Legacy Document

atan

You can use the atan function to compute the arc tangent of a real number.

double_t atan (double_t x);

x

Any floating-point number.

DESCRIPTION

The atan function returns the arc tangent of its argument. The return value is expressed in radians in the range [ $-pi/2$ , + $pi/2$ ]. This function is antisymmetric.

 such that  for all x 

The tan function performs the inverse operation $(tan(y))$ .

EXCEPTIONS

When x is finite and nonzero, the result of $atan(x)$ might raise one of the following exceptions:

• inexact (for all nonzero values of x)
• underflow (if the result is inexact and must be represented as a denormalized number or 0)

SPECIAL CASES

Table 10-25 shows the results when the argument to the atan function is a zero, a NaN, or an Infinity.

Special cases for the atan function

Operation	Result	Exceptions raised
$atan(+0)$	+0	None
$atan(-0)$	$-0$	None
$atan(NaN)$	NaN	None[46]
$atan(+$ )	+ $pi/2$	Inexact
$atan(-$ )	- $pi/2$	Inexact

EXAMPLES

z = atan(1.0);    /* z = arctan 1 = \x86/4 */
z = atan(-1.0);   /* z = arctan -1 = -\x86/4. The inexact exception 
                     is raised. */

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[46] If the NaN is a signaling NaN, the invalid exception is raised.