Convert \input to number

Question

I'm reading several txt-files from disk and I want to do some simple calculations on them and be able to print them on the page.

Basically I have:

\newcommand\getinput[2]{
    \immediate\write18{do-some-things > input.txt}
    \input{input.txt}}

And now I want to do use that input for simple additions, like this:

\newcounter{mycounter}
My Input: \getinput
\addtocounter{mycounter}{\getinput}
My Sum: \themycounter

Answer 1

You will need this to work by expansion if you want to use it directly for a counter, so cannot use \write. Instead, it's possible to exploit the piped input ability of modern engines. Using a working example with echo

\documentclass{article}
\makeatletter
\newcommand\getinput{\@@input|"echo 1"}
\makeatother
\begin{document}

\newcounter{mycounter}
My Input: \getinput
\addtocounter{mycounter}{\getinput}
My Sum: \themycounter

\end{document}

Here, we need the 'raw' behaviour of the \input primitive, hence needing to use it's LaTeX name, \@@input.

---

If you are happy with a two-step process, first saving the information then using it, you might use the more robust expl3 wrapper around the same idea.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand \saveinput { m m }
  { \sys_get_shell:nnN {#1} { } #2 }
\ExplSyntaxOff
\begin{document}

\newcounter{mycounter}
\saveinput{echo 1}\savedinput

My Input: \savedinput
\addtocounter{mycounter}{\savedinput}
My Sum: \themycounter

\end{document}

Answer 2

\documentclass{article}
\usepackage{readarray}
\newcounter{tmpcnt}
\newwrite\notesfile
\newcommand\getinput[1]{
  \immediate\openout\notesfile=input.txt
  \immediate\write\notesfile{\the\numexpr#1+\thetmpcnt\relax}
  \immediate\closeout\notesfile
  \readdef{input.txt}\inputvar
  \inputvar
}
\begin{document}
\setcounter{tmpcnt}{5}
\getinput{3}

\addtocounter{tmpcnt}{\inputvar}

\getinput{4}

\getinput{-2}
\end{document}