From: Charles Krug <charles AT pentek DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: math.h sin() function returns wrong value
Date: Wed, 23 Jul 1997 08:50:54 -0400
Lines: 56
Message-ID: <33D5FE2E.2A08@pentek.com>
References: <01bc8d93$99a3f7a0$2a39868b AT dgmdavies>
NNTP-Posting-Host: mail.pentek.com
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk

Gareth Davies wrote:
> 
> I'm pretty new to C,

(snip)

> /* Test program for sin() function and angle conversion, by Gareth Davies
> 1997
>    It probably isn't great code, but I was trying to figure out the problem
> */
> #include <stdio.h>
> #include <stdlib.h>
> #include <math.h>
> 
> void main()
> {
>   char textangle;                       /* the text version of angle */
>   int angle;                    /* the angle to be passed to sin() */
>   float trigx, trigy, trigangle;        /* the results of the trig and pythagoras
>                            calculations */
> 
>   printf("sin(45) = %f\n", sin(45));
> 
>   printf("angle = ");
>   gets(textangle);                      /* get an angle out of 256, and store it in

Your difficulty lies in a misunderstanding of trig functions under C, or
BASIC or FORTRAN for tha t matter.  Trig functions only understand
radian measure (180degrees = PI).  So instead of:

		printf("%f\n", sin(45));

What you really need is:

	const PI=3.141592

	main()
	{
		float angle_deg, angle_rad;
		(etc)
			
		angle degrees = 45;
		angle radians = PI * (45/180);

		printf ("The sine of angle %f in degrees, %f in radians is %f\n",
angle_deg,
                            angle_rad, sin(angle_rad));

		(etc)
	}


Don't worry, you'll get it--everyone else here was new to C once too


Charles