"missing \begin{document}" when setting new command

Question

I'm trying to get this command into my LaTeX document but it causes the compiler to fail, saying "Missing \begin{document}":

\newcommand{\C*}{\mathbb{C} \setminus \lbrace 0 \rbrace}

I have also tried using \C0 instead of \C* thinking the asterisk might cause an issue, but to no change in the error message. The error message also says I can't use \mathbb{.} outside math mode, which I am not trying to do. Using \newcommand{\CC}{\mathbb{C}} is working fine in my document, I don't understand the difference to be honest and thus I can't figure out how to fix the issue.

Answer 1

Command names have to be letters or a single non letter you can not have a command \C* or \C0

After the first couple of expansions your input is more or less the same as this, which gives the same error.

\documentclass{article}


\newcommand{\C}{}{*}{\mathbb{C} \setminus \lbrace 0 \rbrace}

\begin{document}

\end{document}

So \C gets defined

 \newcommand{\C}{}

then it starts a paragraph of text with the symbol * which gives the missing document error as you can not typeset text before \begin{document}

{*}

Then if you scroll past that it tries to typeset

{\mathbb{C} \setminus \lbrace 0 \rbrace}

which gives an error because it is not in math mode.

Answer 2

You already know why it doesn't work. If you really want to have \C to do \mathbb{C} and \C* to do \mathbb{C}\setminus\{0\}, you can do it easily:

\usepackage{xparse}

\NewDocumentCommand{\C}{s}{%
  \mathbb{C}\IfBooleanT{#1}{\setminus\{0\}}%
}

With \IfBooleanT{#1} we check for the presence of *.

Full example

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

\NewDocumentCommand{\C}{s}{%
  \mathbb{C}\IfBooleanT{#1}{\setminus\{0\}}%
}

\begin{document}

The set of complex numbers is $\C$. If we remove~$0$
we get $\C*$.

\end{document}

If you prefer \C0, here it is:

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

\NewDocumentCommand{\C}{t{0}}{%
  \mathbb{C}\IfBooleanT{#1}{\setminus\{0\}}%
}

\begin{document}

The set of complex numbers is $\C$. If we remove~$0$
we get $\C0$.

\end{document}

The output is the same.

Answer 3

can't figure out how to fix the issue

If by "fix" you mean define a command using one "other" character, like the * or 0, you could use \@ifnextchar

\documentclass[a4paper, twocolumn]{article}
\begin{document}
  \makeatletter
    % What to do if there is a *
    % Need an argument to "eat" the *.
    % (so in the below, the argument #1
    % is the asterisk)
    \newcommand\CStar[2]{GotAStar and #2}
    % What to do otherwise
    \def\CNoStar{\relax}
    % Define \C and delegat what should happen
    % if it's followed by a *
    \def\C{\@ifnextchar*\CStar\CNoStar}
  \makeatother
  \C*{ARG}
\end{document}

The answers following question explains what happens here pretty good: Understanding \@ifnextchar