# std::acos(std::valarray)

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< cpp‎ | numeric‎ | valarray

Defined in header `<valarray>` |  |
template< class T > valarray<T> acos( const valarray<T>& va ); |  |

For each element in `va` computes arc cosine of the value of the element.

### Parameters

va | - | value array to apply the operation to

### Return value

Value array containing arc cosines of the values in `va`.

### Notes

Unqualified function (acos) is used to perform the computation. If such function is not available, std::acos 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:

  - All const member functions of std::valarray are provided.
  - std::valarray, std::slice_array, std::gslice_array, std::mask_array and std::indirect_array can be constructed from the replacement type.
  - All functions accepting an argument of type const std::valarray & except `begin()` and `end()` (since C++11) should also accept the replacement type.
  - All functions accepting two arguments of type const std::valarray & should accept every combination of const std::valarray & and the replacement type.
  - The return type does not add more than two levels of template nesting over the most deeply-nested argument type.

### Possible implementation

    template<class T>
    valarray<T> acos(const valarray<T>& va)
    {
        valarray<T> other = va;
        for (T &i : other) {
            i = acos(i);
        }
        return other;
    }

### Example

Run this code

    #include <cmath>
    #include <iostream>
    #include <valarray>

    const double pi = 4.0 * atan(1.0);

    int main()
    {
        // take common x-values from unit circle
        std::valarray<double> v1 = {-1.0, -sqrt(3.0)/2.0, -sqrt(2.0)/2.0, -0.5, 0.0, 0.5, sqrt(2.0)/2.0, sqrt(3.0)/2.0, 1.0};
        std::valarray<double> v2 = std::acos(v1) * 180.0 / pi;;

        for(double n : v2)
            std::cout << std::fixed << n << ' ';
        std::cout << '\n';
    }

Output:

    180.000000 150.000000 135.000000 120.000000 90.000000 60.000000 45.000000 30.000000 0.000000

### See also

asin(std::valarray) | applies the function std::asin to each element of valarray (function template)
atan(std::valarray) | applies the function std::atan to each element of valarray (function template)
atan2(std::valarray) | applies the function std::atan2 to a valarray and a value (function template)
cos(std::valarray) | applies the function std::cos to each element of valarray (function template)
acosacosfacosl (C++11)(C++11) | computes arc cosine (arccos(x)) (function)
acos(std::complex) (C++11) | computes arc cosine of a complex number (arccos(z)) (function template)

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