In C, tan(30) gives me a negative value! Why?

Question

I observe that my tan(float) function from the cmath library is returning a negative value.

The following piece of code, when run :

    #include <cmath>
    ....

    // some calculation here gives me a value between 0.0 to 1.0.
    float tempSpeed = 0.5; 

    float tanValue = tan(tempSpeed * 60);

    __android_log_print(ANDROID_LOG_INFO, "Log Me", "speed: %f", tanValue);

Gives me this result in my Log file:

    Log Me: speed `-6.4053311966`

As far as I remember

    tan(0.5*60) = tan(30) = 1/squareroot(3);

Can someone help me here as in why I am seeing a negative value? Is it related to some floating point size error? Or am I doing something really dumb?

Awesome, such quick response. Now I can go home and get some sleep. :-) Thanks guys!! — Hari Krishna Ganji, Jun 25 '12 at 18:53
Yeah, I Should have.. and I agree that's dumb. But, sometimes when you are deep and lost.. [like figuring out the right ways of mapping swipe length and swipe speed to the ball's trajectory], the simplest things don't occur to your head. :-) — Hari Krishna Ganji, Jul 03 '12 at 04:41

score 46 · Accepted Answer · answered Jun 25 '12 at 18:25

46

In C, tan and other trigonometric functions expect radians as their arguments, not degrees. You can convert degrees to radians:

tan( 30. * M_PI / 180. ) == 0.57735026918962576450914878050196

answered Jun 25 '12 at 18:25

K-ballo

80,396
20
159
169

Multiply the argument by `pi / 180` to convert from degrees to radians – Useless Jun 25 '12 at 18:26
1

Just so you know, `M_PI` isn't part of the standard: http://stackoverflow.com/questions/1727881/how-to-use-the-pi-constant-in-c – Electro Oct 04 '12 at 17:27

score 7 · Answer 2 · answered Jun 25 '12 at 18:25

7

That's the tangent of your angle (30 radians.) if you are looking for the tangent of 30 degrees, you must convert your angle to radians first.

answered Jun 25 '12 at 18:25

Jonathan Grynspan

43,286
8
74
104

rav3451 · Answer 3 · 2012-06-27T13:26:02.343

1

I guess in C the tan Function requires you to Input Radians as an argument and not The actual degree value.

so for Tan 30 , you would have to convert your 30 degrees to radian. Keep in Mind that 360 degrees is 2*Pi radian so 30 degress would be (1\6 * Pi)th of a radian.

so tan(1\6 * Pi) would give you the correct answer. where Pi is 3.142

edited Jun 27 '12 at 13:26

answered Jun 25 '12 at 18:47

rav3451

41
6

@HighPerformanceMark .. Yup.. you are right, I seemed to have left out the 2 there.. I have corrected it now... thanks – rav3451 Jun 27 '12 at 13:26

In C, tan(30) gives me a negative value! Why?

3 Answers3