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Showing content from https://cplusplus.com/reference/valarray/log/ below:

function template

std::log

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

Compute natural logarithm of valarray elements

Returns a valarray containing the natural logarithm (base-e logarithm) of all the elements of x, in the same order.

The function calls log (unqualified) once for each element.

This function overloads cmath's log.

Parameters

x: valarray containing elements of a type for which the unary function log is defined.

Return value A valarray object with the natural logarithms of the elements of x.

Example

// log valarray example
#include <iostream>     // std::cout
#include <cstddef>      // std::size_t
#include <cmath>        // std::log(double)
#include <valarray>     // std::valarray, std::log(valarray)

int main ()
{
  double val[] = {5.0, 6.6, 8.0, 9.9};
  std::valarray<double> foo (val,4);

  std::valarray<double> bar = log (foo);

  std::cout << "foo:";
  for (std::size_t i=0; i<foo.size(); ++i)
    std::cout << ' ' << foo[i];
  std::cout << '\n';

  std::cout << "bar:";
  for (std::size_t i=0; i<bar.size(); ++i)
    std::cout << ' ' << bar[i];
  std::cout << '\n';

  return 0;
}

Output:

foo: 5 6.6 8 9.9
bar: 1.60944 1.88707 2.07944 2.29253