\documentclass{article}
\textwidth=4.5in
\textheight=7.125in
\usepackage{pseudocode}
%
%
%    September 2, 1999
%
%
\title{Pseudocode: A \LaTeX\  Style File for Displaying Algorithms}
\author{
D.L.\ Kreher\\
Department of Mathematical Sciences\\
Michigan Technological University \\ 
Houghton, MI 49931\\
{\tt kreher@mtu.edu}\\[1ex]
and \\[1ex]
D.R.\ Stinson\\
Department of Combinatorics and Optimization\\
University of Waterloo\\
Waterloo ON, N2L 3G1\\
{\tt dstinson@uwaterloo.ca}
}

\newcommand{\keyword}[1]{\texttt{#1}}
\begin{document}
%\makeatletter
%\setlength{\pcode@width}{2.75in}
%\addtolength{\pcode@width}{-40pt}
%\makeatother

\maketitle

\section{Introduction}

This paper describes a \LaTeX\  environment named \keyword{pseudocode} that can be used
for describing algorithms in pseudocode form.  This is the style used
in our %recently published 
textbook {\em Combinatorial Algorithms: Generation, Enumeration and Search}
\cite{cages}.  The style file \keyword{pseudocode.sty} is available for free
downloading from the web page
\verb+http://www.math.mtu.edu/~kreher/cages.html+

This package is quite easy to use, and allows algorithms to be described 
in a \LaTeX\  document using a natural Pascal-like syntax.  In the remaining sections
of this note, we describe how to use the \keyword{pseudocode} environment
and we present some examples.  Readers familiar with \LaTeX\ 
(see \cite{lamport}) should be
able to easily customize the style file to include additional desired features.

The \keyword{pseudocode} environment requires the \keyword{fancybox}
package by Timothy Van Zandt.  This package is described in 
Section 10.1.3 of \cite{LC}.
Other environments for describing algorithms include
\keyword{alg}, \keyword{algorithmic}, \keyword{newalg} and \keyword{program}.
These style files, as well as \keyword{fancybox}, are all available from the CTAN web site
\verb+http://www.ctan.org/+

\section{The \keyword{pseudocode} Environment}

Within the \keyword{pseudocode} environment, a number of commands for 
popular algorithmic constructs are available.
In general, the commands provided can be nested to
describe quite complex algorithms. 

The \keyword{pseudocode} environment is  invoked
as follows:
\begin{verbatim}
\begin{pseudocode}{<Name>}{<Parameters>}
  pseudocode constructs
\end{pseudocode}
\end{verbatim}

\noindent{}The argument  \verb+<Name>+ is the name of the algorithm, and
 \verb+<Parameters>+ is a list of parameters for the algorithm.
For example, the commands

\begin{verbatim}
\begin{pseudocode}{CelsiusToFahrenheit}{c}
  f \GETS {9c/5} + 32\\
  \RETURN{f}
\end{pseudocode}
\end{verbatim}

\noindent{}produce the following output when included in
a \LaTeX\  document:

\begin{pseudocode}{CelsiusToFahrenheit}{c}
  f \GETS {9c/5} + 32\\
  \RETURN{f}
\end{pseudocode}

Notice that the command \verb+\GETS+ produces a 
left arrow, which we use to indicate an assignment of
a variable.  The user could use instead some other symbol, if desired.
For example, \verb+\GETS+ could be replaced by \verb+=+,
as is done in the ``C'' programming language.

\subsection{ The {\em begin-end} Construct}

To form compound statements from simple statements, the {\em begin-end} 
construct is used as follows:
\begin{verbatim}
 \BEGIN
    some statement\\
    another statement\\
    yet another statement
 \END
\end{verbatim}
This generates the following:

\medskip
\begin{pseudocode}[display]{}{}
\BEGIN
\mbox{some statement}\\
\mbox{another statement}\\
\mbox{yet another statement}
\END
\end{pseudocode}

\noindent{}The effect of this construct is to group 
a collection of statements
using a left brace bracket of the appropriate size.

