if statements in latex

Question

I'm trying to understand why the \mymap function in the following code does not produce any output when called with a variable in a for loop (see \myloop). It produces output when text is explicitly passed to it. See \myloopb. Sorry for the simple question, I wasn't able to understand explanations of the \if and \ifx statements on stackexchange and other places. If it matters I'm using pdflatex from MikTex on Windows.

\documentclass[12pt]{article}
\usepackage{pgffor}

% define variables
\def\vala{aa}
\def\valb{bb}

% this produces different output depending on #1
\newcommand{\mymap}[1]{
  \def\temparg{#1}
  \ifx\temparg\vala
  Condition 1 is true
  \fi
  \ifx\temparg\valb
  Condition 2 is true
  \fi
}

\newcommand{\myloop}{
  \foreach \myvar in {\vala,\valb}{
    % this does not produce any output
    \mymap{\myvar}
  }
}

\newcommand{\myloopb}{
    % this produces output
    \mymap{aa}
    \mymap{bb}
}

\begin{document}
\section{Using myloop does not produce output}
% no output
\myloop
\section{Using myloopb produces output}
% produces output
\myloopb
\end{document}

Answer 1

The issue is expansion here. \ifx tests whether the two following macros (if macros are passed) have the same meaning. In the first case \temparg has the meaning macro:->\myvar, whereas \vala has the meaning macro:->aa and \valb is macro:->bb.

Your \myvar has macro:->\vala and macro:->\valb as meaning in your loop. If you expand it twice before \mymap takes its argument, you get the correct meaning of \temparg. You can trigger earlier expansion with \expandafter<tokA><tokB>, which will expand <tokB> before <tokA> is expanded. So inside your loop use \expandafter\expandafter\expandafter\mymap\expandafter\expandafter\expandafter{\myvar}.

If your input is always fine to be fully expanded (it is in your example), you could use \edef\temparg{#1} instead, which is much cleaner than the \expandafter-chain proposed above.

\documentclass[12pt]{article}
\usepackage{pgffor}

% define variables
\def\vala{aa}
\def\valb{bb}

% this produces different output depending on #1
\newcommand{\mymap}[1]{%
  \edef\temparg{#1}%
  \ifx\temparg\vala
  Condition 1 is true%
  \fi
  \ifx\temparg\valb
  Condition 2 is true%
  \fi
}

\newcommand{\myloop}{%
  \foreach \myvar in {\vala,\valb}{%
    % this does not produce any output
    \mymap{\myvar}%
  }%
}

\newcommand{\myloopb}{%
    % this produces output
    \mymap{aa}%
    \mymap{bb}%
}

\begin{document}
\section{Using myloop does not produce output}
% no output
\myloop
\section{Using myloopb produces output}
% produces output
\myloopb
\end{document}

Answer 2

You can define a general purpose string case comparison.

\documentclass[12pt]{article}
\usepackage{pgffor}

% a string comparison function
\ExplSyntaxOn

\NewExpandableDocumentCommand{\StringCaseTF}{mm+m+m}
 {% #1 = string input, #2 = cases,
  % #3 = code to execute after a match
  % #4 = code to execute after no match
  \str_case_e:nnTF { #1 } { #2 } { #3 } { #4 }
 }

\ExplSyntaxOff

% define variables (not with \def)
\newcommand\vala{aa}
\newcommand\valb{bb}

% this produces different output depending on #1
\newcommand{\mymap}[1]{%
  \StringCaseTF{#1}{
    {\vala}{Condition 1 is true}
    {\valb}{Condition 2 is true}
  }{\par There was a match!}{\par There was no match!}%
  \par
}

\newcommand{\myloop}{%
  \foreach \myvar in {\vala,\valb,x}{
    % this does not produce any output
    \mymap{\myvar}
  }
}

\begin{document}

\myloop

\end{document}

Load xparse if you don't have the most recent LaTeX kernel.

The third or fourth arguments to \StringCaseTF can be left empty, of course.