A looping device in plain Teχ

tex-loop.tex

% Input: we define \loop to print some text the number of given times
  
\newcount\c%
\long\def\loop#1 for #2 times{
    \c=#2\relax%
    \begingroup%
        \def\iterate{%
            \ifnum\c>0 %
                \advance\c by -1%
                {#1}\par%
                \iterate%
            \fi%
        }%
        \iterate
    \endgroup%
}

\loop
    this sentence.
for 10 times

% Output: “this sentence.
% this sentence.
% this sentence.
% this sentence.
% this sentence.
% this sentence.
% this sentence.
% this sentence.
% this sentence.
% this sentence.”

% Input: We define \ifundefined to determine
% whether the inner macro \iterate propagated outside

{\tt iterate} is defined?
% via “Teχ by Topic” page 142
\def\ifundefined#1{\expandafter
    \ifx\csname#1\endcsname\relax}
\ifundefined{iterate}No\else Yes\fi

% Output: Yes

%\iterate
%% fails with “Undefined control sequence.” though

\bye