Initial Query

In latex when I force a newline in a caption with \\ it gives an error saying \Hy@tempa has an extra } runaway argument?

LLM Generation

Query

In latex when I force a newline in a caption with \\ it gives an error saying \Hy@tempa has an extra } runaway argument?

Output

Okay, that error message involving `\Hy@tempa` is a classic sign that the `hyperref` package is involved and is struggling with the `\\` command inside the `\caption`. Here's why it happens and how to fix it: **The Problem: `\caption` and `hyperref`** 1. **Moving Argument:** The argument of `\caption{...}` is what LaTeX calls a "moving argument." This means LaTeX doesn't just typeset it immediately; it also writes it to auxiliary files (like `.lof` for the List of Figures or `.lot` for the List of Tables) and potentially uses it for PDF bookmarks if you're using `hyperref`. 2. **Fragile Commands:** The `\\` command is a "fragile" command in LaTeX. Fragile commands can break or cause unexpected behavior when they are processed inside moving arguments or during certain complex macro expansions. 3. **`hyperref` Processing:** The `hyperref` package needs to process the caption text to create links and bookmarks. When it encounters the fragile `\\` command during this processing (often when reading from the `.lof`/`.lot` file or preparing bookmark data), it gets confused, leading to errors like the one you see (`\Hy@tempa` is an internal temporary macro used by `hyperref`). **Solutions** Here are the standard ways to fix this, ordered from most recommended to less common: 1. **Use the Optional Argument for `\caption` (Recommended)** This is the cleanest and most robust solution. The `\caption` command takes an optional argument that is used *only* for the List of Figures/Tables and bookmarks, and a mandatory argument that is used for the actual caption printed in the document. * Put the "clean" version (without `\\`) in the optional argument. * Put the version with the line break (`\\`) in the mandatory argument. 

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    \documentclass{article}
    \usepackage{graphicx}
    \usepackage{hyperref} % To demonstrate the issue and solution

    \begin{document}

    \listoffigures % Add LoF to see the effect

    \begin{figure}[htbp]
      \centering
      % \includegraphics{your_image} % Placeholder for your image
      \rule{5cm}{3cm} % Placeholder rectangle
      \caption[Short caption for LoF and bookmarks]% Optional argument (no \\)
              {This is the longer caption text \\ that needs a forced line break.}% Mandatory argument (with \\)
      \label{fig:example}
    \end{figure}

    \end{document}
*Why it works:* `hyperref` and the LoF/LoT mechanism only see the clean text from the optional argument. The fragile `\\` is only processed when the main caption is typeset directly in the document, where it's safe. 2. **Protect the Command with `\protect`** You can tell LaTeX explicitly not to expand the `\\` command when it's being written to the auxiliary files by prefixing it with `\protect`. 
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    \documentclass{article}
    \usepackage{graphicx}
    \usepackage{hyperref}

    \begin{document}

    \listoffigures

    \begin{figure}[htbp]
      \centering
      \rule{5cm}{3cm} % Placeholder rectangle
      \caption{This is the caption text \protect\\ with a protected line break.}
      \label{fig:example_protect}
    \end{figure}

    \end{document}
*Why it works:* `\protect` prevents `\\` from being expanded prematurely. It gets written literally as `\\` (or a safe equivalent) to the aux files. *Caveat:* While this often fixes the error, it *might* still lead to undesirable output (like a literal `\\` or unexpected spacing) in the PDF bookmarks or the List of Figures/Tables, depending on how `hyperref` handles `\protect\\` in those contexts. The optional argument method gives you more control. 3. **Use `\newline` instead of `\\`** Sometimes, `\newline` might work where `\\` fails, as it has slightly different internal workings. 
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    \documentclass{article}
    \usepackage{graphicx}
    \usepackage{hyperref}

    \begin{document}

    \listoffigures

    \begin{figure}[htbp]
      \centering
      \rule{5cm}{3cm} % Placeholder rectangle
      \caption{This is the caption text \newline using newline instead.}
      \label{fig:example_newline}
    \end{figure}

    \end{document}
*Caveat:* Similar caveats to `\protect\\` apply regarding bookmarks and LoF/LoT appearance. Also, `\newline` doesn't take an optional argument for extra vertical space like `\\` does (`\\[<dimen>]`). **Recommendation** Always prefer **Solution 1 (using the optional argument)**. It's the most idiomatic LaTeX way to handle captions that need different representations in the document versus the LoF/LoT/bookmarks, and it avoids potential issues with fragile commands interacting with packages like `hyperref`.

Substring Evaluation

Testing if the previous output contains the string \protect: True