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atan, atanf, atan2, atan2f

Calculates the arctangent of x (atan or atanf) or the arctangent of y/x (atan2 or atan2f).

double atan( 
   double x 
float atan(
   float x 
);  // C++ only
long double atan(
   long double x
);  // C++ only
double atan2( 
   double y, 
   double x 
float atan2(
   float y,
   float x
);  // C++ only
long double atan2(
   long double y,
   long double x
);  // C++ only
float atanf( 
   float x 
float atan2f(
   float y,
   float x


x, y
Any numbers.

Return Value

atan returns the arctangent of x in the range of –π/2 to π/2 radians. atan2 returns the arctangent of y/x in the range –π to π radians. If x is 0, atan returns 0. If both parameters of atan2 are 0, the function returns 0.

atan2 uses the signs of both parameters to determine the quadrant of the return value.

Input SEH Exception Matherr Exception


The atan function calculates the arctangent of x. atan2 calculates the arctangent of y/x. atan2 is well defined for every point other than the origin, even if x equals 0 and y does not equal 0.

atan has an implementation that uses Streaming SIMD Extensions 2 (SSE2). See _set_SSE2_enable for information and restrictions on using the SSE2 implementation.

C++ allows overloading, so you can call overloads of atan and atan2. In a C program, atan and atan2 always take and return doubles.


Routine Required header Compatibility
atan, atan2, atanf, atan2f <math.h> ANSI, Win 98, Win Me, Win NT, Win 2000, Win XP

For additional compatibility information, see Compatibility in the Introduction.


All versions of the C run-time libraries.


// crt_atan.c
// arguments: 0.5 5
#include <math.h>
#include <stdio.h>
#include <errno.h>

int main( int ac, char* av[] ) 
   double x1, x2, y;
   if( ac != 3 ){
      fprintf( stderr, "Usage: %s <x1> <x2>\n", av[0] );
   x1 = atof( av[1] );
   y = atan( x1 );
   printf( "Arctangent of %f: %f\n", x1, y );
   x2 = atof( av[2] );
   y = atan2( x1, x2 );
   printf( "Arctangent of %f / %f: %f\n", x1, x2, y ); 


Arctangent of 0.500000: 0.463648
Arctangent of 0.500000 / 5.000000: 0.099669

See Also

Floating-Point Support Routines | acos | asin, cos | _matherr | sin | tan | Run-Time Routines and .NET Framework Equivalents