Understanding Expansion Control

Oct 13, 2025 by miodeya.com 57 views

Introduction

When working with LaTeX3, it's not uncommon to encounter issues related to expansion control. One such issue arises when using the \str_if_eq:nnTF function, which is used to compare two strings. In this article, we'll delve into the world of expansion control and explore ways to manage the expansion of arguments in \str_if_eq:nnTF.

Understanding Expansion Control

Expansion control is a crucial aspect of LaTeX programming, especially when working with functions that take arguments. In LaTeX3, expansion control is achieved through the use of various functions and techniques. The \str_if_eq:nnTF function, in particular, requires careful management of expansion to ensure that it works as intended.

The Problem with \str_if_eq:nnTF

The \str_if_eq:nnTF function is used to compare two strings and return a boolean value indicating whether they are equal. However, when using this function, you may encounter issues related to expansion control. For example, if you pass a macro as an argument to \str_if_eq:nnTF, it may be expanded prematurely, leading to incorrect results.

A Real-World Example

Let's consider a real-world example to illustrate the issue. Suppose we have a main TeX file that contains content for two different languages. We want to create two separate PDFs, one for each language. The content for each language is prepared already, but we need to use the \str_if_eq:nnTF function to compare the language codes and determine which PDF to create.

\documentclass{article}
\usepackage{expl3}
\begin{document}
\newcommand{\languagecode}{en}
\newcommand{\languagecode2}{fr}
\str_if_eq:nnTF {\languagecode} {\languagecode2}
{\message{Creating French PDF}}
{\message{Creating English PDF}}
\end{document}

In this example, we define two macros, \languagecode and \languagecode2, which contain the language codes for English and French, respectively. We then use the \str_if_eq:nnTF function to compare these two macros and determine which PDF to create.

The Issue with Expansion

However, when we run this code, we may encounter issues related to expansion control. The \str_if_eq:nnTF function may expand the macros prematurely, leading to incorrect results. For example, if we define the macros as follows:

\newcommand{\languagecode}{\en}
\newcommand{\languagecode2}{\fr}

The \str_if_eq:nnTF function will expand the macros prematurely, leading to incorrect results. To fix this issue, we need to use techniques to control the expansion of the macros.

Controlling Expansion with \exp_not:N

One way to control the expansion of macros is to use the \exp_not:N function. This function takes a single argument and returns a token list that is not expanded. We can use this function to prevent the macros from being expanded prematurely.

\documentclass{article}
\usepackage{expl3}
\begin{document}
\newcommand{\languagecode}{\en}
\newcommand{\languagecode2}{\fr}
\str_if_eq:nnTF \exp_notN \languagecode \exp_notN \languagecode2
{\message{Creating French PDF}}
{\message{Creating English PDF}}
\end{document}

In this example, we use the \exp_not:N function to prevent the macros from being expanded prematurely. This ensures that the \str_if_eq:nnTF function receives the correct arguments and returns the correct result.

Controlling Expansion with \group_begin:nn

Another way to control the expansion of macros is to use the \group_begin:nn function. This function takes two arguments and returns a group that is not expanded. We can use this function to prevent the macros from being expanded prematurely.

\documentclass{article}
\usepackage{expl3}
\begin{document}
\newcommand{\languagecode}{\en}
\newcommand{\languagecode2}{\fr}
\group_begin:nn \languagecode} {\languagecode2}
\str_if_eqnnTF {\languagecode {\languagecode2}
{\message{Creating French PDF}}
{\message{Creating English PDF}}
\group_end:
\end{document}

In this example, we use the \group_begin:nn function to prevent the macros from being expanded prematurely. This ensures that the \str_if_eq:nnTF function receives the correct arguments and returns the correct result.

Conclusion

In conclusion, controlling expansion in \str_if_eq:nnTF is crucial when working with LaTeX3. By using techniques such as \exp_not:N and \group_begin:nn, we can prevent macros from being expanded prematurely and ensure that the function receives the correct arguments. This ensures that the function returns the correct result and helps to prevent errors in our LaTeX code.

