Identification of an empty macro from pgfplots using \ifx

Question

Consider following MWE and the output thereafter. I have defined an empty macro and used \ifx to identify it as such. However, when an empty macro is received from \pgfkeysgetvalue, \ifx doesn't work.

\documentclass{article}
\usepackage{pgfplots, filecontents}
\pgfplotsset{compat=1.6}
\begin{filecontents}{test.dat}
  Time Distance
  0 0
  1 1
\end{filecontents}

\pgfplotstableset{
  columns/Distance/.style={
     column name={$D_{\alpha}$},
  }
}
\begin{document}
%
\def\tmp{}
\ifx\tmp\empty
\noindent Tmp is \tmp.\\
\fi
%
\pgfplotstableread{test.dat}{\loadedtable}%
\pgfplotstableforeachcolumn\loadedtable\as\col{%
  \pgfplotstableset{columns/\col/.try}%
  \pgfkeysgetvalue{/pgfplots/table/column name}\temp%
  \ifx\temp\empty%
    \noindent Temp is empty.\\%
  \else%
    \noindent Macro col is \col. Macro temp is \temp.\\%
  \fi%
}%
\end{document}

Output of MWE

I am not sure whether macro \temp returned by \pgfkeysgetvalue is \empty. But it doesn't print anything. I have looked at these related posts:

How to check if a macro value is empty or will not create text with plain TeX conditionals?

Why doesn't \ifx work inside a node command in TikZ?

What is the difference between \pgfutil@empty and \empty in an \ifx environment

I've taken the liberty to reenter the links such that they are slightly more informative and also parsed automatically. Please revert back if you don't feel that it's proper. — percusse, Nov 02 '12 at 21:20

score 7 · Accepted Answer · answered Nov 02 '12 at 21:20

7

If you modify your code to look like

  \pgfkeysgetvalue{/pgfplots/table/column name}\temp%
\show\temp
  \ifx\temp\empty%
    \noindent Temp is empty.\\%

You will see \temp is \relax in the no value case, so test for

\ifx\temp\relax

Don't forget to take the \show out of the production code.

answered Nov 02 '12 at 21:20

David Carlisle

757,742

Wonderful! Did the trick. – devendra Nov 02 '12 at 21:32
2

It only does for the MWE... but that does not load pgfplotstable - in which case /pgfplots/table/column name is undefined (=\relax). If you load pgfplotstable, it has a different value. – Christian Feuersänger Nov 03 '12 at 11:14
@ChristianFeuersänger thanks for the clarification, I know nothing I just scattered some \show in the MWE:-) – David Carlisle Nov 03 '12 at 11:46
@DavidCarlisle sure - I would have done the some. Thanks for the answer! – Christian Feuersänger Nov 03 '12 at 11:57

score 3 · Answer 2 · edited Jul 15 '17 at 16:19

Your MWE is incomplete: it does not load pgfplotstable. However, column name is only available within pgfplotstable.

What beats me is that you did not get an error although you used an undefined key. Ah - it is the /.try handler. Apparently, it swallows the nested error message. I was unaware of that behavior ...

Anyway, I checked what happens if you load pgfplotstable (as you did for sure in your larger production code). And: column name is not empty initially. It only expands to an empty value (more precisely: to \pgfkeysnovalue). Apparently, this has some special support which is (alas) not documented: - if you say column name={}, the column's display name is the empty string. - if you say column name=\pgfkeysnovalue, the column's display name defaults to the column's name. This is the default.

I will adjust the documentation.

Here is a suitable solution:

\documentclass{article}
\usepackage{pgfplots, filecontents}
\usepackage{pgfplotstable}% ---------- CF
\pgfplotsset{compat=1.6}
\begin{filecontents}{test.dat}
  Time Distance
  0 0
  1 1
\end{filecontents}

\pgfplotstableset{
  columns/Distance/.style={
     column name={$D_{\alpha}$},
  }
}

% --------- CF
\def\emptycolname{\pgfkeysnovalue}

\begin{document}
%
\def\tmp{}
\ifx\tmp\empty
\noindent Tmp is \tmp.\\
\fi
%
\pgfplotstableread{test.dat}{\loadedtable}%
\pgfplotstableforeachcolumn\loadedtable\as\col{%
  \begingroup% ---- CF
  \pgfplotstableset{columns/\col/.try}%
  \pgfkeysgetvalue{/pgfplots/table/column name}\temp%
  \ifx\temp\emptycolname% ----- CF
    \noindent Temp is empty.\\%
  \else%
    \noindent Macro col is \col. Macro temp is \temp.\\%
  \fi%
  \endgroup% ---- CF
}%

\pgfplotstabletypeset\loadedtable
\end{document}

I introduced four changes: first, I loaded pgfplotstable. Second, I defined \emptycolname to contain the value that pgfplotstable assumes if you never assigned a column name: \pgfkeysnovalue. Third, a used that value in the \ifx comparison. Last (but not least), I introduced \begingroup ... \endgroup inside of your loop. Otherwise, you will inherit the column name of the previous loop iteration. I also added \pgfplotstabletypeset to see that it works.

See http://pgfplots.sourceforge.net/TeX-programming-notes.pdf for how to transport local variables outside of a TeX group (if you need it).

Identification of an empty macro from pgfplots using \ifx

2 Answers2

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