Passing command output as a parameter to the other command

Question

I created some kind of switch statement with a help of \ifthenelse command. Now, I would like to pass its output to another command. In my example I want to colorize each sentence with a color specified by a number. My code is:

\documentclass{article}

\usepackage{ifthen}
\usepackage{color}

\newcommand{\num}{1}

 \newcommand{\mySwitch}[1]{
 \ifthenelse{#1=1}{red}{        % color no 1 is red
 \ifthenelse{#1=2}{blue}{}}}    % color no 2 is blue ... and so on 

\begin{document}

\color{\mySwitch{\num}}
\mySwitch{\num} is the color no \num.

\renewcommand{\num}{2}
\color{\mySwitch{\num}}
\mySwitch{\num} is the color no \num.

\end{document}

I would like to obtain something like this:

but the result of compilation is:

What is more, I keep getting a number of errors from which the first one is

Missing \endcsname inserted \color{\NoToColor{\num}}.

I suppose the proper solution should have something to do with \expandafter command but simply putting

 \expandafter\color\expandafter{\mySwitch{\num}} 

gives no results. I must be missing something but I can't get it to work and I can't find any similar example. Please give some hints.

Answer 1

We need expanable switch statement. When egreg use \ExplSyntaxOn ... \ExplSyntaxOff then I can use NormalTeXSyntaxOn ... NormalTeXSynatxOff in my example, because NormalTeXSyntax is more natural than whatever else syntax. It is syntax of TeX program itself. If you know this syntax well then you needn't to know any other syntax.

I hope that somebody will understand me...

\documentclass{article}
\usepackage{color}

% NormalTeXSyntaxON
\def\setcase#1 {\expandafter\def\csname col:#1\endcsname}
\def\mySwitch#1{\expandafter\ifx\csname col:#1\endcsname\relax \coldefault 
                \else \csname col:#1\endcsname\fi}

\def\coldefault {black}    
\setcase 1      {red} 
\setcase 2      {blue}  
\setcase word   {green}

% NormalTeXSyntaxOff

\begin{document}

\newcommand\num{1}
\color{\mySwitch{\num}}
\mySwitch{\num} is the color no \num.

\renewcommand\num{2}
\color{\mySwitch{\num}}
\mySwitch{\num} is the color no \num.

\renewcommand\num{word}
\color{\mySwitch{\num}}
\mySwitch{\num} is the color no \num.

\end{document}

I mean that the accepted solution here is overkilled. The \usepackage{xparse} used in this solution reads 24 files with 16648 lines in total but my solution needs only three lines without any aditional packages. The effect of both solutions are the same. But people tend to select more complicated solutions here. Unfortunatelly.

Answer 2

The argument to \color (when called without the optional color space argument) must directly expand to a string of characters, which should be a color name (or a tint specification if xcolor is used).

There are several expandable switch statement available. I suggest one using expl3:

\documentclass{article}
\usepackage{xparse}
\usepackage{color}

\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\mySwitch}{m}
 {
  \martino_switch:n { #1 }
 }

\cs_new:Npn \martino_switch:n #1
 {
  \str_case:onF { #1 } % expand once the argument
   {
    {1}{red}
    {2}{blue}
   }
   {black} % none of the above
 }
\ExplSyntaxOff

\begin{document}

\newcommand{\test}{1}

\textcolor{\mySwitch{\test}}{Some text}

\renewcommand{\test}{2}
\textcolor{\mySwitch{\test}}{Some text}

\end{document}

As you see, adding new cases is quite easy.

The strings are not limited to numbers:

\documentclass{article}
\usepackage{xparse}
\usepackage{color}

\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\mySwitch}{m}
 {
  \martino_switch:n { #1 }
 }

\cs_new:Npn \martino_switch:n #1
 {
  \str_case:onF { #1 } % expand once the argument
   {
    {1}{red}
    {2}{blue}
    {zot}{yellow}
   }
   {black} % none of the above
 }
\ExplSyntaxOff

\begin{document}

\newcommand{\test}{1}

\textcolor{\mySwitch{\test}}{Some text}

\renewcommand{\test}{2}
\textcolor{\mySwitch{\test}}{Some text}

\renewcommand{\test}{zot}
\textcolor{\mySwitch{\test}}{Some text}

\renewcommand{\test}{blurb}
\textcolor{\mySwitch{\test}}{Some text}

\end{document}