pgfplotstable: correct way to generate new columns using custom commands in a table that can be plotted with tikz

Question

Sorry if the title is too big, but I didn't know how to express it as something specific, I try to generate new columns in a table using custom commands, for instance:

\documentclass{scrbook}
\usepackage[nomessages]{fp}
\usepackage{ifthen}
\usepackage{pgfplotstable}
\usepackage{pgfplots}

\newcommand{\newcoltest}[1]%my custom command
{
\ifthenelse{\isodd{#1}}
{\FPeval{\result}{clip(#1+1)}}
{\FPeval{\result}{clip(#1*3/2-1)}}
\result
}

\begin{document}

\pgfplotstableread{
T
1
2
3
4
5
6
7
8
9
10
}\loadedtable
\pgfplotstablecreatecol[
   create col/assign/.code={%
     \getthisrow{T}\vala
     \edef\newentry{\noexpand\newcoltest{\vala}}%
     \pgfkeyslet{/pgfplots/table/create col/next content}\newentry
   }%
]{Sa}{\loadedtable}
\pgfplotstabletypeset[string type]\loadedtable%critic line

\end{document}

the critic line is \pgfplotstabletypeset[string type]\loadedtable, which works only with string type, when I change to another type or even do not put anything, an error ocurrs.

I put \noexpand based on pgfplotstable: Use command when defining new column with create col/assign/.code?, since without this I think that my command is not recognized.

I also can't plot it using tikz, and I think this is related

Answer 1

Something like this?

\documentclass{scrbook}
\usepackage{xfp}
\usepackage{pgfplotstable}
\usepackage{pgfplots}

\newcommand{\newcoltest}[1]%my custom command
{%
\ifodd#1
\fpeval{trunc(#1+1)}%
\else
\fpeval{trunc(#1*3/2-1)}%
\fi
}

\begin{document}

\pgfplotstableread{
T
1
2
3
4
5
6
7
8
9
10
}\loadedtable
\pgfplotstablecreatecol[
   create col/assign/.code={%
     \getthisrow{T}\vala
     \edef\newentry{\noexpand\newcoltest{\vala}}%
     \pgfkeyslet{/pgfplots/table/create col/next content}\newentry
   }%
]{Sa}{\loadedtable}
\pgfplotstabletypeset\loadedtable%critic line

\end{document}

All I did was to make the stuff expandable by

• going from fp to xfp, and
• dropping ifthen in favor of a simple \ifodd.

Answer 2

Your idea of using \noexpand seems good to me, but prevents expansion of:

• the next token;

• only once, i.e.: if your \newentry is subject to another \edef, the \noexpand won't be there anymore to prevent expansion of \newcoltest—I guess this must be the cause of your troubles here.

To counter the second point, LaTeX2e's \protected@edef and \protect mechanism uses a clever trick to make the protection reinstate itself every time it is used inside one of the dedicated macros \protected@edef, \protected@write, etc. But e-TeX and derived engines (including pdfTeX, LuaTeX and XeTeX) have the \protected primitive which is more powerful, because it protects macros even inside engine primitives such as \edef and \write, and doesn't require one to use \protect. You just need to declare a macro as \protected and voilà, it won't be expanded inside \edef, \write, etc.

xparse's \NewDocumentCommand macro (as opposed to \NewExpandableDocumentCommand) creates \protected macros, because that is generally the safest thing to do unless you really know your macro is expandable. You can very simply use \NewDocumentCommand to define \newcoltest in order to solve your problem, even keeping \ifthenelse, \FPeval or other “non-expandable stuff” inside the replacement text of \newcoltest. Of course, there is no need for \noexpand in this case, since \NewDocumentCommand defines engine-protected macros—as if you had used e-TeX's \protected primitive.

\documentclass{article}
\usepackage[nomessages]{fp}
\usepackage{ifthen}
\usepackage{pgfplotstable}
\usepackage{pgfplots}
\usepackage{xparse}

\ExplSyntaxOn % No spurious spaces this way!

\NewDocumentCommand \newcoltest { m }
  {
    \ifthenelse{\isodd{#1}}
      {\FPeval{\result}{clip(#1+1)}}
      {\FPeval{\result}{clip(#1*3/2-1)}}
    \result
  }

\ExplSyntaxOff

\begin{document}

\pgfplotstableread{
T
1
2
3
4
5
6
7
8
9
10
}\loadedtable
\pgfplotstablecreatecol[
   create col/assign/.code={%
     \getthisrow{T}\vala
     \edef\newentry{\newcoltest{\vala}}%
     \pgfkeyslet{/pgfplots/table/create col/next content}\newentry
   }%
]{Sa}{\loadedtable}
\pgfplotstabletypeset[string type]\loadedtable

\end{document}

Besides, since you were talking conditionals with @Schrödinger'scat, you might be interested in knowing that expl3 has powerful expandable conditionals, for instance \bool_if:nTF, \str_case:nnTF and all their variants (see interface3.pdf after reading expl3.pdf).