How can I sum two values and store the result in other variable?

Question

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

Answer 1

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}

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}

Answer 2

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

Answer 3

Since LuaTeX is available, forget all that complicated stuff and do something like:

\directlua{
a = 0
a = a + 1

tex.print(a)
}

Answer 4

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

Answer 5

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.

Answer 6

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.

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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?