In the sections that follow we will use the notation
\verb+<stmt>+
to indicate  a simple statement or a compound statement.
Note that the contents of statements are typeset in math mode.
Therefore, non-math mode text must be enclosed in an
\verb+\mbox{}+.

Observe that the double backslash \verb+\\+ plays the same
role as the semicolon in Pascal, i.e., it is used to
separate statements, and should never appear before
\verb+\END+.

\subsection{The {\em if-then-else} Construct}

The {\em if-then-else} construct takes various forms,
such as the following:
\begin{verbatim}
\IF <condition> \THEN <stmt> 
\IF <condition> \THEN <stmt> \ELSE <stmt> 
\IF <condition> \THEN <stmt> \ELSEIF <stmt> \THEN <stmt> 
\end{verbatim}
Note that there is no limit placed on the number
of \verb+\ELSEIF+s that may be used in an {\em if-then-else} construct.
For example,  the commands:
\begin{verbatim}
\IF some condition is true
\THEN
 \BEGIN
    some statement\\
    another statement\\
    yet another statement
 \END
\ELSEIF some other condition is true
\THEN
 \BEGIN
    some statement\\
    another statement\\
    yet another statement
 \END
\ELSEIF some even more bizarre condition is met
\THEN 
  do something else
\ELSE
  do the default actions
\end{verbatim}

\noindent{}would produce the following output:

\medskip
\begin{pseudocode}[display]{}{}
\IF \mbox{some condition is true}
\THEN 
\BEGIN
\mbox{some statement}\\
\mbox{another statement}\\
\mbox{yet another statement}
\END
\ELSEIF \mbox{some other condition is true}
\THEN 
\BEGIN
\mbox{some statement}\\
\mbox{another statement}\\
\mbox{yet another statement}
\END
\ELSEIF\mbox{some even more bizarre condition is met}
\THEN \mbox{do something else}
\ELSE
\mbox{do the default actions}
\end{pseudocode}

\subsection{The {\em for} Loop}

The {\em for} loop takes the following forms:
\begin{verbatim}
\FOR <var> \GETS <lower>  \TO <upper> \DO <stmt> 
\FOR <var> \GETS <upper>  \DOWNTO <lower> \DO <stmt> 
\FOREACH <condition> \DO  <stmt> 
\end{verbatim}
For example,
\begin{verbatim}
\FOR i \GETS 0 \TO 10 \DO
  some processing
\end{verbatim}
produces

\medskip
\begin{pseudocode}[display]{}{}
\FOR i \GETS 0 \TO 10 \DO
\mbox{some processing}
\end{pseudocode}

\noindent{}and

\begin{verbatim}
\FOREACH x \in \mathcal{S}  \DO
  some processing 
\end{verbatim}

\noindent{produces}

\medskip
\begin{pseudocode}[display]{}{}
\FOREACH x \in \mathcal{S}  \DO
  \mbox{some processing}
\end{pseudocode}

\subsection{The {\em while} Loop}

The {\em while} loop takes the following form:

\begin{verbatim}
\WHILE <condition> \DO <stmt> 
\end{verbatim}

\noindent{}For example,

\begin{verbatim}
\WHILE some condition holds \DO 
  some processing 
\end{verbatim}

\noindent{}produces

\medskip
\begin{pseudocode}[display]{}{}
\WHILE \mbox{some condition holds} \DO 
\mbox{some processing }
\end{pseudocode}

\subsection{The {\em repeat-until} Loop}

The {\em repeat-until} loop takes the following form:

\begin{verbatim}
\REPEAT <stmt> \UNTIL <condition> 
\end{verbatim}

\noindent{}For example,

\begin{verbatim}
\REPEAT
  some processing 
\UNTIL some condition is met 
\end{verbatim}

\noindent{}produces

\medskip
\begin{pseudocode}[display]{}{}
\REPEAT
\mbox{some processing}
\UNTIL \mbox{some condition is met}
\end{pseudocode}

