atan problems in c++

Question

before I get into my problem, I want to point out a couple of things: 1)I know there is already an atan2 function in the cmath library, this is purely as an exercise and for my own practice, and 2) I know that the code does not account for 0.

ok, so tan(theta) = y / x, where y and x are coordinates on a plane... that means:

theta = atan(y/x) in Quads I and IV, and theta = atan(y/x) + 180 in Quads II and III

so why when I use the following code:

float atan(float y, float x)
 {
 float result = 0.0f;

if (x > 0) //quads I and IV if x is positive
 { 
result = atanf(y/x);
 }
 else if (x < 0)
 {
 result = atan(y/x) + 180; //quads II and III if x is negative
 }

return result;

 }

does it spit me junk? example, for coordinates (-4,4) it gave me the result of: 179.215, when it should be 135:

atan(4/-4) = -45 degrees + 180 degrees = 135 degrees

but what's happening is it is computing

atan(4.0f/-4.0f) = -0.785398 + 180 degrees = 179.215.

am I missing some step here?

score 11 · Accepted Answer · answered Jul 14 '12 at 14:59

11

The standard atan and atan2 functions, as well as all other C functions that work with angles, work with radians, not degrees.

If you want your own function to output degrees, you have to multiply the return value from atanf with 180/pi; to keep everything in radians, add pi instead of 180.

answered Jul 14 '12 at 14:59

Michael Madsen

54,231
8
72
83

I had originally thought it was in radians, but messed up the formula to go rads to degrees, which gave me second guesses. thanks. – David Torrey Jul 14 '12 at 15:02

score 3 · Answer 2 · answered Jul 14 '12 at 14:57

3

atan speaks in radians, not in degrees...

answered Jul 14 '12 at 14:57

GL770

2,910
1
14
9

nope, (atan(y/x) * 180 / 3.14) + 180. See http://stackoverflow.com/questions/1727881/how-to-use-the-pi-constant-in-c to get the value of pi – GL770 Jul 14 '12 at 15:00
yea i had it backwards. thats probably why when i tried it the first time I ruled it out, because I was thinking that it was in radians before – David Torrey Jul 14 '12 at 15:02

score 3 · Answer 3 · answered Jul 14 '12 at 14:58

3

atan returns a result in radians. You can convert to degrees with degrees=180*radians/π.

answered Jul 14 '12 at 14:58

arx

16,686
2
44
61

atan problems in c++

3 Answers3