In this post, I want to summarize what I have learned about creating algorithmic pseudo code in LaTeX.

In LaTeX, there are several packages which can help you to write pseudo code, notably algorithmicx and algorithm2e. It seems that algorithm2e is more actively maintained1. I decided to give algorithm2e a try. The end result is shown in the title image. The code to create the title image is

\documentclass{article}

\usepackage[ruled, lined, linesnumbered, commentsnumbered, longend]{algorithm2e}
\usepackage{xcolor}

\begin{document}

\newcommand\mycommfont[1]{\small\ttfamily\textcolor{blue}{#1}}
\SetCommentSty{mycommfont}

\begin{algorithm}
    \SetKwFunction{isOddNumber}{isOddNumber}
    % \SetKwInput{Input}{Input}
    % \SetKwInput{Output}{Output}
    \SetKwInOut{KwIn}{Input}
    \SetKwInOut{KwOut}{Output}

    \KwIn{A list $[a_i]$, $i=1, 2, \cdots, n$, where each element is an
    integer.}
    \KwOut{Processed list.}

    $newList = [\ ]$

    \tcc{For odd elments in the list, we add 1, and for even elments, we add 2.
    After the loop, all elements are even.}
    \For{$i \leftarrow 0$ \KwTo $n-1$}{
        \eIf{$\isOddNumber(a_i)$}{

            $newList.append(a_i + 1)$ \tcp*[f]{Some thought-provoking comment.}
         }{
            \tcp{Another comment}
            $newList.append(a_i + 2)$
         }
    }

    \KwRet{$newList$}
    \caption{A plain algorithm}
\end{algorithm}
\end{document}

Now I am going to explain the code in detail.

Controling the algorithm style

There are certain options for the algorithm2e package which control the algorithm style. They can be specified when you import the package:

\usepackage[ruled, lined, linesnumbered, commentsnumbered, longend]{algorithm2e}

The meaning of these options are list below:

  • ruled: display algorithm like three-line table.
  • lined: display indentation line so that you can clearly see the indentation level.
  • linenumbered: number the statement line.
  • commentnumbered: number the comment line.
  • longend: used end for to end for loop and use end if to end if condition.

Keyword input, output and function

Define keyword input and output

First we need to specify the input and output of the algorithm. algorithm2e provides \KwIn and \KwOut command for input and output respectively:

\KwIn{A list $[a_i]$, $i=1, 2, \cdots, n$, where each element is an integer.}
\KwOut{Processed list.}

These keyword parameters are not aligned in its : symbol. If you prefer an aligned keyword, you need to define the input and output keyword yourslef like the following code:

\SetKwInOut{KwIn}{Input}
\SetKwInOut{KwOut}{Output}

\KwIn{A list $[a_i]$, $i=1, 2, \cdots, n$, where each element is an
integer.}
\KwOut{Processed list.}

In the above code, we redefine the \KwIn and \KwOut command with \SetKwInOut. \SetKwInOut will make sure that the defined keyword command will algin at the : character.

Define keyword function

You can define a keyword function via \SetKwFunction{COMMAND}{TEXT} command.

COMMAND is used to define the keyword function command, and TEXT is the text that will be shown when you invoke the command, for example:

\SetKwFunction{isOddNumber}{isOddNumber}

Then you can invokde the command via \isOddNumber.

Comment

By default, the comment used by algorithm2e has its default style (font size, character color, etc.). If you are not statisfied with comment style, you can change it via \SetCommentSty{} command. For example, the following code, we change the comment font size, font style and color:

\usepackage{xcolor}
\newcommand\mycommfont[1]{\small\ttfamily\textcolor{blue}{#1}}
\SetCommentSty{mycommfont}

Algorithm2e provides \tcc and \tcp command to create multi-line and single line C-style comments respectively.

For example, you can use \tcc to create a multi-line comment:

\tcc{For odd elments in the list, we add 1, and for even elments, we add 2.
After the loop, all elements are even.}

The created comment looks like:

You can use \tcp to create single line comment. You can put \tcp{} command on a separate line to the statement you want to comment:

\tcp{Another comment}
$newList.append(a_i + 2)$

The produced comment is like:

When you want to put \tcp in the same line with the statement, there is also a stared version \tcp*[OPTION] {} with options. The options is used to change the comment style slightly. For example, the following code:

$newList.append(a_i + 1)$ \tcp*[f]{Some thought-provoking comment.}

will create inline comment shown below.

References


  1. More discussions about different packages to produce pseudo in LaTeX can be found here and here.