# math.h trig functions questions (and some forgotten high school math)

Discussion in 'C Programming' started by Mark Healey, May 21, 2006.

1. ### Mark HealeyGuest

Do the trig functions in math.h work in degrees, radians or what? For
some reason it doesn't say which in "man math.h"

IIRC the arctan of a slope gives the angle. So, shouldn't atanf((float)1)
number?

--
Mark Healey
marknews(at)healeyonline(dot)com

Mark Healey, May 21, 2006

2. ### Chris McDonaldGuest

Mark Healey <> writes:

>Do the trig functions in math.h work in degrees, radians or what? For
>some reason it doesn't say which in "man math.h"

>IIRC the arctan of a slope gives the angle. So, shouldn't atanf((float)1)
>number?

If on a Unix/Linux system - man 3 sin

--
Chris.

Chris McDonald, May 21, 2006

3. ### Jack KleinGuest

On Sun, 21 May 2006 03:43:36 GMT, Mark Healey <> wrote
in comp.lang.c:

> Do the trig functions in math.h work in degrees, radians or what? For
> some reason it doesn't say which in "man math.h"
>
> IIRC the arctan of a slope gives the angle. So, shouldn't atanf((float)1)
> return 45 or .7something (degrees or radians) instead of some negative
> number?

Doesn't your C reference book tell you? All C trigonometric functions
work in radians. If you want to work in degrees, you need to provide
functions or macros to do the conversions back and forth.

--
Jack Klein
Home: http://JK-Technology.Com
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Jack Klein, May 21, 2006
4. ### Keith ThompsonGuest

Re: math.h trig functions questions (and some forgotten high schoolmath)

Mark Healey <> writes:
> Do the trig functions in math.h work in degrees, radians or what? For
> some reason it doesn't say which in "man math.h"
>
> IIRC the arctan of a slope gives the angle. So, shouldn't atanf((float)1)
> return 45 or .7something (degrees or radians) instead of some negative
> number?

Some negative number?

This program:

#include <stdio.h>
#include <math.h>
int main(void)
{
printf("atanf((float)1.0) = %f\n", atanf((float)1.0));
return 0;
}

gives me:

atanf((float)1.0) = 0.785398

Did you remember the "#include <math.h>"? Did you make sure to link
in the math library (on a Unix-like system, you might need a "-lm"
option).

--
Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Keith Thompson, May 21, 2006
5. ### Joe SmithGuest

"Keith Thompson" <> wrote in message
news:...
> Mark Healey <> writes:
>> Do the trig functions in math.h work in degrees, radians or what? For
>> some reason it doesn't say which in "man math.h"
>>
>> IIRC the arctan of a slope gives the angle. So, shouldn't
>> atanf((float)1)
>> return 45 or .7something (degrees or radians) instead of some negative
>> number?

>
> Some negative number?
>
> This program:
>
> #include <stdio.h>
> #include <math.h>
> int main(void)
> {
> printf("atanf((float)1.0) = %f\n", atanf((float)1.0));
> return 0;
> }
>
> gives me:
>
> atanf((float)1.0) = 0.785398
>
> Did you remember the "#include <math.h>"? Did you make sure to link
> in the math library (on a Unix-like system, you might need a "-lm"
> option).

And would atanf be in there, in particular one that accepts a float as
opposed to double? joe

Joe Smith, May 22, 2006
6. ### Keith ThompsonGuest

Re: math.h trig functions questions (and some forgotten high schoolmath)

"Joe Smith" <> writes:
> "Keith Thompson" <> wrote in message
> news:...
>> Mark Healey <> writes:
>>> Do the trig functions in math.h work in degrees, radians or what? For
>>> some reason it doesn't say which in "man math.h"
>>>
>>> IIRC the arctan of a slope gives the angle. So, shouldn't
>>> atanf((float)1)
>>> return 45 or .7something (degrees or radians) instead of some negative
>>> number?

>>
>> Some negative number?
>>
>> This program:
>>
>> #include <stdio.h>
>> #include <math.h>
>> int main(void)
>> {
>> printf("atanf((float)1.0) = %f\n", atanf((float)1.0));
>> return 0;
>> }
>>
>> gives me:
>>
>> atanf((float)1.0) = 0.785398
>>
>> Did you remember the "#include <math.h>"? Did you make sure to link
>> in the math library (on a Unix-like system, you might need a "-lm"
>> option).

>
> And would atanf be in there, in particular one that accepts a float as
> opposed to double? joe

Possibly not, since atanf was added in C99 -- but if atanf weren't in
the library, that wouldn't explain the OP's result of "some negative
number".

