Newtcolorbox and foreach

Question

I have defined 2 Boxes:

\documentclass{article}
\usepackage[most]{tcolorbox}

\newtcolorbox{one}{}
\newtcolorbox{two}{}

\begin{document}

\begin{one}
The new command works fine
\end{one}

\begin{two}
The new command works fine
\end{two}

\end{document}

But with \foreach result error:

\foreach \i in {one,two}{
\newtcolorbox{\i}{}
}

Approach:

\foreach \i in {one,two}{
\expandafter\newtcolorbox{\csname\i\endcsname}{ }}

Any hint would be appreciated.

Answer 1

Every cycle in a \foreach loop is done inside a group, so the \newtcolorbox declaration is lost as soon as the group ends.

You may appreciate a general loop that doesn't need groups:

\documentclass{article}
\usepackage{tcolorbox}

\ExplSyntaxOn

\NewDocumentCommand{\lforeach}{mm}
 {% #1 = list, #2 = template
  \cs_set:Nn \__lforeach_temp:n { #2 }
  \clist_map_function:nN { #1 } \__lforeach_temp:n
 }

\ExplSyntaxOff

\lforeach{one,two}{\newtcolorbox{#1}{}}

\begin{document}

\begin{one}
The new command works fine
\end{one}

\begin{two}
The new command works fine
\end{two}

\end{document}

If you're running LaTeX with a release date prior to 2020-10-01, you also need \usepackage{xparse}.

The idea is the same as with \foreach, but you don't need to use a scratch macro such as \i to denote the current item (which is the main reason why \foreach uses grouping), using instead #1.

You can use this loop in various contexts.

Answer 2

\foreach creates groups and I am not sure what the \csname...\endcsname is for, but the following works.

\documentclass{article}

\usepackage[most]{tcolorbox}
\pgfkeys{new tcolorbox/.code=\newtcolorbox{#1}{},
new tcolorbox/.list={one,two}}

\begin{document}

\begin{one} The new command works fine \end{one}

\begin{two} The new command works fine \end{two}

\end{document}

Answer 3

An approach using only \expandafters. It helps to define \addto and \xaddto. Here, the \foreach just strings together the future commands into the global \tmp, which is executed after the group is exited.

\documentclass{article}
\usepackage[most]{tcolorbox}

\def\addto#1#2{\expandafter\gdef\expandafter#1\expandafter{\tmp#2}}
\def\xaddto#1#2{\expandafter\addto\expandafter#1\expandafter{#2}}
\def\tmp{}
\foreach \i in {one,two}{
  \addto\tmp{\newtcolorbox}
  \xaddto\tmp{\expandafter{\i}{}}
}\tmp
\begin{document}

\begin{one}
The new command works fine
\end{one}

\begin{two}
The new command works fine
\end{two}

\end{document}