing an integer division, since integer
division indeed truncates, portably. So our new function is

  FUNCTION Trunc (X: Float) RETURN Integer IS
  BEGIN
    RETURN Integer(2.0 * X) / 2;
  END Trunc;

which, I suppose, can be optimized efficiently, but seems like a
kludgy way to do a simple truncation. I think this is an example of
an operation that's much more easily done as an intrinsic than as
a programmer-defined function.

Any thoughts in net-land?

By the way - I am all the more red-faced because I let the dangerous
Trunc monster loose into a textbook. 'Course, I haven't run into 
another Ada text that even discusses the question...

Always in search of the portable Ada program, I remain

Mike

-------------------------------------------------------------------------------
Michael B. Feldman
Visiting Professor 1991-92               Professor
Dept. of Comp. Sci. and Engrg.           Dept. of Elect. Engrg. and Comp. Sci.
University of Washington FR-35           The George Washington University
Seattle, WA 98105                        Washington, DC 20052

mfeldman@cs.washington.edu               mfeldman@seas.gwu.edu
(206) 632-3794 (voice)                   (202) 994-5253 (voice)
(206) 543-2969 (fax)                     (202) 994-5296 (fax)
-------------------------------------------------------------------------------