Functions.tex

\section{Functions}
\label{sec:functions}

When you find yourself doing the same task several times, it may be a good time to create a reusable function. A function, as you learned in the Enclosures section, is something written within curly braces. David Touretzky introduces the idea of function in this way: ``think of a function as a box through which data flows. The function operates on the data in some way, and the result is what flows out.''\footnote{Touretzky, David. COMMON LISP: A Gentle Introduction to Symbolic Computation. The Benjamin/Cummings Publishing Company, Inc, 1990, p. 1. That's the book that inspired the title of this tutorial.}

\begin{figure}[h]
\centerline{\framebox{
	\includegraphics[scale=0.9]{fig-function-box.pdf}}}
\caption{General idea of a function.}
\label{fig:function-box}
\end{figure}

The first line in the example below defines a function and assigns it to the variable \texttt{f}. The second line puts the function to work.
 
\begin{lstlisting}[style=SuperCollider-IDE, basicstyle=\scttfamily\footnotesize]
f = { 2 + 2 }; // define the function
f.value; // put the function to work
\end{lstlisting}
 
The function above is not terribly useful, as it only knows how to do one thing (add 2 and 2). Normally you will want to define functions that can give you different results depending on the input arguments you give to it. We use the keyword \texttt{arg} to specify the inputs that a function can accept. The example below looks more like the drawing from figure \ref{fig:function-box}.
 
\begin{lstlisting}[style=SuperCollider-IDE, basicstyle=\scttfamily\footnotesize]
f = {arg a, b; ["a plus b", a+b, "a times b", a*b].postln}; // define function
f.value(3, 7); // now you can give any two numbers as arguments to the function
f.value(10, 14);

// Compare:
~sillyRand = rrand(0, 10); // not a function
~sillyRand.value; // evaluate several times
~sillyRand2 = {rrand(0, 10)}; // a function
~sillyRand2.value; // evaluate several times
\end{lstlisting}
 

As a last example, here's one very useful function.

 
\begin{lstlisting}[style=SuperCollider-IDE, basicstyle=\scttfamily\footnotesize]
// Use this function to decide how to spend your Summer days
(
~whatToDo = { 
var today, dayName, actions;
	today = Date.getDate.dayOfWeek;
	dayName = 
	case
	{today==0} {"Sunday"}
	{today==1} {"Monday"}
	{today==2} {"Tuesday"}
	{today==3} {"Wednesday"}
	{today==4} {"Thursday"}
	{today==5} {"Friday"}
	{today==6} {"Saturday"};
	actions = ["boomerang throwing", "arm wrestling", "stair climbing", "playing chess", "underwater hockey", "pea shooting", "a nap marathon"];
	"Ah, " ++ dayName ++ "...! " ++ "What a good day for " ++ actions.choose;
};
)

// Run it in the morning
~whatToDo.value;
\end{lstlisting}

\bigskip
\todo[inline, color=green!40]{ TIP: Another common notation to declare arguments at the beginning of a Function is: \texttt{f = \{|a, b| a + b\}}. This is equivalent to \texttt{f = \{arg a, b; a + b\}}}