Best Practices

When working with LaTeX3, it's essential to follow best practices to ensure that your code is efficient and error-free. Here are some best practices to keep in mind:

Use \exp_not:N to prevent macros from being expanded prematurely.
Use \group_begin:nn to prevent macros from being expanded prematurely.
Use \str_if_eq:nnTF with caution and ensure that the arguments are not expanded prematurely.
Test your code thoroughly to ensure that it works as intended.

Introduction

In our previous article, we explored the importance of controlling expansion in \str_if_eq:nnTF when working with LaTeX3. We discussed various techniques, such as using \exp_not:N and \group_begin:nn, to prevent macros from being expanded prematurely. In this article, we'll answer some frequently asked questions related to controlling expansion in \str_if_eq:nnTF.

Q: What is the difference between \exp_not:N and \group_begin:nn?

A: Both \exp_not:N and \group_begin:nn are used to prevent macros from being expanded prematurely. However, they work in different ways. \exp_not:N returns a token list that is not expanded, whereas \group_begin:nn returns a group that is not expanded. In general, \exp_not:N is used when you need to prevent a single macro from being expanded, whereas \group_begin:nn is used when you need to prevent a group of macros from being expanded.

Q: How do I use \exp_not:N to prevent a macro from being expanded?

A: To use \exp_not:N to prevent a macro from being expanded, simply wrap the macro in \exp_not:N. For example:

\newcommand{\languagecode}{\en}
\str_if_eq:nnTF {\exp_not:N \languagecode} {\en}
  {\message{Creating English PDF}}
  {\message{Creating French PDF}}

In this example, the \languagecode macro is wrapped in \exp_not:N to prevent it from being expanded prematurely.

Q: How do I use \group_begin:nn to prevent a group of macros from being expanded?

A: To use \group_begin:nn to prevent a group of macros from being expanded, simply wrap the group of macros in \group_begin:nn and \group_end:. For example:

\newcommand{\languagecode}{\en}
\newcommand{\languagecode2}{\fr}
\group_begin:nn {\languagecode} {\languagecode2}
  \str_if_eq:nnTF {\languagecode} {\languagecode2}
    {\message{Creating French PDF}}
    {\message{Creating English PDF}}
\group_end:

In this example, the group of macros \languagecode and \languagecode2 are wrapped in \group_begin:nn and \group_end: to prevent them from being expanded prematurely.

Q: What happens if I don't use \exp_not:N or \group_begin:nn?

A: If you don't use \exp_not:N or \group_begin:nn, the macros may be expanded prematurely, leading to incorrect results. For example, if you define the macros as follows:

\newcommand{\languagecode}{\en}
\newcommand{\languagecode2}{\fr}
\str_if_eq:nnTF {\languagecode} {\languagecode2}
  {\message{Creating French PDF}}
  {\message{Creating English PDF}}

The \languagecode and \languagecode2 macros may be expanded prematurely, leading to incorrect results.

Q: Can I use \exp_not:N and \group_begin:nn together?

A: Yes, you can use \exp_not:N and \group_begin:nn together to prevent a group of macros from being expanded prematurely. For example:

\newcommand{\languagecode}{\en}
\newcommand{\languagecode2}{\fr}
\group_begin:nn {\exp_not:N \languagecode} {\exp_not:N \languagecode2}
  \str_if_eq:nnTF {\languagecode} {\languagecode2}
    {\message{Creating French PDF}}
    {\message{Creating English PDF}}
\group_end:

In this example, the group of macros \languagecode and \languagecode2 are wrapped in \exp_not:N and then wrapped in \group_begin:nn and \group_end: to prevent them from being expanded prematurely.

Conclusion

In conclusion, controlling expansion in \str_if_eq:nnTF is crucial when working with LaTeX3. By using techniques such as \exp_not:N and \group_begin:nn, we can prevent macros from being expanded prematurely and ensure that the function receives the correct arguments. We hope this Q&A article has been helpful in answering some of the most frequently asked questions related to controlling expansion in \str_if_eq:nnTF.