C in a Nutshell

Name

atan

Synopsis

Calculates the inverse tangent of a number

#include <math.h>
doubleatan( double x );
float atanf( float x );        (C99)
long double atanl( long double x );        (C99)

atan() implements the inverse tangent function, commonly called arc tangent.

The return value is given in radians, and is thus in the range -π/2 ≤ atan(x) ≤ π/2.

Example

#ifdef PI
  printf("The symbol PI was already defined.\n");
  long double pi = (long double) PI;
#else
  long double pi = 4.0L *atanl( 1.0L );   // Because tan(pi/4) = 1
#endif
  printf( "Assume pi equals %.17Lf.\n", pi);

This code produces the following output:

Assume pi equals 3.14159265358979324.

See Also

The arc tangent functions for complex numbers: catan(), catanf(), and catanl()