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More on: Dealing with errors

Errors reported at ends of environments.

Some environments (notably amsmath alignments and tabularx tables) Scan the whole envionment body before processing the content. This means that any error within the environment is reported on the last line. However as seen in the main lesson, TeX’s display of the error context should still pinpoint the error location.

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align}
\alpha &= \frac{1}{2}\\
\beta  &= \frak{2}{3}\\
\gamma &= \frac{3}{4} 
\end{align}

\end{document}

Here the error will be reported on line 11

l.11 \end{align}

Although the real error is on line 9 as shown by the context lines:

! Undefined control sequence.
<argument> ...ha &= \frac {1}{2}\\ \beta &= \frak 
                                                  {2}{3}\\ \gamma &= \frac {...

Spurious errors due to earlier errors.

When calling LaTeX interactively from the command line it is possible to stop the processing at the first error with x, edite the document and re-run. However if you scroll past the error or use an editor or online system that does this for you then TeX will try to recover, however this may lead to several more errors being reported.

So do not be too concerned about the number of errors reported and always concentrate on fixing the first reported error.

\documentclass{article}

\begin{document}
Text_word  $\alpha + \beta$.

More text.
\end{document}

The error here is the underscore _ which should be entered as \_.

TeX does report this correctly with the first error message

! Missing $ inserted.
<inserted text> 
                $
l.4 Text_
         word  $\alpha + \beta$.
?

However if you scroll past the ? prompt then TeX recovers by adding a $ so the _ is seen in math mode as a subscript. The math mode then continues until the $ which ends math, so the following \alpha is seen in text mode generating another error

! Missing $ inserted.
<inserted text> 
                $
l.4 Text_word  $\alpha
                       + \beta$.
? 

Errors that do not trigger an error prompt

Some errors, especially errors that are not detected until the end of the file, do not generate an error prompt but just give a warning in the log.

If you try this example using the LaTeX CGI server it will return a PDF by default, to see the error message in the log add %!TeX log.

\documentclass{article}

\begin{document}

 Text {\large some large text)  normal size?

\end{document}

In this example the size change was mistakenly ended with ) rather than } This is not detected until the end of the file when TeX detects that there is still an unclosed group. It reports here the line at which the group was opened { It can not detect the actual error as the ) is seen as “normal text”.

(\end occurred inside a group at level 1)

### simple group (level 1) entered at line 5 ({)

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