Missing $ and }?

Question

I'm getting a "Missing $ inserted" and "Missing } inserted. Extra }, or forgotten \endgroup"

Here's the code:

\documentclass{article}

\usepackage{calc}
\usepackage{ifthen}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{mathtools}
\usepackage{amsmath}
\usepackage{amssymb}
\begin{document}
\begin{center}

\begin{flushleft}
As is clearly shown in this bar chart that is absolutely,
incontrovertibly, and significantly different from all other
figures shown previously, the doramus bar goes ``Woo!'' because
it can fly, but notice how tidanme is actually quite sad due to a
physical disorder that has prevented it from being taller. The
disruptive selection is clear here though, as sound frequencies
that are really really high (RRH) are very common while sound
frequencies that are really really low (RRL) are common when
added together and multiplying by $(i(2z)+42)$ where $z$ \in
\mathbb{R}\backslash\frac{\pi}{2}.

\end{flushleft}

\end{document}

Why is it telling me these? The code works fine. Thanks!

Answer 1

If I minimise your example and run pdflatex on it

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

\begin{center}

\begin{flushleft}
As is clearly shown in this bar chart that is absolutely,
incontrovertibly, and significantly different from all other
figures shown previously, the doramus bar goes ``Woo!'' because
it can fly, but notice how tidanme is actually quite sad due to a
physical disorder that has prevented it from being taller. The
disruptive selection is clear here though, as sound frequencies
that are really really high (RRH) are very common while sound
frequencies that are really really low (RRL) are common when
added together and multiplying by $(i(2z)+42)$ where $z$ \in
\mathbb{R}\backslash\frac{\pi}{2}.

\end{flushleft}

\end{document}

I do indeed get the error

! Missing $ inserted.
<inserted text> 
                $
l.17 ... multiplying by $(i(2z)+42)$ where $z$ \in

This error message has actually shown me where the problem is, it's with, as Christian Hupfer points out in the comments, \in.

Since \in is a math mode command, it needs to be in math mode, which is why the missing $ has been inserted for you. This is why the code appears to work fine, the missing $ has been put it and your code has been corrected, but you cannot rely on it to make a very good job of cleaning up any mistakes in the general case. Nevertheless if we ignore the error and plough on through, with the missing $ in place, I get another

! Missing $ inserted.
<inserted text> 
                $
l.20

error. This is because the first missing $ was put in, but of course an opening $ needs a closing $ which hadn't been put in yet. So now it will put one in and we can plough on through again.

You will then pick up a few more errors caused, I guess, by ignoring the first one and the fact that

! LaTeX Error: \begin{center} on input line 7 ended by \end{document}.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.23 \end{document}

Or, in other words, you've forgotten the \end{center}, as LShaver points out in the comments. Once again these errors can be corrected for successfully in this case, which is why you do get an essentially functioning output, but this is not to be relied upon in general. Instead we need to fix our errors when we encounter them.

I have a few general comments to make. The first is that we need to enclose the entire mathematical expression in $ ... $. You are quite right, actually, to enclose the z in $ ... $ and what you have would be correct if you just had the mathematical variable on its own in a sentence, e.g.:

where $z$ is real.

Here you have used the correct logical markdown to mark out z as a mathematical object, it will be in the correct typeface and you have the whole of the single, logical mathematical unit within one pair of $ ... $.

However in your example, the mathematical unit does not end at the z and it needs to take into account everything that follows. Beware of the temptation many new users have to see $ ... $ as a kind of hack to get special symbols. Things like

where z $\in$ $\mathbb{R}$ \textbackslash $\frac{$\pi$}{2}$

are easy mistakes that people who are new to LaTeX sometimes make. Instead, it's important to view $ ... $ as marking out a logical, semantic unit, it says, this is my mathematical object/unit and the whole lot needs to go in there because $ ... $ doesn't just give special symbols, it sets you up for mathematics, giving you proper spacing, the proper fonts, proper kerning and so and and so forth.

So, instead we should have

added together and multiplying by $(i(2z) + 42)$ where $z \in
\mathbb{R} \backslash \frac{\pi}{2}$.

Also, I think you should use \setminus instead of \backslash. \setminus is more logical semantically, the command name does what it says on the tin, but more importantly it gives you the correct spacing around a binary operator. Compare:

\[
  z \in \mathbb{R} \setminus \frac{\pi}{2} \quad \text{and} \quad z
  \in \mathbb{R} \backslash \frac{\pi}{2}
\]

If you use hard-wrapping, as I have done in the question and answer, it can make the error messages (which include a line number, as, for example, l.17) more useful, but this is a matter of choice.

If you indent your code it can be easier to spot problems such as missing \end{environment}s

For example, if I indent the minimised version of your code I get

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

\begin{center}
  \begin{flushleft}
    As is clearly shown in this bar chart that is absolutely,
    incontrovertibly, and significantly different from all other
    figures shown previously, the doramus bar goes ``Woo!'' because it
    can fly, but notice how tidanme is actually quite sad due to a
    physical disorder that has prevented it from being taller. The
    disruptive selection is clear here though, as sound frequencies
    that are really really high (RRH) are very common while sound
    frequencies that are really really low (RRL) are common when added
    together and multiplying by $(i(2z) + 42)$ where
    $z \in \mathbb{R} \setminus \frac{\pi}{2}$.
  \end{flushleft}

\end{document}

We can see that something is wrong because we're still one level of indentation in. You might also like to use an editor which can insert a complete environment for you so that you don't forget. Try reading this list.

With those corrections applied, we should get no errors and good output:

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

\begin{center}
  \begin{flushleft}
    As is clearly shown in this bar chart that is absolutely,
    incontrovertibly, and significantly different from all other
    figures shown previously, the doramus bar goes ``Woo!'' because it
    can fly, but notice how tidanme is actually quite sad due to a
    physical disorder that has prevented it from being taller. The
    disruptive selection is clear here though, as sound frequencies
    that are really really high (RRH) are very common while sound
    frequencies that are really really low (RRL) are common when added
    together and multiplying by $(i(2z) + 42)$ where
    $z \in \mathbb{R} \setminus \frac{\pi}{2}$.
  \end{flushleft}
\end{center}

\end{document}