Floating point

scott andrew franco samiam at moorecad.com
Tue Sep 1 05:26:43 CEST 2020


In any case, yes, I see the test does not work. Its 1735 (from the Pascal-P5 test catalog):
 
{
PRT test 1735: For trunc(x), the value of trunc(x) is such that if x is
 positive or zero then 0 < x-trunc(x) < 1; otherwise 1 <x-
 trunc(x) < 0. It is an error if such a value does not exist.
 ISO 7185 reference: 6.6.5.3
 }
 program iso7185prt1735;
 var a: integer;
 b: real;
 begin
 { assign maximum value of integer }
 b := maxint;
 { now move it completely out of range in floating point only }
 b := b+10.0;
 { now the assignment is invalid }
 a := trunc(b)
 end.
 I'm open to suggestions. ISO 7185 does not have a "maxreal" or similar.
 I guess b := maxint; b := b*2 would do it. I was trying for something more subtle
 (which clearly didn't work).
 Regards,
 Scott Franco
 
--------- Original Message --------- Subject: RE: Floating point
From: "scott andrew franco" <samiam at moorecad.com>
Date: 8/31/20 8:06 pm
To: "Waldek Hebisch" <hebisch at math.uni.wroc.pl>, "gpc at gnu.de" <gpc at gnu.de>

 Waldek,
 
Sure, 48 bits vs 64 bits. Why didn't it truncate the mantissa on conversion to float, ie, b := maxint?
I would have expected something like zeros on the right side.
 
Thanks,
 
Scott
 
--------- Original Message --------- Subject: Floating point
From: "Waldek Hebisch" <hebisch at math.uni.wroc.pl>
Date: 8/31/20 7:58 pm
To: "gpc at gnu.de" <gpc at gnu.de>

On Mon, Aug 31, 2020 at 01:07:42PM -0700, scott andrew franco wrote:
 > samiam at samiam-home-pc:~/projects/pascal/pascal-p5$ cat test.pas
 > program test(output);
 > var b,c: real;
 > begin
 > b := maxint;
 > c := 10.0;
 > b := b+c;
 > writeln(b:40:1);
 > writeln(maxint:38);
 > end.
 > samiam at samiam-home-pc:~/projects/pascal/pascal-p5$ gpc -o test test.pas
 > samiam at samiam-home-pc:~/projects/pascal/pascal-p5$ ./test
 > 9223372036854775808.0
 > 9223372036854775807
 
 Note that floating point results are approximate and on
 64-bit machines real accuracy is lower than integer
 accuracy.
 
 -- 
 Waldek Hebisch
 
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