Documents produced with the commands you have learned up to this point will look acceptable to a large audience. While they are not fancy-looking, they obey all the established rules of good typesetting, which will make them easy to read and pleasant to look at. However, there are situations where LaTeX does not provide a command or environment that matches your needs, or the output produced by some existing command may not meet your requirements.

In this chapter, we will try to give some hints on how to teach LaTeX new tricks and how to make it produce output that looks different from what is provided by default.

LaTeX is a fairly high-level language compared to Plain TeX and thus is more limited. The next chapter will focus on Plain TeX and will explain advanced techniques for programming.

New commands

To add your own commands, use the

\newcommand{\name}[num]{definition}

command. Basically, the command requires two arguments: the name of the command you want to create (preceded by a backslash), and the definition of the command. Note that the command name can but need not be enclosed in braces, as you like. The num argument in square brackets is optional and specifies the number of arguments the new command takes (up to 9 are possible). If missing it defaults to 0, i.e. no argument allowed.

The following two examples should help you to get the idea. The first example defines a new command called \wbal that will print “The Wikibook about LaTeX”. Such a command could come in handy if you had to write the title of this book over and over again.

\newcommand{\wbal}{The Wikibook about \LaTeX}
This is ‘‘\wbal'' \ldots{} ‘‘\wbal''

The next example illustrates how to define a new command that takes one argument. The #1 tag gets replaced by the argument you specify. If you wanted to use more than one argument, use #2 and so on, these arguments are added in an extra set of brackets.

\newcommand{\wbalsup}[1] {
  This is the Wikibook about LaTeX 
  supported by #1}
\newcommand{\wbalTwo}[2] {
  This is the Wikibook about LaTeX
  supported by #1 and #2}
% in the document body:
\begin{itemize}
\item \wbalsup{Wikimedia}
\item \wbalsup{lots of users!}
\item \wbalTwo{John Doe}{Anthea Smith}
\end{itemize}

Name your new command \wbalTwo and not \wbal2 as digits cannot be used to name macros — invalid characters will error out at compile-time.

LaTeX will not allow you to create a new command that would overwrite an existing one. But there is a special command in case you explicitly want this: \renewcommand. It uses the same syntax as the \newcommand command.

In certain cases you might also want to use the \providecommand command. It works like \newcommand, but if the command is already defined, LaTeX will silently ignore the new command.

With LaTex2e, it is also possible to add a default parameter to a command with the following syntax:

\newcommand{name}[num][default]{definition}

If the default parameter of \newcommand is present, then the first of the number of arguments specified by num is optional with a default value of default; if absent, then all of the arguments are required.

\newcommand{\wbalTwo}[2][Wikimedia]{
  This is the Wikibook about LaTeX
  supported by {#1} and {#2}!}
% in the document body:
\begin{itemize}
\item \wbalTwo{John Doe}
\item \wbalTwo[lots of users]{John Doe}
\end{itemize}

Note

When the command is used with an explicit first parameter it is given enclosed with brackets (here "[lots of users]").

Here is a common example: if you are writing a book about Mathematics and you have to use vectors, you have to decide how they will look. There are several different standards, used in many books. If a is a vector, some people like to add an arrow over it (${\displaystyle {\vec {a}}}$), other people write it underlined (a); another common version is to write it bold (a). Let us assume you want to write your vectors with an arrow over them; then add the following line in your mystyle.sty.

\newcommand{\myvec}[1]{\vec{#1}}

and write your vectors inside the new \myvec{...} command. You can call it as you wish, but you'd better choose a short name because you will probably write it very often. Then, if you change your mind and you want your vectors to look differently you just have to change the definition of your \myvec{...}. Use this approach whenever you can: this will save you a lot of time and increase the consistency of your document.

New environments

Just as with the \newcommand command, there is a command to create your own environments. The \newenvironment command uses the following syntax:

\newenvironment{name}[num][default]{before}{after}

Again \newenvironment can have an optional argument. When the \begin{name} command (which starts the environment) is encountered, the material specified in the before argument is processed before the text in the environment gets processed. The material in the after argument gets processed when the \end{name} command (which ends the environment) is encountered.

The optional num and default arguments are used the same way as in the \newcommand command. LaTeX makes sure that you do not define an environment that already exists. If you ever want to change an existing environment, you can use the \renewenvironment command. It uses the same syntax as the \newenvironment command.

The example below illustrates the usage of the \newenvironment command:

\newenvironment{king}
{ \rule{1ex}{1ex}\hspace{\stretch{1}} }
{ \hspace{\stretch{1}}\rule{1ex}{1ex} }

\begin{king}
My humble subjects \ldots
\end{king}

\newenvironment{simple}%
{\noindent}%
{\par\noindent}

\begin{simple}
See the space\\to the left.
\end{simple}
Same\\here.

  See the space
to the left.

  Same
here.

\newenvironment{correct}%
{\noindent\ignorespaces}%
{\par\noindent%
\ignorespacesafterend}

\begin{correct}
No space\\to the left.
\end{correct}
Same\\here.

No space
to the left.

Same
here.

\begin{correct}\input{somefile.tex}\end{correct}

\begin{correct}%
\input{somefile.tex}%
\end{correct}

Declare commands within new environment

New commands can be declared within newenvironment. Commands declared within the newenvironment refer to their arguments by doubling the # character. In the following example, a new environment is declared along with a nested command:

\newenvironment{topics}{
\newcommand{\topic}[2]{ \item{##1 / ##2} }
Topics:
\begin{itemize}
}
{
\end{itemize}
}

If, by mistake, the arguments passed to the \topics macro are defined with a single # character, the following error message will be thrown:

   ! Illegal parameter number in definition of \topics.

