I'm trying some calculations in double precision, and I'd like the
output rounded off so that errors in calculation are not shown. For
instance, the result of COS(90 * 3.141592653589793238d0 / 180.) is
-4.37113886E-06. I'd like it to be zero. Thanks for any help.

On Mar 29, 11:43 am, Bamm gmail.com> wrote:

For that matter, I also wonder why the result of Cosine is real and
not double precision. Thanks for any clarification.

Bamm gmail.com> wrote:

Well...

1. You can't get "output rounded so that errors in calculation are not
shown." That is equivalent to asking for there to be no errors in
calculations. Ain't gonna happen. If you can establish what the order of
magnitude of the errors will be, then you can round the output
appropriately if you use an F edit descriptor. You aren't ever going to
get exactly zero out of an E edit descriptor (or list-directed output)
unless the number is indeed exactly zero.

In article
<3277fc93-726f-428f-82cd-aae971d0711f@d4g2000prg.googlegroups.com>,
Bamm gmail.com> wrote:

The closest number is "never" going to be zero because there are
simply too many nonzero numbers near zero compared to floating point
numbers near the exact value of pi/2. However, I get
6.12323399573676604E-017 for the result of your expression, which is
a lot closer to zero than your value.

$.02 -Ron Shepard

Here's the answer I get for the same program above using g77 on Linux:

6.12303177E-17

Am I doing anything wrong?

In article e6g2000prf.googlegroups.com>,
Bamm gmail.com> wrote:

Yes, you failed to show us the source code of the original program
which showed a much larger value.

-- greg

On Mar 29, 12:48 am, Bamm gmail.com> wrote:

Bamm wrote:

You may want to investigate using fixed-point arithmetic?

http://en.wikipedia.org/wiki/Fixed-point_arithmetic

Don't know; here's what I'm seeing:

program precision_trial
integer, parameter :: dp = selected_real_kind(15,307)
integer, parameter :: sp = selected_real_kind(6,37)
write(*,*) cos( 90 * 3.141592653589793238_dp / 180. )
write(*,*) cos( 90 * 3.141592653589793238_sp / 180. )
end program precision_trial

Bamm wrote:

Probably not, different compilers print out a different number
of digits for list directed output. The standard doesn't
specify how many digits to print out for an * format. If ...

No I am not. I understand how (binary) floating points are used to
represent actual (decimal) numbers. If I thought they were exact, I
wouldn't be asking how to round it off properly so that the errors are
not shown.

However I have since found where I went wrong. I was using

double precision Speed, Azimuth, V
Azimuth = 90.
parameter(Pi = 3.141592653589793238d0)
V = Speed * cos(Azimuth * Pi / 180.)
print *, V