atan, atanf, atanl

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Defined in header <math.h>
float atanf ( float arg ) ;
(1) (since C99)
double atan ( double arg ) ;
(2)
long double atanl ( long double arg ) ;
(3) (since C99)
_Decimal32  atand32 ( _Decimal32 arg ) ;
(4) (since C23)
_Decimal64  atand64 ( _Decimal64 arg ) ;
(5) (since C23)
_Decimal128 atand128 ( _Decimal128 arg ) ;
(6) (since C23)
Defined in header <tgmath.h>
#define atan( arg )
(7) (since C99)
1-6) Computes the principal value of the arc tangent of arg .
7) Type-generic macro: If the argument has type long double , (3) ( atanl ) is called. Otherwise, if the argument has integer type or the type double , (2) ( atan ) is called. Otherwise, (1) ( atanf ) is called. If the argument is complex, then the macro invokes the corresponding complex function ( catanf , catan , catanl ).

The functions (4-6) are declared if and only if the implementation predefines __STDC_IEC_60559_DFP__ (i.e. the implementation supports decimal floating-point numbers).

(since C23)

Parameters

arg - floating-point value

Return value

If no errors occur, the arc tangent of arg ( arctan(arg) ) in the range [-
π
2
; +
π
2
] radians, is returned.

If a range error occurs due to underflow, the correct result (after rounding) is returned.

Error handling

Errors are reported as specified in math_errhandling .

If the implementation supports IEEE floating-point arithmetic (IEC 60559):

  • if the argument is ±0, it is returned unmodified;
  • if the argument is +∞, +π/2 is returned;
  • if the argument is -∞, -π/2 is returned;
  • if the argument is NaN, NaN is returned.

Notes

POSIX specifies that in case of underflow, arg is returned unmodified, and if that is not supported, an implementation-defined value no greater than DBL_MIN , FLT_MIN , and LDBL_MIN is returned.

Example

Run this code 
#include <math.h>
#include <stdio.h>
 
int main(void)
{
    printf("atan(1) = %f, 4*atan(1)=%f\n", atan(1), 4 * atan(1));
    // special values
    printf("atan(Inf) = %f, 2*atan(Inf) = %f\n", atan(INFINITY), 2 * atan(INFINITY));
    printf("atan(-0.0) = %+f, atan(+0.0) = %+f\n", atan(-0.0), atan(0));
}

Output: 

atan(1) = 0.785398, 4*atan(1)=3.141593
atan(Inf) = 1.570796, 2*atan(Inf) = 3.141593
atan(-0.0) = -0.000000, atan(+0.0) = +0.000000

References

  • C23 standard (ISO/IEC 9899:2024):
  • 7.12.4.3 The atan functions (p: TBD)
  • 7.25 Type-generic math <tgmath.h> (p: TBD)
  • F.10.1.3 The atan functions (p: TBD)
  • C17 standard (ISO/IEC 9899:2018):
  • 7.12.4.3 The atan functions (p: 174)
  • 7.25 Type-generic math <tgmath.h> (p: 272-273)
  • F.10.1.3 The atan functions (p: 378)
  • C11 standard (ISO/IEC 9899:2011):
  • 7.12.4.3 The atan functions (p: 238-239)
  • 7.25 Type-generic math <tgmath.h> (p: 373-375)
  • F.10.1.3 The atan functions (p: 519)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.12.4.3 The atan functions (p: 219)
  • 7.22 Type-generic math <tgmath.h> (p: 335-337)
  • F.9.1.3 The atan functions (p: 456)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 4.5.2.3 The atan function

See also

(C99) (C99)
computes arc tangent, using signs to determine quadrants
(function)
(C99) (C99)
computes arc sine ( arcsin(x) )
(function)
(C99) (C99)
computes arc cosine ( arccos(x) )
(function)
(C99) (C99)
computes tangent ( tan(x) )
(function)
(C99) (C99) (C99)
computes the complex arc tangent
(function)
C++ documentation for atan
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