Extending the number of arguments

The xkeyval packages will let you define key/value options for commands.

\mycommand[key1=value1, key3=value3]{some text}

The package is quite complete and documentation is exhaustive. We recommend that package developers read it.

Let's provide a simple example^[1]:

\usepackage{xkeyval}
% ...

\makeatletter
\def\my@emphstyle#1{\csname my@style@#1\endcsname}
%% Predefined styles
\providecommand\my@style@default{\em}
\providecommand\my@style@bold{\bfseries}

\define@key{myemph}{code}{%
  \def\my@emphstyle{#1}
}
\define@key{myemph}{style}{%
  \def\my@emphstyle{\csname my@style@#1\endcsname}
}
\newcommand\setemph[1]{%
  \setkeys{myemph}{#1}
}

\renewcommand\emph[1]{%
  {\my@emphstyle #1}
}

\makeatother

Something \emph{important}

\setemph{style=bold}
Something \emph{important}

\setemph{code=\Large\sffamily}
Something \emph{important}

Arithmetic

LaTeX can manipulate numbers.

The calc package provides the common infix notation.

\usepackage{calc}
% ...
\newcounter{mine}
\setcounter{mine}{2*17}
\themine

For high-precision computations, you can use the fp^[2] package.

\usepackage{fp}

% Clip
\[
\FPmul\result{2}{7}
\FPclip\result\result
2*7 = \result
\]

% Infix
\[
\newcommand\result{11}
\sqrt{\sin(2+\result)} \approx
\FPeval\result{round(root(2,sin(result + 2.5)),2)}
\result
\]

% Postfix
\[
\FPupn\result{17 2.5 + 17.5 swap - 2 1 + * 2 swap /} % or \FPupn\result{2 17.5 17 2.5 + - 2 1 + * /}
\FPclip\result\result
(17+2.5 - 17.5) * (2+1) / 2  = \result
\]

% High precision
\[
\FPdiv\result{17}{7}
\frac{17}{7} \approx \FPtrunc\result\result{3}
\result
\]

Conditionals

LaTeX can use conditionals thanks to the ifthen package.

\usepackage{ifthen}
% ...

\ifthenelse{ \equal{\myvar}{true} }{
  This is true.
}{
  This is false.
}

FYI: For newer projects e-TeX is recommended^[3], available in LaTeX from etoolbox. See section 3.6 of their manual.

Following a short example, a bool that switches its state every time it is used.^[4]

\documentclass{article}
\usepackage{etoolbox}
\usepackage{parskip}
\usepackage{tikz}
\newbool{volt}
\newcommand{\onoff}{%
    \ifbool{volt}{%
\boolfalse{volt}\candleOn\space ON}{%
    \booltrue{volt}\candleOff\space OFF}%
}
\newcommand{\candleOff}{\tikz \draw (0,0) rectangle
(.2,.8);}
\newcommand{\candleOn}{\begin{tikzpicture}\draw (0,0) rectangle
(.2,.8);\draw [fill=orange] (.1,.9) circle
[radius=.1];\end{tikzpicture}
}
\begin{document}
\onoff\par
\onoff\par
\onoff\par
\onoff\par
\end{document}

Loops

The PGF/TikZ extension provides the \foreach command.

\usepackage{tikz}
% ...

\foreach \i/\q in {wheat/50g, water/1L, yeast/2g}{
  \noindent\i\dotfill\q\\
}

If you are only using \foreach and not drawing graphics, you may instead use the pgffor package directly.

Alternatively you can check out the multido package.

Strings

xstring provides a lot of features. From CTAN:

• testing a string’s contents
• extracting substrings
• substitution of substrings
• string length
• position of a substring
• number of recurrences of a substring

Examples:

\usepackage{xstring}
% ...

\newcommand\mystr{Hello World!}

The string ``\mystr'' has \StrLen{\mystr}{} characters.

Predicate ``\mystr{} contains the word Hello'' is \IfSubStr{\mystr}{Hello}{true}{false}.

LaTeX Hooks

LaTeX provide two hooks:

• \AtBeginDocument will let you specify a set of commands that will be executed when \begin{document} is met.
• \AtEndDocument does the same for \end{document}.

This gives you some more flexiblity for macros. It can be useful to override settings that get executed after the preamble. These hooks can be called several times. The commands will be executed in the order they were set.

For instance, let's replace the page numbers with oldstylenums:

\usepackage{textcomp}

\AtBeginDocument{%
  % Make the page numbers in text figures
  \let\myThePage\thepage
  \renewcommand{\thepage}{ \oldstylenums{\myThePage} }
}

There are also hooks for classes and packages. See Creating Packages.

Command-line LaTeX

If you work on a Unix-like OS, you might be using Makefiles or any kind of script to build your LaTeX projects. In that connection it might be interesting to produce different versions of the same document by calling LaTeX with command-line parameters. If you add the following structure to your document:

\usepackage{ifthen}
%...

% default value.
\providecommand\blackandwhite{false}
%...

\ifthenelse{ \equal{\blackandwhite}{true} }{
% "black and white" mode; do something..
}{
% "color" mode; do something different..
}

Now you can call LaTeX like this:

latex '\providecommand{\blackandwhite}{true}\input{test.tex}'

First the command \blackandwhite gets defined and then the actual file is read with input. By setting \blackandwhite to false the color version of the document would be produced.

Notes and References