From: Christoph Kukulies <kuku AT gilberto DOT physik DOT rwth-aachen DOT de>
Message-Id: <199703260838.JAA13050@gilberto.physik.rwth-aachen.de>
Subject: Re: Newbie troubles with Sin and Cos
In-Reply-To: <Pine.SUN.3.95.970325193134.25918B-100000@ernie.eecs.uic.edu> from Andrew Deren at "Mar 25, 97 07:43:23 pm"
To: aderen AT eecs DOT uic DOT edu
Date: Wed, 26 Mar 1997 09:38:24 +0100 (MET)
Cc: DBerry1 AT dca DOT gov DOT au, djgpp AT delorie DOT com
Reply-To: Christoph Kukulies <kuku AT gilberto DOT physik DOT rwth-aachen DOT de>
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> Your problem is that math sin and cosine function use radians not degrees.
> To convert from degrees to radians use 
> radians = (pi * degrees) / 180.0;
> where pi 3.14 or you can use math #define PI to get it more accurate
> 
> Than your main function would be:
> > main ()
> > {
> > while (theta < 360)
> > {
> > result = sin((PI * theta) / 180.0);
> > printf ("Sin of %i is %f\n",theta,result);
> > theta++;
> > }
> > return 0;
> > }
> The other way to do it would be to say
> while (theta < 2 * PI){

The original program contained some other errors,
like

1/  result should be defined double (rather than float) though
    from the nature of sine/cosine they never are absolute > 1.

2/  theta should have been defined double as well since the argument
    of sin is double as well as the return value is.

3/  All calculations (like 2 * PI) and comparisons (like theta < 360)
    should be done in doubles resp. the appropriate types or proper casts
    should be used.  C is not FORTRAN. 


> ..
> theta += 0.0174532925; // (2*PI)/360
> }
> I hope that helps
> 
> On 26 Mar 1997 DBerry1 AT dca DOT gov DOT au wrote:
> 
> > G'day
> > 
> > I've just started playing around with DJGPP and have come across the 
> > following problem.
> > 
> > I'm trying to write a routine that spins a pixel around in a circle.  That 
> > works ok so far, except that the X and Y points are never next to each 
> > other - I think I have narrowed it down to the results I am getting from 
> > using COS and SIN.
> > 
> > for instance here's the output from a simple prog I did to test my idea
> > 
> > Sin of 0 is 0.000000
> > Sin of 1 is 0.841471
> > Sin of 2 is 0.909297
> > Sin of 3 is 0.141120
> > Sin of 4 is -0.756802
> > 
> > But using a calculator I get
> > 
> > Sin of 0 is 0
> > Sin of 1 is 0.017452
> > Sin of 2 is 0.034899
> > Sin of 3 is 0.052335
> > Sin of 4 is 0.069756
> > 
> > Here's the simple prog...
> > 
> > #include <math.h>
> > #include <stdlib.h>
> > #include <stdio.h>
> > 
> > float result = 0.0;
> > int theta = 0;
> > 
> > main ()
> > {
> > while (theta < 360)
> > {
> > result = sin(theta);
> > printf ("Sin of %i is %f\n",theta,result);
> > theta++;
> > }
> > return 0;
> > }
> > 
> > Am I using the wrong variable types to hold the results ?  Do I need to do 
> > something special in DJGPP to get the magic numbers ?
> > 
> > Thanks
> > dberry AT dca DOT gov DOT au
> > 
> > 
> 
> 

--
Christoph P. U. Kukulies kuku AT gil DOT physik DOT rwth-aachen DOT de