\subsection{Main Programs and Procedures}

We can describe a main program that calls one (or more) procedures as follows:

\begin{verbatim}
\begin{pseudocode}{<Name>}{<Parameters>}
\PROCEDURE{<ProcedureName>}{<ProcedureParameters>}
   some stuff
\ENDPROCEDURE
\MAIN
  some stuff\\
  \CALL{<ProcedureName>}{<ActualParameters}>\\
  more stuff
\ENDMAIN
\end{pseudocode}
\end{verbatim}

Here is a simple example to illustrate the 
use of a main program calling a procedure.
The commands

\begin{verbatim}
\begin{pseudocode}{TemperatureTable}{lower, upper}
\PROCEDURE{CelsiusToFahrenheit}{c}
  f \GETS {9c/5} + 32\\
  \RETURN{f}
\ENDPROCEDURE 
\MAIN
x \GETS lower \\
\WHILE x \leq upper \DO
\BEGIN
 \OUTPUT{x, \CALL{CelsiusToFahrenheit}{x}}\\
 x \GETS x+1
\END
\ENDMAIN
\end{pseudocode}
\end{verbatim}

\noindent{}produce the following output:

\begin{pseudocode}{TemperatureTable}{lower, upper}
\PROCEDURE{CelsiusToFahrenheit}{c}
  f \GETS {9c/5} + 32\\
  \RETURN{f}
\ENDPROCEDURE 

\MAIN
x \GETS lower \\
\WHILE x \leq upper \DO
\BEGIN
 \OUTPUT{x, \CALL{CelsiusToFahrenheit}{x}}\\
 x \GETS x+1
\END
\ENDMAIN
\end{pseudocode}


\subsection{Comments}

A comment statement may be inserted in an algorithm using the
following command:
\begin{verbatim}
\COMMENT{<stmt>}
\end{verbatim}
For example, the commands
\begin{verbatim}
A \GETS B\\
\COMMENT{Now increment the value of $A$}\\
A \GETS A+1
\end{verbatim}

\noindent{}produce the output

\medskip
\begin{pseudocode}[display]{}{}
A \GETS B\\
\COMMENT{Now increment the value of $A$}\\
A \GETS A+1
\end{pseudocode}

\medskip

Note that comments are assumed to be text. Thus, in order to include
mathematical expressions in a comment, math mode must be used explicitly.

\subsection{Other Predefined Keywords}

Several other predefined keywords are available.
We summarize their usage in Table \ref{other predefined keywords}.

\begin{table}[h]
\caption{Other Predefined Keywords}
\label{other predefined keywords}
\begin{center}
\begin{tabular}{@{}ll@{}}
\hline
command & output\\
\hline
\verb+\LOCAL{list of variables}+&
$\LOCAL{\mbox{list of variables}}$\\
\verb+\GLOBAL{list of variables}+&
$\GLOBAL{\mbox{list of variables}}$\\
\verb+\EXTERNAL{list of procedures}+&
$\EXTERNAL{\mbox{list of procedures}}$\\
\verb+\RETURN{list of values}+&
$\RETURN{\mbox{list of values}}$\\
\verb+\OUTPUT{list of values}+&
$\OUTPUT{\mbox{list of values}}$\\
\verb+\EXIT+&
$\EXIT$\\
\verb+\AND+&
$\AND$\\
\verb+\OR+&
$\OR$\\
\verb+\NOT+&
$\NOT$\\
\verb+\TRUE+&
$\TRUE$\\
\verb+\FALSE+&
$\FALSE$\\
\verb+\GETS+&
$\GETS$\\
\hline
\end{tabular}
\end{center}
\end{table}

Also note 
that all of the keywords 
\verb+\IF+, \verb+\WHILE+, \verb+\CALL{}{}+, \verb+\NOT+, etc.\
are available for
use outside of the \keyword{pseudocode} environment, but they must
be input in math mode. For example,
\begin{center}
\verb+The  $\WHILE$ loop is our friend.+
\end{center}
generates
\begin{center}
The  $\WHILE$ loop is our friend.
\end{center}


