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From: "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu>
Newsgroups: comp.lang.fortran
Subject: Re: Angle Units For Trig Functions
Date: Thu, 24 Oct 2024 02:17:15 -0000 (UTC)
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On Wed, 23 Oct 2024 23:13:38 +0000, Lawrence D'Oliveiro wrote:

> On Wed, 23 Oct 2024 16:38:41 -0600, Louis Krupp wrote:
> 
>> Seriously, though, if you're interfacing with people, degrees are easier
>> and more familiar than radians.
> 
> But trig calculations are easier in radians. And it is easy to convert 
> back and forth, as I explained in the posting that started this thread.

Easier?

program foo
   real*8 x, y
   real*8, parameter :: deg2rad = 4 * atan(1.d0) / 180
   x = 137*180
   y = x * deg2rad
   print *, sind(x), sin(y)
end program foo

% gfcx -o z -Wall a.f90 && ./z
  -0.0000000000000000       -9.8590724568376608E-016

One of these values is wrong.

For x = (n + 1) * 180 and n < 2**23, sind(x) = +-0, exactly.
For y = x * pi / 180, sin(x) never equals +-0.

You seem to be missing that argument reduction for sind(x)
is much easier than argument reduction for sin(x).  

-- 
steve