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.


va - value array to apply the operation to

Return value

Value array containing natural logarithms of the values in va.


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:

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


#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

See also

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

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.