## 15.6. atan(), atanf(), atanl(), atan2(), atan2f(), atan2l()

Calculate the arc tangent of a number.

### Prototypes

```#include <math.h>

double atan(double x);
float atanf(float x);
long double atanl(long double x);

double atan2(double y, double x);
float atan2f(float y, float x);
long double atan2l(long double y, long double x);```

### Description

Calculates the arc tangent of a number in radians. (That is, the value whose tangent is x.)

The atan2() variants are pretty much the same as using atan() with y/x as the argument...except that atan2() will use those values to determine the correct quadrant of the result.

For those of you who don't remember, radians are another way of measuring an angle, just like degrees. To convert from degrees to radians or the other way around, use the following code:

```degrees = radians * 180.0f / M_PI;
radians = degrees * M_PI / 180;
```

### Return Value

The atan() functions return the arc tangent of x, which will be between PI/2 and -PI/2. The atan2() functions return an angle between PI and -PI.

### Example

```double atanx;
long double ldatanx;

atanx = atan(0.2);
ldatanx = atanl((long double)0.3);

atanx = atan2(0.2);
ldatanx = atan2l((long double)0.3);
```