--
Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Keith Thompson, May 22, 2006
7. ### Joe SmithGuest

"Keith Thompson" <> wrote in message
news:...
> "Joe Smith" <> writes:
>> "Keith Thompson" <> wrote in message
>> news:...
>>> Mark Healey <> writes:
>>>> Do the trig functions in math.h work in degrees, radians or what? For
>>>> some reason it doesn't say which in "man math.h"
>>>>
>>>> IIRC the arctan of a slope gives the angle. So, shouldn't
>>>> atanf((float)1)
>>>> return 45 or .7something (degrees or radians) instead of some negative
>>>> number?
>>>
>>> Some negative number?
>>>
>>> This program:
>>>
>>> #include <stdio.h>
>>> #include <math.h>
>>> int main(void)
>>> {
>>> printf("atanf((float)1.0) = %f\n", atanf((float)1.0));
>>> return 0;
>>> }
>>>
>>> gives me:
>>>
>>> atanf((float)1.0) = 0.785398
>>>
>>> Did you remember the "#include <math.h>"? Did you make sure to link
>>> in the math library (on a Unix-like system, you might need a "-lm"
>>> option).

>>
>> And would atanf be in there, in particular one that accepts a float as
>> opposed to double? joe

>
> Possibly not, since atanf was added in C99 -- but if atanf weren't in
> the library, that wouldn't explain the OP's result of "some negative
> number".
>

I'm surprised that my implementation, circa ten years old, has it. joe

#ifdef _M_MRX000

/* MIPS fast prototypes for float */
/* ANSI C, 4.5 Mathematics */

/* 4.5.2 Trigonometric functions */

_CRTIMP float __cdecl acosf( float );
_CRTIMP float __cdecl asinf( float );
_CRTIMP float __cdecl atanf( float );
_CRTIMP float __cdecl atan2f( float , float );
_CRTIMP float __cdecl cosf( float );
_CRTIMP float __cdecl sinf( float );
_CRTIMP float __cdecl tanf( float );

Joe Smith, May 22, 2006
8. ### Tim PrinceGuest

Re: math.h trig functions questions (and some forgotten high schoolmath)

Joe Smith wrote:
> "Keith Thompson" <> wrote in message
> news:...
>> "Joe Smith" <> writes:
>>> "Keith Thompson" <> wrote in message
>>> news:...
>>>> Mark Healey <> writes:
>>>>> Do the trig functions in math.h work in degrees, radians or what? For
>>>>> some reason it doesn't say which in "man math.h"
>>>>>
>>>>> IIRC the arctan of a slope gives the angle. So, shouldn't
>>>>> atanf((float)1)
>>>>> return 45 or .7something (degrees or radians) instead of some negative
>>>>> number?
>>>> Some negative number?
>>>>
>>>> This program:
>>>>
>>>> #include <stdio.h>
>>>> #include <math.h>
>>>> int main(void)
>>>> {
>>>> printf("atanf((float)1.0) = %f\n", atanf((float)1.0));
>>>> return 0;
>>>> }
>>>>
>>>> gives me:
>>>>
>>>> atanf((float)1.0) = 0.785398
>>>>
>>>> Did you remember the "#include <math.h>"? Did you make sure to link
>>>> in the math library (on a Unix-like system, you might need a "-lm"
>>>> option).
>>> And would atanf be in there, in particular one that accepts a float as
>>> opposed to double? joe

>> Possibly not, since atanf was added in C99 -- but if atanf weren't in
>> the library, that wouldn't explain the OP's result of "some negative
>> number".
>>

> I'm surprised that my implementation, circa ten years old, has it. joe
>
> #ifdef _M_MRX000
>
> /* MIPS fast prototypes for float */
> /* ANSI C, 4.5 Mathematics */
>
> /* 4.5.2 Trigonometric functions */
>
> _CRTIMP float __cdecl acosf( float );
> _CRTIMP float __cdecl asinf( float );
> _CRTIMP float __cdecl atanf( float );
> _CRTIMP float __cdecl atan2f( float , float );
> _CRTIMP float __cdecl cosf( float );
> _CRTIMP float __cdecl sinf( float );
> _CRTIMP float __cdecl tanf( float );
>
>

The math functions for float data type were reserved and made optional
in C89, and mandatory in C99. This seems to have produced some
confusion as to whether restricting a compiler to C89 mode, (gcc
default), may suppress the float functions, or break them, as in some
MSVC versions. Clearly, the provision for them in C89 was produced by
strong demand and intention of certain vendors to provide them.

Tim Prince, May 22, 2006