Wrapping a for loop inside an environment

Question

I'm trying to make an environment that wraps a for loop:

\documentclass{article}
\usepackage[utf8]{inputenc}
\title{Testing}
\author{This is a Name}
\date{June 2022}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\maketitle
\newenvironment{fora}{
  \pgfplotsforeachungrouped \i in {0,1,...,5}  {
} {
  }
}
\begin{fora}
\textit{meow}
\end{fora}
\end{document}

I get this error on overleaf at \end{fora}:

Missing \endcsname inserted.
TeX capacity exceeded, sorry [input stack size=5000].

My intended goal was for "meow" to be printed 5 times. Any ideas on what is wrong? I'm guessing it's an issue related to the nested curly brackets.

Or one of the alternatives in macros - Collecting contents of environment and store them for later retrieval - TeX - LaTeX Stack Exchange — user202729, Jun 19 '22 at 17:55

egreg · Answer 1 · 2022-06-20T08:22:19.547

You seem to be trying something like

\newcommand{\mycommand}[1]{something with #1}
\newenvironment{fora}
  {\mycommand{}% begin part
  {}}% end part

where the { in the begin part should be matched by the } in the end part.

Sorry, no, it cannot work: a “begin part” like that would have unmatched braces and, actually, TeX matches them in a way that you might find surprising, but it isn't.

What you maybe want is to grab the environments content and to pass it to the macro.

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\NewDocumentEnvironment{fora}{+b}
  {\pgfplotsforeachungrouped \i in {0,1,...,5}  {#1}}
  {}
\begin{document}
\begin{fora}
\textit{meow}
\end{fora}
\end{document}

The argument type b roughly means “make #1 to be the whole content of the environment, after stripping off leading and trailing spaces”; the + prefix means that blank lines are allowed in the environment.

A few other comments:

inputenc is no longer necessary, if the input files are UTF-8
first load packages, then do declarations. Stating \title before \usepackage{pgfplots} makes it hard to find things during document maintenance
declare your commands and environments in the preamble, not after \begin{document}

+1 I tested changing your \texit{meow} in \textit{meow now\test \i\}. Wonderfull! — Raffaele Santoro, Jun 20 '22 at 08:31

score 0 · Answer 2 · answered Jun 19 '22 at 19:40

To understand the error it helps to indent the input to better show the structure

You get the same error from

\documentclass{article}
\usepackage[utf8]{inputenc}
\title{Testing}
\author{This is a Name}
\date{June 2022}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\maketitle
\newenvironment{fora}
  {\pgfplotsforeachungrouped \i in {0,1,...,5}  {} {}}
  \begin
{fora}
\textit{meow}
\end{fora}
\end{document}

which can be simplified to

\documentclass{article}
\begin{document}
\newenvironment{fora}
  {}
  \begin
\end{fora}
\end{document}

So the begin code of the evironment was loop with a empty body (but this is never called so I demonstrate an empty begin code here)

the end code of the environment is \begin

so \end starts by inserting \begin and you get an internal loop of unwanted expansions.

Wrapping a for loop inside an environment

2 Answers2