Dear all,
I came across some behaviour of floats in a C-style loop which I don't
understand. In the following test function, I want to collect N floats
between 0.0 and 1.0, including these boundaries.
fun {SampleInts N}
Incr = 1.0 / {IntToFloat N-1}
in
for X in 0.0;X=<1.0;X+Incr
collect:C
do {C X}
end
end
{SampleInts 10}
% -> [0.0, 0.11111 ... 0.88889]
However, the last value (i.e., 1.0) is left out in the result. It appears that
this last value is actually already slightly greater then 1.0 (due to the
typical float problems I assume). So, I came up with the following hack,
where I test against a number which is slightly larger than 1.0.
fun {SampleInts2 N}
Incr = 1.0 / {IntToFloat N-1}
Max = 1.0+1.0E~13
in
for X in 0.0;X=<Max;X+Incr
collect:C
do {C X}
end
end
{SampleInts2 10}
% -> [0.0, 0.11111 ... 0.88889 1.0]
Now, the surprising aspect for me is that with such a hack, the last element
is indeed displayed as 1.0 and not anything slightly larger.
I should perhaps mention that I am porting some Common Lisp program to Oz,
where the equivalent of the first function works totally fine in Lisp. So, is
there some magic at the Lisp side going on (Lisp supports ratios, not only
floats), and the Oz behaviour is the usual thing to happen when you use
floats? Any idea, how to write a version of SampleInts which works for an
arbitrary high N?
Thank you very much!
Best
Torsten
--
Torsten Anders
http://strasheela.sourceforge.net/
http://torsten-anders.de/
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