CONTR47@NOSC-TECR.ARPA (07/04/88)
Norm Cohen writes:
Sam Harbaugh expects the addition in
my_dime : dollar_type :=
0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01;
-- 1 2 3 4 5 6 7 8 9 10
to be performed using universal_real addition rather than dollar_type
addition. Actually, it is performed using dollar_type addition: Each
real literal is implicitly converted to dollar_type and the ten resulting
dollar_type values are summed.
If Sam had written
my_dime : constant :=
0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01;
then exact universal_real addition would have been performed.
The Ada rule is that conversion takes place only at the bottom of the
expression tree: Only numeric literals, named numbers, and attributes
with universal results are convertible.
The rule is given in RM paragraph 4.6(15). Paragraph 4.6(20) provides
enlightening examples.
Norman Cohen
IBM Research
------------------------------
Wellll. I tried Norm's suggestion (assuming he meant
"constant dollar_type" instead of just "constant") so that
my_dime : constant dollar_type := 0.01 + 0.01 etc. for a total of ten 0.01s
Janus said my_dime is worth 0.08
Alsys said my_dime is worth 0.10
Dec said my_dime is worth 0.08
How about other people trying this in case I'm
missing something and also to see how other
compilers handle this.
regards,sam harbaugh
-------------------dik@cwi.nl (Dik T. Winter) (07/04/88)
In article <8807031845.AA01533@ajpo.sei.cmu.edu> CONTR47@NOSC-TECR.ARPA writes: > Norm Cohen writes: ... > The Ada rule is that conversion takes place only at the bottom of the > expression tree: Only numeric literals, named numbers, and attributes ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > with universal results are convertible. > ------------------------------ > Wellll. I tried Norm's suggestion (assuming he meant > "constant dollar_type" instead of just "constant") so that > my_dime : constant dollar_type := 0.01 + 0.01 etc. for a total of ten 0.01s ... > How about other people trying this in case I'm > missing something and also to see how other Yes, you are. Norm did *not* mean constant dollar_type, but just constant. You still are trying to get an implicit conversion from a universal real expression to something of type dollar_type. This is not possible, as noted by Norm. Oh, BTW, our system (DG) gives: My dime is .102 Your dime is .078 which is wrong on two counts. -- dik t. winter, cwi, amsterdam, nederland INTERNET : dik@cwi.nl BITNET/EARN: dik@mcvax
murphy@beatnix.UUCP (Michael Murphy) (07/06/88)
The more serious problem here which is what *really* causes the bad numbers is that most (all?) Ada compilers only support fixed point numbers that are a power of two. So despite the type declaration with a delta of 0.01, the actual delta used is 0.0078125 (2e-7). This makes decimal arithmetic using fixed point unworkable. -- Michael Murphy P.S. The VADS 5.5 compiler for the ELXSI prints out 0.10 for all my_dime calculations (constant or not) because they are all done with universal arithmetic, but your_dime is 0.08 for the above reason.