\subsection{Statement Numbering}

Statements can be numbered and given a reference key so that
they can be referenced in a \LaTeX\ document using a \verb+\ref{}+ command
(see section 4.2 of \cite{lamport}).
This is done as follows:

\begin{verbatim}
\STMTNUM{<space>}{<key>}
\end{verbatim}

\noindent{}The argument \verb+<space>+ is the amount of space to be left between
the text and the statement number.  This is a length that is specified by the
user, and generally will require some experimentation in order for it to look nice.
The argument \verb+<key>+ is the reference key
used in the LaTeX\  \verb+\ref{}+ command to refer to the given
statement.

The default numbering for statements is arabic. However, it can be changed
by a suitable \verb+\renewcommand{}+.
An example is provided in the next section.

\section{An example}

The following example demonstrates the use of the
\keyword{pseudocode} environment to describe a complete
algorithm, the familiar ``mergesort'' algorithm. The \LaTeX\ input

\begin{verbatim}
\renewcommand{\thepseudonum}{\roman{pseudonum}}
\begin{pseudocode}{MergeSort}{n,X}
\label{MergeSort}
\COMMENT{Sort the array $X$ of length $n$}\\
\IF n=2 \THEN
\BEGIN
  \IF X[0]>X[1] \THEN
  \BEGIN
    T \GETS X[0]\\
    X[0]\GETS X[1]\\
    X[1]\GETS T
  \END
\END
\ELSEIF n>2 \THEN
\BEGIN
  m\GETS \lfloor n/2 \rfloor\\
  \FOR i\GETS 0 \TO m-1 \DO A[i] \GETS X[i]\\
  \FOR i\GETS m \TO n-1 \DO B[i] \GETS X[i]\\
  \COMMENT{Now sort the subarrays $A$ and $B$}\\
  \CALL{MergeSort}{m,A}\\
  \CALL{MergeSort}{n-m,B}\\
  i\GETS 0\\
  j\GETS 0\\
  \FOR k \GETS 0 \TO n-1 \DO
  \BEGIN
    \IF A[i] \leq B[j] \THEN
    \BEGIN
       X[k]\GETS A[i] \STMTNUM{1in}{st.1}\\
       i\GETS i+1
    \END
    \ELSE
    \BEGIN
       X[k]\GETS B[j] \STMTNUM{1.03in}{st.2}\\
       j\GETS j+1
    \END
  \END
\END
\end{pseudocode}
\end{verbatim}

\noindent{}produces the following output:

\renewcommand{\thepseudonum}{\roman{pseudonum}}
\begin{pseudocode}{MergeSort}{n,X}
\label{MergeSort}
\COMMENT{Sort the array $X$ of length $n$}\\
\IF n=2 \THEN
\BEGIN
  \IF X[0]>X[1] \THEN
  \BEGIN
    T \GETS X[0]\\
    X[0]\GETS X[1]\\
    X[1]\GETS T
  \END
\END
\ELSEIF n>2 \THEN
\BEGIN
  m\GETS \lfloor n/2 \rfloor\\
  \FOR i\GETS 0   \TO m-1 \DO A[i] \GETS X[i]\\
  \FOR i\GETS m \TO n-1 \DO B[i] \GETS X[i]\\
  \COMMENT{Now sort the subarrays $A$ and $B$}\\
  \CALL{MergeSort}{m,A}\\
  \CALL{MergeSort}{n-m,B}\\
  i\GETS 0\\
  j\GETS 0\\
  \FOR k \GETS 0 \TO n-1 \DO
  \BEGIN
    \IF A[i] \leq B[j] \THEN
    \BEGIN
       X[k]\GETS A[i] \STMTNUM{.9in}{st.1}\\
       i\GETS i+1
    \END
    \ELSE
    \BEGIN
       X[k]\GETS B[j] \STMTNUM{.93in}{st.2}\\
       j\GETS j+1
    \END
  \END
\END
\end{pseudocode}

