par-reduce.tex

\subsection{Reductions}

\begin{frame}[t]{Reduction pattern}
\begin{itemize}
  \item A \textmark{reduction} computes the sum of all elements
        in a data set.

  \vspace{1em}\pause
  \item \textbad{Note}: \cppid{std::reduce} looks quite similar
        to \cppid{std::accumulate} on the surface.
    \begin{itemize}
      \item Result is not deterministic unless the sum opration
            is both associative and commutative.
    \end{itemize}
\end{itemize}
\end{frame}

\begin{frame}[t,fragile]{Add all elements in a vector}
\begin{lstlisting}
void print_add(const std::vector<double> & v)
{
  double r = std::reduce(std::execution::par,
    v.begin(), v.end());

  std::cout << "sum= " << r << "\n";
}
\end{lstlisting}
\vfill
\begin{itemize}
  \item Initial value is \cppid{value\_type\{\}}.
  \item Binary operation is \cppid{std::plus<>()}.
\end{itemize}
\end{frame}

\begin{frame}[t,fragile]{Providing initial value}
\begin{lstlisting}
void print_add(const std::vector<double> & v)
{
  double r = std::reduce(std::execution::par,
    v.begin(), v.end(), 100.0);

  std::cout << "sum= " << r << "\n";
}
\end{lstlisting}
\begin{itemize}
  \item Still reduction operation is \cppid{std::plus<>()}.
\end{itemize}
\end{frame}

\begin{frame}[t,fragile]{Providing reduction operator}
\begin{lstlisting}
void print_add(const std::vector<double> & v)
{
  double r = std::reduce(std::execution::par,
    v.begin(), v.end(), 0.0,
    [](double x, double y) { return x+y; }
  );

  std::cout << "sum= " << r << "\n";
}
\end{lstlisting}
\end{frame}