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The latest version of this topic can be found at 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 ).

Syntax

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  
Parameters
x, y

Any numbers.
Return Value
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.
Input
SEH Exception
Matherr Exception
Remarks
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.
Requirements
Routine
Required header
   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  
.NET Framework Equivalent
System::Math::Atan
System::Math::Atan2
See Also
Floating-Point Support

acos, acosf, acosl

asin, asinf, asinl

cos, cosf, cosl, cosh, coshf, coshl

_matherr

sin, sinf, sinl, sinh, sinhf, sinhl

tan, tanf, tanl, tanh, tanhf, tanhl

_CIatan

_CIatan2