std::log - cppreference.com
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      std::log

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      <tbody> </tbody>
      Definido no cabeçalho <cmath>
      float log( float arg );
      double log( double arg );
      long double log( long double arg );
      double log( Integral arg );
      		 (desde C++11)

      Computes the natural (base e) logarithm of arg.

      Parâmetros

      arg -
      flutuando valor de ponto
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      floating point value
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      Valor de retorno

      natural logarithm of arg.

      Erro de domínio ocorre se arg é negativo. NAN é devolvido em caso.
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      Domain error occurs if arg is negative. NAN is returned in that case.
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      Faixa de erro ocorre se arg é 0. -HUGE_VAL é devolvido em caso.
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      Range error occurs if arg is 0. -HUGE_VAL is returned in that case.
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      Exemplo

      The following code computes the binary (base 2) logarithm with help of natural logarithm.

      #include <cmath>
#include <iostream>
 
int main()
{
    double base = 2.0;
    double arg  = 256.0;
    double result = std::log(arg) / std::log(base);
 
    std::cout << result << '\n';
}

      Saída: 

      8

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