# std::log(std::valarray)

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< cpp‎ | numeric‎ | valarray

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

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

### Parameters

va | - | value array to apply the operation to

### Return value

Value array containing natural logarithms of the values in `va`.

### 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.
  - 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> log( const valarray<T>& va )
    {
        valarray<T> other = va;
        for (T &i : other)
            i = log(i);

        return other; // proxy object may be returned
    }

### Example

Run this code

    #include <iomanip>
    #include <iostream>
    #include <valarray>

    void show(char const* title, const std::valarray<double>& va)
    {
        std::cout << title << " :" << std::right << std::fixed;
        for (double x : va) {
            std::cout << std::setw(10) << x;
        }
        std::cout << '\n';
    }

    int main()
    {
        const std::valarray<double> n { 0.0, 1.0, 2.0, 3.0 };
        const std::valarray<double> exp_n { std::exp(n) };
        const std::valarray<double> log_exp_n { std::log(exp_n) };

        show("n      ", n);
        show("eⁿ     ", exp_n);
        show("log(eⁿ)", log_exp_n);
    }

Output:

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

### See also

log10(std::valarray) | applies the function std::log10 to each element of valarray (function template)
exp(std::valarray) | applies the function std::exp to each element of valarray (function template)
log logf logl (C++11)(C++11) | computes natural (base e) logarithm (\({\small \ln{x} }\)ln(x)) (function)
log(std::complex) | complex natural logarithm with the branch cuts along the negative real axis (function template)

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