< 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.


[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:

[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

#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);


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

[edit] See also

applies the function std::log10 to each element of valarray
(function template) [edit]
applies the function std::exp to each element of valarray
(function template) [edit]
computes natural (base e) logarithm (ln(x))
(function) [edit]
complex natural logarithm with the branch cuts along the negative real axis
(function template) [edit]