I am trying to convert \textwidth to other units I want and do calculations with the number (without the unit).
How does one do this?
2
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1What do you want to do exactly? A number can't be a command… – Bernard Jun 18 '16 at 17:49
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The pgfmath package does math without units. \pgfmathparse{\textwidth/1mm} will put the value you want into \pgfmathresult. – John Kormylo Jun 18 '16 at 18:49
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\strip@pt removes the unit pt and eTeX's \dimexpr can be used for the calculation:
\documentclass{article}
\usepackage{siunitx}
\makeatletter
% #1: macro, which gets the result of the conversion without unit
% #2: length expression
\newcommand*{\converttomm}[2]{%
\edef#1{%
\strip@pt\dimexpr(#2)*2540/7227\relax % 72.27 pt = 1 in = 25.4 mm
}%
}
\makeatother
\begin{document}
\converttomm{\mmTextWidth}{\textwidth}
The text width is \SI{\mmTextWidth}{\milli\meter}.
\end{document}
answered Jun 18 '16 at 19:46
You can use my calculator package.
The \LENGTHDIVIDE command divides two lengths and stores result as a number in a new command, as you need.
Try this code:
\documentclass[a4paper]{article}
\usepackage{calculator}
\begin{document}
\LENGTHDIVIDE{\textwidth}{1mm}{\size}
\size
\end{document}
In a standard a4 article \size returns 121.25427
Length conversion to decimal using LaTeX3:
\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
%\cs_new_eq:NN \calc \fp_eval:n
\cs_new_eq:NN \convertlen \dim_to_decimal_in_unit:nn
\ExplSyntaxOff
\begin{document}
\verb|\textwidth| in \verb|pt|s: \the\textwidth
\makeatletter
\verb|1mm| in \verb|pt|s: \setlength{\@tempdima}{1mm}\the\@tempdima
\makeatother
$\frac{\texttt{\string\textwidth}}{\texttt{1mm}} = \convertlen{\textwidth}{1mm}$
\end{document}
The above defines the user interface \convertlen{<fromlen>}{<tounit>} using LaTeX3's \dim_to_decimal_in_unit.
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Interestingly there is a significant drop in precision. Seems that the calculation does basically the quotient with
1mmfirst approximated by186467spwhich is what\number\dimexpr 1mm\relaxreturns (I obtain same result as you if I do\xinttheiexpr [5]\textwidth/\dimexpr 1mm\relax\relax.) The computation as in Heiko's answer is more accurate: that would be\xinttheiexpr [5] \textwidth*2540/(7227*65536)\relax, where the ratio is also in smallest terms635/118407168. – user4686 Sep 11 '16 at 19:45 -
for
345pt, the exact value inmmis345*2540/7227=292100/2409=121.253632212536322125363221253632...– user4686 Sep 11 '16 at 19:50 -
the exact value of
1mminspis186467.9811..., but TeX replaces it by186467as it truncates when one assigns dimensions to register. That means basically a relative error of order0.5*10^-5. And indeed the\dim_to_decimal_in_unitprovides here a result which is wrong at the sixth decimal. – user4686 Sep 11 '16 at 19:59
