Re: Arithmetic with durations

[stt@houdini.camb.inmet.com (Tucker Taft)]

Jerry van Dijk (jerry@jvdsys.nextjk.stuyts.nl) wrote:

: In article <34149060.2E21@home.com> lmcoon@home.com writes:

: >The line that calculates the amount of simulation
: >time to wait for each event won't compile.  My
: >compiler (Thomson ObjectAda running in Windows 95)
: >produces the following diagnostic: "LRM:4.5.5(20)
: >The fixed-fixed multiplying operators shall not be
: >used in a context where the expected type for the
: >result is universal_fixed."

: ...
:      Sim_Delay_Time := Real_Delay_Time * Total_Sim_Time / Total_Real_Time;

: If you looked up 4.5.5 you will have found that the rule used for
: the multiplication is given in 4.5.5 (19):

:    function "*" (Left, Right : universal_fixed) return universal_fixed.

: However, line (20) then makes the exception that this will only work
: if the destination type is something other then universal_fixed, either
: im- or explicit.

: However, as the result of the multiplication will be used for a
: division also defined in 4.5.5 (19) the resulting type _is_
: implicitly an universal_fixed, so the multiplication is illegal.

: To fix the problem, simply make the return type explicit, like:
: ...
:      Sim_Delay_Time := Duration (Real_Delay_Time * Total_Sim_Time)
:        / Total_Real_Time;

: Now, perhaps one of our resident RM guru's can answer the really
: interesting question: why this exception ?

The fixed-fixed multiplying operators are defined to return
infinite precision.  Of course, that is a bit inefficient to do
at run-time.  Hence, you are required to coerce the result to
some particular finitely precise numeric type.  Hence the
rule that you can't use the output of a fixed-fixed multiplying
operator as an input to another one, without first coercing it
to a particular, non-infinite-precision, numeric type.

: --

: -- Jerry van Dijk | Leiden, Holland
: -- Consultant     | Team Ada
: -- Ordina Finance | jdijk@acm.org

--
-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Burlington, MA  USA