How to check if a macro is at the end of a line or paragraph

Question

I need a macro to behave differently in case it's at the end of a line. Is there any way to check something like that?

\documentclass[11pt]{memoir}
\newcommand{\test}{
    % if at the end of line
        % Print A
    % else
        % Print B
    }
\begin{document}
    Hello, \test how are you doing? % Should print B
Hello, how are you doing? \test % Should print A

\test Hello, how are you doing? % Should print B

\end{document}

Thank you

Edit: I really meant at the end of a paragraph, but I'm going to leave the question as is, because there are good answers to check both the end of a line and the end of a paragraph.

You can store the location (\pdfsavepos or \tikzmark) in the aux file, then check it in the next run. See https://tex.stackexchange.com/questions/413580/how-to-correctly-change-the-side-that-marginpar-appears-per-case for example. — John Kormylo, Feb 05 '23 at 03:28
Somewhat related sectioning - Macro that knows it is at the end of a section? - TeX - LaTeX Stack Exchange — user202729, Feb 05 '23 at 06:19

score 3 · Accepted Answer · answered Feb 04 '23 at 17:28

Checking whether a control sequence is at the “end of a line” is generally not possible in TeX, because endlines are converted to spaces (or ignored altogether if they follow a control word) during tokenization. You can check whether \test is followed by \par, though.

\documentclass[11pt]{memoir}
\makeatletter
\newcommand{\test}{%
  @ifnextchar\par{A}{B}%
}
\makeatother
\begin{document}
Hello, \test how are you doing? % Should print B
Hello, how are you doing? \test % Should print A
\test Hello, how are you doing? % Should print B
\end{document}

score 1 · Answer 2 · answered Feb 05 '23 at 06:15

It's possible if you assume that the content are not already tokenized (loosely speaking, the command is not used inside the "argument" of other commands, or the places where \verb|...| fails)

The basic idea is to store the current inputlineno, tokenize one more token, compare inputlineno with it.

\documentclass{article}
\usepackage{tikz}
\def\test{%
   \count0=\inputlineno
   \futurelet\tmptoken\testb
}
\def\testb{%
   \ifnum\count0=\inputlineno
      middle of line%
   \else
      end of line%
   \fi
}
\usepackage{hyperref}
\begin{document}
middle of line = \test.
end of line = \test
= \test%
...
\textbf{inside a ``normal'' command argument it will always return middle of line: \test
}
\begin{tikzpicture}
   \node [text width = 10cm] at (0, 0) {(I mean, Ti\emph{k}Z node text is not a ``normal'' command argument, see \url{https://tex.stackexchange.com/q/345871/250119}) \test
      };
\end{tikzpicture}
\end{document}

How to check if a macro is at the end of a line or paragraph

2 Answers2