On Sun, 11 Aug 2013 10:43:06 +0200 Tina Harriott wrote:
> On Wed, Jul 24, 2013 at 7:28 PM, Glenn Fowler <[email protected]> wrote:
> >
> > On Wed, 24 Jul 2013 19:02:39 +0200 Tina Harriott wrote:
> >> Here's one of my little tough problems which I am unable to solve
> >> myself, even after looking at the source code of ast-ksh. The problem
> >> below requires a good understanding how floating point numbers are
> >> implemented in computers.
> >
> >> I'm trying to prototype code and like to iterate over a small, linear
> >> area by using the C library function nextafter() to step forward the
> >> smallest possible distance between each step, and print the number of
> >> iterations needed to cover the distance between 4 and 4.000000000001:
> >> ksh -c 'float v ; integer iter ; for ((iter=0,v=4 ; v < 4.000000000001
> >> && iter < 10000000; v=nextafter(v,4.000000000001))) ; do
> >> ((iter++));done;print $iter '
> >> 2305843
> >
> >> The result 2305843 is correct.
> >
> >> However, if I use typeset -E (or just typeset) to declare the variable
> >> v the loop runs forever, or in this case until it hits iter < 10000000
> >> which I added as safeguard later:
> >> ksh -c 'typeset -E v ; integer iter ; for ((iter=0,v=4 ; v <
> >> 4.000000000001 && iter < 10000000; v=nextafter(v,4.000000000001))) ;
> >> do ((iter++));done;print $iter '
> >> 10000000
> >
> >> Can anyone explain this?
> >
> > float is an alias
> > this shows the alias definition
> > type float
> > which is
> > typeset -lE
> > this documents -l
> > typeset --?l
> >
> > so for your example
> > typeset -E
> > gave you a double v and
> > typeset -lE
> > would give you a "long double" v
> But why does nextafter() misbehave if I want to use a datatype smaller
> than "long double"? Accuracy is a good thing, but in this case we
> iterate too fine-grained, meaning the code should iterate over the
> smallest possible steps of a double, but not over the smallest
> possible steps of a long double.
here is a general question about C floating point that will determine
how to handle different floating point types in ksh arithmetic
given
float a;
long double b;
float c;
a = <some valid float number>;
b = a;
c = b;
is this always true
a == c
_______________________________________________
ast-users mailing list
[email protected]
http://lists.research.att.com/mailman/listinfo/ast-users
