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atan, atanf, atanl, atan2, atan2f, atan2l

Calculates the arctangent of x (atan, atanf, and atanl) or the arctangent of y/x (atan2, atan2f, and atan2l).

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 
long double atanl(
   long double x
float atan2f(
   float y,
   float x
long double atan2l(
   long double y,
   long double x

x, y

Any numbers.

atan returns the arctangent of x in the range –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. All results are in radians.

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


SEH Exception

Matherr Exception




The atan function calculates the arctangent (the inverse tangent function) of x. atan2 calculates the arctangent of y/x (if x equals 0, atan2 returns π/2 if y is positive, -π/2 if y is negative, or 0 if y is 0.)

atan has an implementation that uses Streaming SIMD Extensions 2 (SSE2). For information and restrictions about using the SSE2 implementation, see _set_SSE2_enable.

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


Required header

atan, atan2, atanf, atan2f, atanl, atan2l


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

int main( int ac, char* av[] ) 
   double x, y, theta;
   if( ac != 3 ){
      fprintf( stderr, "Usage: %s <x> <y>\n", av[0] );
      return 1;
   x = atof( av[1] );
   theta = atan( x );
   printf( "Arctangent of %f: %f\n", x, theta );
   y = atof( av[2] );
   theta = atan2( y, x );
   printf( "Arctangent of %f / %f: %f\n", y, x, theta ); 
   return 0;
Arctangent of 5.000000: 1.373401
Arctangent of 0.500000 / 5.000000: 0.099669
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