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In LaTeX2e,how can I sum two values and assign them to other variable?

I want to compute something like:

var=\textwidth - 1cm 

And if both were constants:

var=1+1
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2  
Please explain your intention. \textwidth is also constant. Or do you mean number vs. dimension? –  Marco Daniel Sep 30 '11 at 17:05
1  
Depending on where you want to use it, there might be different options (pure TeX, TikZ/PGF, Lua, etc.) –  ℝaphink Sep 30 '11 at 17:07
3  
Why do you use the tag plain-tex together with latex3? –  Leo Liu Sep 30 '11 at 17:08
1  
@voodoomsr: The current version of LaTeX is LaTeX2e, not LaTeX3. LaTeX3 is in development and only a few experimental functions are available. Since Plain TeX and LaTeX are totally different TeX formats, you can't use them together. If you are using LaTeX, you don't need to use any tag for this, and don't use latex3 tag if you are not using it. –  Leo Liu Sep 30 '11 at 17:27
1  
@LeoLiu thanks Leo I wasn't clear that tex and latex can't be mixed, so I'm going to retag the question –  mjsr Sep 30 '11 at 17:32
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6 Answers

up vote 32 down vote accepted
+50

In regular LaTeX, the calc package allows for easy manipulation of length arithmetic:

\documentclass{article}
\usepackage{calc}% http://ctan.org/pkg/calc
\newlength{\mylength}
\begin{document}
\setlength{\mylength}{\textwidth}%
\noindent\rule{\mylength}{20pt}

\bigskip
\setlength{\mylength}{\textwidth-1cm}%
\noindent\rule{\mylength}{20pt}

\bigskip
\setlength{\mylength}{\textwidth-80pt+5mm-1bp}%
\noindent\rule{\mylength}{20pt}
\end{document}

Lines of different lengths using calc package

The above deals with lengths. For basic arithmetic using numbers, the fp package. Here is an example using infix notation (Reverse Polish Notation/RPN is also possible via \FPupn):

\documentclass{article}
\usepackage[nomessages]{fp}% http://ctan.org/pkg/fp
\begin{document}
The following arithmetic is easy:
\begin{itemize}
  \item \FPeval{\result}{clip(5+6)}%
    $5+6=\result$
  \item \FPeval{\result}{round(2+3/5*pi,5)}%
    $2+3/5\times\pi=\result$
\end{itemize}
\end{document}

Regular arithmetic using fp package

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4  
I think it is simpler to use \FPeval{result}{round(2+3/5*pi,5)} –  Leo Liu Sep 30 '11 at 17:33
    
@LeoLiu: Thanks; sure is easier to read/follow. –  Werner Sep 30 '11 at 18:59
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In classical Knuth TeX,

\newdimen\len
\len=\hsize
\advance\len by -1cm

\newcount\cnt
\cnt=1
\advance\cnt by 1

eTeX,

\newdimen\len
\len=\dimexpr\hsize-1cm\relax

\newcount\cnt
\cnt=\numexpr1+1\relax

LaTeX with calc,

\usepackage{calc}

\newlength\len
\setlength{\textwidth+1cm}

\newcounter{cnt}
\setcounter{cnt}{1+1}

LaTeX2e with expl3 (LaTeX3),

\usepackage{expl3}
\ExplSyntaxOn
\dim_new:N \l_len_dim
\dim_set:Nn \l_len_dim {\textwidth + 1cm}

\int_new:N \l_cnt_int
\int_set:Nn \l_cnt_int {1+1}
\ExplSyntaxOff
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Since LuaTeX is available, forget all that complicated stuff and do something like:

\directlua{
a = 0
a = a + 1

tex.print(a)
}
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In LaTeX, if you just want to subtract one known length (say, 1cm) from another (say, \textwidth) to obtain a new length variable, you can do so using the \newlength, \setlength, and \addtolength instructions, as in the following example:

\newlength\mylength
\setlength\mylength\textwidth
\addtolength\mylength{-1cm} %% note the minus sign
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Good and simple solution, but I prefer to wrap the command names in curly brackets to make it clearer. –  erickrf Jan 13 '13 at 13:16
    
@erickrf: Thanks for this feedback. In the cod I provided, using or omitting the curly braces around the macros are entirely equivalent. Feel free to use whichever method you feel provides more clarity (or, equivalently, less ambiguity). :-) –  Mico Jan 13 '13 at 14:36
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With a fairly recent TeX distribution

\newdimen\len
\len=\dimexpr\textwidth-1cm\relax

\newcount\cnt
\cnt=\numexpr1+1\relax

It's not quite clear what's the framework you're interested in, though.

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This is a bit overkill for the particular examples that you mention, but since this works for more complicated expressions I tend to use it:

\documentclass{article}
\usepackage{pgf}

\begin{document}
\pgfmathsetmacro{\var}{\textwidth - 1cm}

The value of var is \var
\end{document}

Note that with pgfmathsetmacro the result is a decimal without units. If you are only interested in lengths, then you can use a similar macro \pgfmathsetlength.


If you want to minimize what gets loaded and still use pgfmath, then see Is it possible to load pgfmath without loading the full pgf package?

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