The counter \verb+pseudonum+ keeps track of the statement numbers.
The style of the counter values can be changed using the method
described in Section 6.3 of \cite{lamport}. For example,
we used the command
\begin{center}
\verb+ \renewcommand{\thepseudonum}{\roman{pseudonum}} +
\end{center}
in our example so that statements were numbered with 
lowercase Roman numerals.
We also assigned a label to the algorithm using the
\verb+\label{}+ command that is described in Section 4.2 of \cite{lamport}.
Finally, by trial and error, we determined 
spacing so that the statement numbers would be vertically aligned.

We now give an example of how the numbered statements in the above
algorithm can be referenced in a \LaTeX document.  The commands

\begin{verbatim}
On lines (\ref{st.1}) and (\ref{st.2}) of Algorithm 
\ref{MergeSort}, we determine the $k$th element of the 
sorted array.
\end{verbatim}

\noindent{}produce the following output:

\medskip 

\noindent{}On lines (\ref{st.1}) and (\ref{st.2}) of Algorithm \ref{MergeSort},  
we determine the $k$th element of the 
sorted array.

%\medskip 



\section{Framing}
The \keyword{pseudocode} environment also has an optional parameter, 
\verb+<frame>+. The complete form of the \keyword{pseudocode} environment is

\begin{verbatim}
\begin{pseudocode}[<frame>]{<Name>}{<Parameters>}
  pseudocode constructs
\end{pseudocode}
\end{verbatim}

The possible values of  \verb+<frame>+ are:
\begin{center}
\begin{tabular}{llll}
\verb+shadowbox+ &
\verb+doublebox+ &
\verb+ovalbox+ &
\verb+Ovalbox+  \\
\verb+framebox+ &
\verb+plain+ &
\verb+ruled+ &
\verb+display+
\end{tabular}
\end{center}
The values ending with ``box'' draw various types of frames around the algorithm.
The value \verb+plain+ is the default and adds no frame to the algorithm.
The value \verb+display+ is used for displaying sections of code
without the algorithm name or parameters. Here are some examples with
input:

\begin{verbatim}
\begin{pseudocode}[<frame>]{SquareAndMultiply}{x,b,n}
 \COMMENT{ Compute $x^b \pmod{n}$}\\
 z\GETS 1\\
 \WHILE b > 0 \DO
 \BEGIN
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd} 
    \THEN z \GETS z \cdot x  \pmod{n} \\
    b \GETS \CALL{ShiftRight}{b}
 \END\\
 \RETURN{z}
\end{pseudocode}
\end{verbatim}

\noindent{}where we give \verb+<frame>+  each of the values described above.

\medskip

\noindent When \verb+<frame>+ is \verb+shadowbox+ we obtain:

\begin{pseudocode}[shadowbox]{SquareAndMultiply}{x,b,n}
 \COMMENT{ Compute $x^b \pmod{n}$}\\
 z\GETS 1\\
 \WHILE b > 0 \DO
 \BEGIN
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd} 
    \THEN z \GETS z \cdot x  \pmod{n} \\
    b \GETS \CALL{ShiftRight}{b}
 \END\\
 \RETURN{z}
\end{pseudocode}

\noindent When \verb+<frame>+ is \verb+doublebox+ we obtain:

\begin{pseudocode}[doublebox]{SquareAndMultiply}{x,b,n}
 \COMMENT{ Compute $x^b \pmod{n}$}\\
 z\GETS 1\\
 \WHILE b > 0 \DO
 \BEGIN
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd} 
    \THEN z \GETS z \cdot x  \pmod{n} \\
    b \GETS \CALL{ShiftRight}{b}
 \END\\
 \RETURN{z}
\end{pseudocode}

