How does this partial-string color-highlighting code work?

Question

I'm using the awesome-cv class, which highlights the first three letters of the section heading like so:

The code that produces this effect looks like this:

\RequirePackage{xcolor}
\RequirePackage{xifthen}
\RequirePackage{etoolbox}
\RequirePackage[quiet]{fontspec}
\RequirePackage{fontawesome}

\definecolor{awesome-red}{HTML}{DC3522}
\colorlet{awesome}{awesome-red}

\newcounter{colorCounter}
\def\@sectioncolor#1#2#3{%
  {%
    \color{%
      \ifcase\value{colorCounter}%
        awesome\or%
        awesome\or%
        awesome\or%
        awesome\or%
        awesome\else%
        awesome\fi%
    } #1#2#3%
  }%
  \stepcounter{colorCounter}%
}

The text size and boldface are added later in

\newcommand*{\sectionstyle}[1]{{\fontsize{15pt}{1em}\bodyfont\bfseries\color{text}\@sectioncolor #1}}

How does the first part of this code highlight the first three letters?

---

I understand that this isn't a MWE, but I wasn't able to create one out of the .cls file and I don't think this code is that complex to someone who knows what they're looking at. If I need and MWE, let me know.

Answer 1

Actually, this trick is quite simple: Arguments to normal TeX commands can either be single tokens (i.e. characters or \controlsequences), or they can be token sequences enclosed in braces. For example this is why, in math mode, you can write \frac 1 2, \frac{1}{2} or even \frac1{2} and get the same result.

Therefore, all you need to do to get the first three characters of a word is a command with three arguments. For example,

\newcommand\mycommand[3]{%
    \textcolor{red}{#1}%
    \textcolor{green}{#2}%
    \textcolor{blue}{#3}%
}

allows you to highlight the first three letters following an invocation like \mycommand hello. The lo in this case is simply not part of the command.

To get the familiar syntax with just one parameter in braces, you can wrap this command in another command:

\newcommand*\myothercommand[1]{%
    \mycommand #1%
}

This allows you to write \myothercommand{hello}.

Here's a MWE that demonstrates a stripped-down version of the command with different colors:

\documentclass{article}
\RequirePackage{xcolor}
\RequirePackage{xifthen}

\definecolor{awesome-red}{HTML}{DC3522}
\colorlet{awesome}{awesome-red}

\newcounter{colorCounter}
\newcommand\sectioncolor[3]{%
  {%
    \color{%
      \ifcase\value{colorCounter}%
        awesome\or%
        green\or%
        blue\or%
        orange\or%
        yellow\else%
        cyan\fi%
    } #1#2#3%
  }%
  \stepcounter{colorCounter}%
}

\newcommand*{\sectionstyle}[1]{{\fontsize{15pt}{1em}\bfseries\sectioncolor #1}}

\begin{document}
    \sectionstyle{first}\par
    \sectionstyle{second}\par
    \sectionstyle{third}\par
    \sectionstyle{fourth}\par
    \sectionstyle{fifth}\par
    \sectionstyle{sixth}\par
    \sectionstyle{seventh}
\end{document}