multido variable in \paragraph prematurely incremented by one

Question

The compilation of the following MCE:

\documentclass{report}
\usepackage{fixltx2e}
\usepackage{multido}
\begin{document}
\multido{\i=1+1}{4}{%
  \chapter{I am the chapter \i}
  \section{I am a section of chapter \i}
  \subsection{I am a subsection of chapter \i}
  \subsubsection{I am a subsubsection of chapter \i}
  % \paragraph{I am a paragraph of chapter \i}
  % \subparagraph{I am a subparagraph of chapter \i}
}
\end{document}

leads to:

1. the expected result,
2. a strange result if the line % \paragraph{...} is commented out because the variable \i is prematurely incremented by one and the last corresponding occurrence doesn't appear. If the line % \subparagraph{...} is additionally commented out, it is subject to this strange behavior, the \paragraph{...} working as expected.

In this thread of the French TeX forum : (in French, sorry), some contributors isolated the trouble that seems to be a real anomaly.

Answer 1

The answer is probably "don't do that". \paragraph is (by default) an inline heading this means that it is held back until the next paragraph starts so that it can be typeset as part of that paragraph. But here the next paragraph does not start until the loop has spun and so \i has incremented.

If the paragraph is within the loop, it all works better:

\documentclass{report}
\usepackage{fixltx2e}
\usepackage{multido}
\begin{document}
\multido{\i=1+1}{4}{%
  \chapter{Je suis le chapitre \i}
  \section{Je suis une section du chapitre \i}
  \subsection{Je suis une sous-section du chapitre \i}
  \subsubsection{Je suis une sous-sous-section du chapitre \i}
   \paragraph{Je suis un paragraphe du chapitre \i}
  % \subparagraph{Je suis un sous-paragraphe du chapitre \i}
\mbox{}
}
\end{document}

Answer 2

(edit) 2017: since xint 1.1 (2014/10/28) one needs here \usepackage{xinttools}. Code updated for that to replace \usepackage{xint} of initial answer.

As an alternative one may use the expandable macro \xintApplyUnbraced from package xint. But we need first to generate expandably an arithmetic sequence, this not being provided in the package, I give one possible construct here.

The result of the code is that the enumeration is as expected, contrarily to the phenomenon with \multido.

\documentclass{report}

\usepackage{fixltx2e}
\usepackage{multido}
\usepackage{xinttools}


% we define an expandable macro \indices {a+b}{N} to generate
% {a}{a+b}{a+2b}...{a+(N-1)b}
\catcode`\_ 11
\def\indices    #1#2{\indices_A #1.#2.}
\def\indices_A  #1+#2.#3.{\indices_B {#3}{#1}{#2}{}}
\def\indices_B  #1#2#3{\ifnum #1>0
                          \expandafter\expandafter\expandafter
                          \indices_C
                       \else
                          \expandafter\indices_end
                       \fi \expandafter
                   {\the\numexpr #1-1\expandafter}\expandafter
                   {\the\numexpr #2+#3}{#3}{#2}}
\def\indices_end #1#2#3#4#5#6#7{#7}
\def\indices_C #1#2#3#4#5{\indices_B {#1}{#2}{#3}{#5{#4}}}
\catcode`\_ 8

\begin{document}

\multido{\i=1+1}{4}{%
  \chapter{Je suis le chapitre \i}
  \section{Je suis une section du chapitre \i}
  \subsection{Je suis une sous-section du chapitre \i}
  \subsubsection{Je suis une sous-sous-section du chapitre \i}
   \paragraph{Je suis un paragraphe du chapitre \i}
  % \subparagraph{Je suis un sous-paragraphe du chapitre \i}
}


\def\stuff #1{{}% this {} is to stop the expansion
  \chapter{Je suis le chapitre #1}
  \section{Je suis une section du chapitre #1}
  \subsection{Je suis une sous-section du chapitre #1}
  \subsubsection{Je suis une sous-sous-section du chapitre #1}
   \paragraph{Je suis un paragraphe du chapitre #1}
  % \subparagraph{Je suis un sous-paragraphe du chapitre #1}
}

\xintApplyUnbraced\stuff{\indices{5+1}{4}}
% generates for 5,6,7,8.


\end{document}