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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../ranges/../numeric/valarray/log.html below:

std::log(std::valarray) - cppreference.com

template< class T > valarray<T> log( const valarray<T>& va );

For each element in va computes natural logarithm of the value of the element.

[edit] Parameters va - value array to apply the operation to [edit] Return value

Value array containing natural logarithms of the values in va.

[edit] Notes

Unqualified function (log) is used to perform the computation. If such function is not available, std::log 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.
For every function taking a const std::valarray<T>& except begin() and end()(since C++11), identical functions taking the replacement types shall be added;
For every function taking two const std::valarray<T>& arguments, identical functions taking every combination of const std::valarray<T>& and replacement types shall be added.
The return type does not add more than two levels of template nesting over the most deeply-nested argument type.

[edit] Possible implementation

template<class T>
valarray<T> log(const valarray<T>& va)
{
    valarray<T> other = va;
    for (T& i : other)
        i = log(i);
 
    return other; // proxy object may be returned
}

[edit] Example

Output:

n       :  0.000000  1.000000  2.000000  3.000000
eâ¿      :  1.000000  2.718282  7.389056 20.085537
log(eâ¿) :  0.000000  1.000000  2.000000  3.000000

[edit] See also

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