# atan, atanf, atanl

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< c‎ | numeric‎ | math

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)
Defined in header `<tgmath.h>` |  |
#define atan( arg ) | (4) | (since C99)

1-3) Computes the principal value of the arc tangent of `arg`.
4) Type-generic macro: If the argument has type long double, `atanl` is called. Otherwise, if the argument has integer type or the type double, `atan` is called. Otherwise, `atanf` is called. If the argument is complex, then the macro invokes the corresponding complex function (catanf, catan, catanl).

### 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 <stdio.h>
    #include <math.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

- 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

atan2 atan2f atan2l (C99)(C99) | computes arc tangent, using signs to determine quadrants (function)
asin asinf asinl (C99)(C99) | computes arc sine (arcsin(x)) (function)
acos acosf acosl (C99)(C99) | computes arc cosine (arccos(x)) (function)
tan tanf tanl (C99)(C99) | computes tangent (tan(x)) (function)
catan catanf catanl (C99)(C99)(C99) | computes the complex arc tangent (function)

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