Logical String Length

Question

How do I do a conditional 'if then else' statement, based on length of string being greater than (or less than, or equal) a specified value.

For example, something Like

\def\mystring{XYZ}
\def\mythresh{1}
\ifthenelse{\stringlength{\mystring} > \mythresh}{TRUE}{FALSE}

where \stringlength is 'pseudo' for the relevant command, which I am unsure which is the most widely used.

Does the logical greater than operator apply here, or is there another command?

Answer 1

Use the xstring package:

\documentclass{article}
\usepackage{xstring,xifthen}
\begin{document}
\def\mystring{XYZ}
\StrLen{\mystring}[\mystringlen]

\def\mythresh{1}
\ifthenelse{\mystringlen > \mythresh}{TRUE}{FALSE}
\def\mythresh{3}
\ifthenelse{\mystringlen > \mythresh}{TRUE}{FALSE}

\end{document}

\StrLen{<stuff>}[<name>] stores the length of <stuff> in the control sequence <name>.

The use of xifthen is not really necessary. See How to form “if … or … then” conditionals in TeX? and the related Why is the ifthen package obsolete? for alternatives.

---

stringstrings provides similar functionality:

\documentclass{article}
\usepackage{stringstrings}
\begin{document}
\def\mystring{XYZ}
\stringlength[q]{\mystring}% Result is stored in \theresult

\def\mythresh{1}
\ifnum\theresult>\mythresh TRUE \else FALSE\fi    
\def\mythresh{3}
\ifnum\theresult>\mythresh TRUE \else FALSE\fi
\end{document}

Answer 2

Werner's answer answers the question as stated, and I'm not suggesting you unaccept it, but as clarified in comments the actual aim was to check that an input argument has more than (say) 3 characters. That is much simpler and doesn't involve counting anything.

To give an example, the following document shows a direct test that the fourth position is not empty followed by the test using xstring.

In both cases the tests show N for ab and Y for abcd but defining the test and executing the two tests one way takes 70 lines of log file, doing it the other way takes 2360 lines of log output. \tracingall output is usually a fair indication of actual TeX speed, and it's an interesting read anyway to see what TeX is doing behind the scenes.

\documentclass{article}

\tracingall
% check at least four characters

\makeatletter

\def\strcheck#1{\xstrcheck#1{}{}{}{}\xstrcheck}
\def\xstrcheck#1#2#3#4#5\xstrcheck{%
   \ifx\valign#3\valign
     \expandafter\@secondoftwo
   \else
     \expandafter\@firstoftwo
   \fi}


\strcheck{ab}{\show Y}{\show N}
\strcheck{abcd}{\show Y}{\show N}


\makeatother

\usepackage{xstring,xifthen}

\def\strcheckb#1{%
\StrLen{#1}[\mystringlen]%
\ifthenelse{\mystringlen > 3}}


\strcheckb{ab}{\show Y}{\show N}
\strcheckb{abcd}{\show Y}{\show N}

\stop

Answer 3

In support of David Carlisle's comment: even a more general solution like the following shows less than 200 lines of trace log.

\documentclass{article}
\makeatletter
% \getStringLength{<string>}{<returned length>}
\def\getStringLength#1#2{%
  \begingroup\@tempcnta\z@
  \let\x\g@tStringLength
  \ifx\g@tStringLength#1\g@tStringLength
    \def\x##1\g@tStringLength{\endgroup\def#2{0}}%
  \fi
  \x#2#1\g@tStringLength
}
\def\g@tStringLength#1#2#3{%
  \advance\@tempcnta\@ne
  \ifx\g@tStringLength#3%
    \edef#1{\endgroup\def\noexpand#1{\the\@tempcnta}}#1%
  \else
    \expandafter\g@tStringLength\expandafter#1\expandafter#3%
  \fi
}
\makeatother
\begin{document}
{\tracingall
   \getStringLength{ab}\length
   \ifnum\length=4 Y\else N\fi
}

\getStringLength{abcd}\length
\ifnum\length=4 Y\else N\fi

\getStringLength{}\length
\end{document}

Answer 4

David, Ahmed, whilst I appreciate the elegance of your demonstration, the solution that solved my problem was achieved in only 3 lines of code...

\def\todayDate{1st January 1999} %leave empty for today's date.
\StrLen{\todayDate}[\mystringlen]
\ifthenelse{\mystringlen > 3}{\renewcommand{\today}{\todayDate}}{}

However, to cover all bases, a command that simply checks if the variable is empty is as follows, in one fewer line and not making a call to the 'StrLen' command.

\def\todayDate{1st January 1999} %leave empty for today's date.
\ifthenelse{\equal{\todayDate}{}}{}{\renewcommand{\today}{\todayDate}}