# std::atan2(std::valarray)

< cpp‎ | numeric‎ | valarray
 Defined in header  template< class T > std::valarray atan2( const std::valarray& y, const std::valarray& x ); (1) template< class T > std::valarray atan2( const std::valarray& y,                         const typename std::valarray::value_type& vx ); (2) template< class T > std::valarray atan2( const typename std::valarray::value_type& vy,                         const std::valarray& x ); (3)

Computes the inverse tangent of y / x using the signs of arguments to correctly determine quadrant.

1) Computes the inverse tangent of each pair of corresponding values from y and x.

The behavior is undefined if x.size() != y.size().

2) Computes the inverse tangent of vx and each value in the numeric array y.
3) Computes the inverse tangent of vy and each value in the numeric array x.

## Contents

### Parameters

 x, y - numeric arrays to compute inverse tangent of vy, vx - values to compute inverse tangent of

### Return value

A numeric array containing the results of computation of inverse tangent.

### Notes

Unqualified function (atan2) is used to perform the computation. If such function is not available, std::atan2 is used due to argument-dependent lookup.

The function can be implemented with the return type different from std::valarray. In this case, the replacement type has the following properties:

### Example

#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <valarray>

void show(char const* title, const std::valarray<double>& va)
{
std::cout << title << ' ';
std::for_each(std::begin(va), std::end(va), [](const double x)
{
std::cout << ' ' << std::right << std::setw(4) << x << "°";
});
std::cout << '\n';
}

const double pi = std::acos(-1.0); // C++20: std::numbers::pi

int main()
{
auto degrees_to_radians = [](double const& x) { return (pi * x / 180); };
auto radians_to_degrees = [](double const& x) { return (180 * x / pi); };

const std::valarray<double> degrees{-90, -60, -45, -30, 0, 30, 45, 60, 90};

}

Output:

(1)   -90°  -60°  -45°  -30°    0°   30°   45°   60°   90°
(2)   -90°  -60°  -45°  -30°    0°   30°   45°   60°   90°
(3)    90°  120°  135°  150°    0°   30°   45°   60°   90°

###  Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3074 C++98 T is deduced from both the scalar and the valarray for (2,3), disallowing mixed-type calls only deduce T from the valarray