Extract number from string and compare problem

Question

I try to extract the year, month and day from a string and write different text depending on the given date. I thought I could use xstring and etoolbox, but I can'st get it to work. Here's the code snippet:

\usepackage{xstring}
\usepackage{etoolbox}

\begin{document}
\newcommand{\Datum}{04.12.2011}

\newcommand{\Jahr}{\StrBehind[2]{\Datum}{.}}
\newcommand{\Monat}{\StrBetween[1,2]{\Datum}{.}{.}}
\newcommand{\Tag}{\StrBefore[1]{\Datum}{.}}

Jahr: \Jahr

\ifnumcomp{\Jahr}{=}{2011}{Jahr eq 2011}{Jahr neq 2011}

Which gives the following error:

pdflatex> ! Missing number, treated as zero.
pdflatex> <to be read again> 
pdflatex>                    \let 
pdflatex> l.11 {305}
pdflatex>           
pdflatex> ! Missing = inserted for \ifnum.
pdflatex> <to be read again> 

But \Jahr seems to correctly contain 2011.

Answer 1

Please avoid providing code snippets; instead, do create a complete MWE.

You are passing the complete xstring-test to ifnumcomp. That fails. You can save the the result of the xstring-test by the optional argument of \StrBehind and pass this to \ifnumcomp:

\documentclass{article}
\usepackage{xstring}
\usepackage{etoolbox}

\begin{document}
\newcommand{\Datum}{04.12.2011}

\StrBehind[2]{\Datum}{.}[\Jahr]
\StrBetween[1,2]{\Datum}{.}{.}[\Monat]
\StrBefore[1]{\Datum}{.}[\Tag]

Jahr: \Jahr

\ifnumcomp{\Jahr}{=}{2011}{Jahr eq 2011}{Jahr neq 2011}
\end{document}

Maybe you want to combine the output and the test you can use:

\documentclass{article}
\usepackage{xstring}
\usepackage{etoolbox}
\makeatletter
\newcommand\Datum[1]{\@datum{#1}\@executetest{#1}}
\def\@datum#1{#1}
\def\@executetest#1{%
\StrBehind[2]{#1}{.}[\Jahr]
\StrBetween[1,2]{#1}{.}{.}[\Monat]
\StrBefore[1]{#1}{.}[\Tag]
}
\begin{document}
\Datum{04.12.2011}

Jahr: \Jahr

\ifnumcomp{\Jahr}{=}{2011}{Jahr eq 2011}{Jahr neq 2011}
\end{document}

Answer 2

Modern parsing packages take care of some things that probably should be taken care of, but sometimes (or just as an illustrative example) it's simpler to use the TeX primitives directly:

\documentclass{article}

\def\datum{04.12.2011}

\def\Jahr{\expandafter\xJahr\datum\relax}
\def\Monat{\expandafter\xMonat\datum\relax}
\def\Tag{\expandafter\xTag\datum\relax}

\def\xJahr#1.#2.#3\relax{#3}
\def\xMonat#1.#2.#3\relax{#2}
\def\xTag#1.#2.#3\relax{#1}

\begin{document}


Jahr: \Jahr

\ifnum\Jahr=2011
Jahr eq 2011
\else
Jahr neq 2011
\fi

\end{document}

Answer 3

A different implementation with xparse:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\datetest}{ O{\Datum} m m m }
  {
   \seq_set_split:Nnx \l_datetest_date_seq { . } { #1 }
   \int_compare:nTF { \datetest_get:n { #2 } #3 #4 }
  }
\cs_new:Npn \datetest_get:n #1
  {
   \prg_case_str:nnn { #1 }
     {
      { Jahr }  { \seq_item:Nn \l_datetest_date_seq { 2 } }
      { Monat } { \seq_item:Nn \l_datetest_date_seq { 1 } }
      { Tag }   { \seq_item:Nn \l_datetest_date_seq { 0 } }
     }
     { 0 }
  }
\seq_new:N \l_datetest_date_seq
\cs_generate_variant:Nn \seq_set_split:Nnn { Nnx }
\ExplSyntaxOff


\newcommand{\Datum}{04.8.2011}

\begin{document}

\datetest{Jahr}{=}{2011}{Jahr eq 2011}{Jahr neq 2011}

\datetest{Monat}{<=}{9}{$\le$}{$>$}

\newcommand{\mydate}{01.02.2003}

\datetest[\mydate]{Monat}{>}{3}{After March}{Before April}

\datetest[01.05.2010]{Monat}{>}{3}{After March}{Before April}
\end{document}

\datetest acts by default on \Datum, but a macro or an explicit date can be passed as optional argument. The item to test is the first argument, the type of test (<, <=, =, >=, or >) as second argument; then the number to test and what to do if the test is either true or false.

Note that the second argument is a string, not a command. One can extend the macros to define \Jahr, \Monat or \Tag, if so desired.