# std::atan, std::atanf, std::atanl

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< cpp‎ | numeric‎ | math

Defined in header `<cmath>` |  |
| (1) |
float atan ( float num ); double atan ( double num ); long double atan ( long double num ); |  | (until C++23)
/*floating-point-type*/ atan ( /*floating-point-type*/ num ); |  | (since C++23) (constexpr since C++26)
float atanf( float num ); | (2) | (since C++11) (constexpr since C++26)
long double atanl( long double num ); | (3) | (since C++11) (constexpr since C++26)
SIMD overload (since C++26) |  |
Defined in header `<simd>` |  |
template< /*math-floating-point*/ V > constexpr /*deduced-simd-t*/<V> atan ( const V& v_num ); | (S) | (since C++26)
Additional overloads (since C++11) |  |
Defined in header `<cmath>` |  |
template< class Integer > double atan ( Integer num ); | (A) | (constexpr since C++26)

1-3) Computes the principal value of the arc tangent of num. The library provides overloads of `std::atan` for all cv-unqualified floating-point types as the type of the parameter.(since C++23)

S) The SIMD overload performs an element-wise `std::atan` on v_num.

    (See `math-floating-point` and `deduced-simd-t` for their definitions.)

(since C++26)

A) Additional overloads are provided for all integer types, which are treated as double. | (since C++11)

### Parameters

num | - | floating-point or integer value

### Return value

If no errors occur, the arc tangent of num (arctan(num)) 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, num 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.

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::atan(num) has the same effect as std::atan(static_cast<double>(num)).

### Example

Run this code

    #include <cmath>
    #include <iostream>

    int main()
    {
        std::cout << "atan(1) = " << std::atan(1) << '\n'
                  << "4*atan(1) = " << 4 * std::atan(1) << '\n';

        // special values
        std::cout << "atan(Inf) = " << std::atan(INFINITY) << '\n'
                  << "2*atan(Inf) = " << 2 * std::atan(INFINITY) << '\n'
                  << "atan(-0.0) = " << std::atan(-0.0) << '\n'
                  << "atan(+0.0) = " << std::atan(0) << '\n';
    }

Output:

    atan(1) = 0.785398
    4*atan(1) = 3.14159
    atan(Inf) = 1.5708
    2*atan(Inf) = 3.14159
    atan(-0.0) = -0
    atan(+0.0) = 0

### See also

asinasinfasinl (C++11)(C++11) | computes arc sine (\({\small\arcsin{x}}\)arcsin(x)) (function)
acosacosfacosl (C++11)(C++11) | computes arc cosine (\({\small\arccos{x}}\)arccos(x)) (function)
atan2atan2fatan2l (C++11)(C++11) | arc tangent, using signs to determine quadrants (function)
tantanftanl (C++11)(C++11) | computes tangent (\({\small\tan{x}}\)tan(x)) (function)
atan(std::complex) (C++11) | computes arc tangent of a complex number (\({\small\arctan{z}}\)arctan(z)) (function template)
atan(std::valarray) | applies the function std::atan to each element of valarray (function template)
C documentation for atan

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