\noindent When \verb+<frame>+ is \verb+ovalbox+ we obtain:

\begin{pseudocode}[ovalbox]{SquareAndMultiply}{x,b,n}
 \COMMENT{ Compute $x^b \pmod{n}$}\\
 z\GETS 1\\
 \WHILE b > 0 \DO
 \BEGIN
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd} 
    \THEN z \GETS z \cdot x  \pmod{n} \\
    b \GETS \CALL{ShiftRight}{b}
 \END\\
 \RETURN{z}
\end{pseudocode}

\noindent When \verb+<frame>+ is \verb+Ovalbox+ we obtain:

\begin{pseudocode}[Ovalbox]{SquareAndMultiply}{x,b,n}
 \COMMENT{ Compute $x^b \pmod{n}$}\\
 z\GETS 1\\
 \WHILE b > 0 \DO
 \BEGIN
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd} 
    \THEN z \GETS z \cdot x  \pmod{n} \\
    b \GETS \CALL{ShiftRight}{b}
 \END\\
 \RETURN{z}
\end{pseudocode}

\noindent When \verb+<frame>+ is \verb+framebox+ we obtain:

\begin{pseudocode}[framebox]{SquareAndMultiply}{x,b,n}
 \COMMENT{ Compute $x^b \pmod{n}$}\\
 z\GETS 1\\
 \WHILE b > 0 \DO
 \BEGIN
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd} 
    \THEN z \GETS z \cdot x  \pmod{n} \\
    b \GETS \CALL{ShiftRight}{b}
 \END\\
 \RETURN{z}
\end{pseudocode}

\noindent When \verb+<frame>+ is \verb+plain+ or if \verb+[<frame>]+ is omitted we obtain:

\begin{pseudocode}[plain]{SquareAndMultiply}{x,b,n}
 \COMMENT{ Compute $x^b \pmod{n}$}\\
 z\GETS 1\\
 \WHILE b > 0 \DO
 \BEGIN
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd} 
    \THEN z \GETS z \cdot x  \pmod{n} \\
    b \GETS \CALL{ShiftRight}{b}
 \END\\
 \RETURN{z}
\end{pseudocode}

\noindent When \verb+<frame>+ is \verb+ruled+ we obtain:

\begin{pseudocode}[ruled]{SquareAndMultiply}{x,b,n}
 \COMMENT{ Compute $x^b \pmod{n}$}\\
 z\GETS 1\\
 \WHILE b > 0 \DO
 \BEGIN
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd} 
    \THEN z \GETS z \cdot x  \pmod{n} \\
    b \GETS \CALL{ShiftRight}{b}
 \END\\
 \RETURN{z}
\end{pseudocode}


The purpose of the value \verb+display+ is to allow portions of
algorithms to be displayed with out the algorithm header. Thus
for example to display the section of code in the $\WHILE$ loop
of the $\CALL{SquareAndMultiply}{}$ algorithm one could write

\begin{verbatim}
\begin{center}
\begin{minipage}{2in}
\begin{pseudocode}[display]{}{}
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd}
    \THEN z \GETS z \cdot x  \pmod{n}\\
    b \GETS \CALL{ShiftRight}{b}
\end{pseudocode}
\end{minipage}
\end{center}
\end{verbatim}

\noindent which would produce the following output:

\begin{center}
\begin{minipage}{2in}
\begin{pseudocode}[display]{}{}
  z \GETS z^2 \pmod{n} \\
  \IF b\mbox{ is odd}
    \THEN z \GETS z \cdot x  \pmod{n}\\
     b \GETS \CALL{ShiftRight}{b}
\end{pseudocode}
\end{minipage}
\end{center}

\begin{thebibliography}{99}

\bibitem{LC}
M.\ Goossens, F.\ Mittelbach and A.\ Samarin, {\it The \LaTeX Companion}, 
Addison-Wesley, 1994.

\bibitem{cages}
D.L.\ Kreher and D.R.\ Stinson,
{\em Combinatorial Algorithms: Generation, Enumeration and Search},
CRC Press, 1999.

\bibitem{lamport}
L.\ Lamport, \LaTeX, 
{\em A Document Preparation System},
Addison-Wesley, 1994.
\end{thebibliography}
\end{document}