[Tex/LaTex] algorithm package: Comment on “for” loop does not work

algorithmsloops

It seems that \COMMENT in the algorithm package is broken. It gives an error when used in a FOR or FORALL line, so the first comment gives a compilation error:

\begin{algorithm}[ht]
  \caption{My algorithm}
  \begin{algorithmic}
   \FORALL{$p \in P$}   \COMMENT{loop over all puppies}
   \STATE PatPuppy($p$) \COMMENT{sooo nice!}
   \ENDFOR
  \end{algorithmic}
\end{algorithm}

The error is:

Something's wrong--perhaps a missing \item.
leading text:     \FORALL{$p \in P$}

Is there a workaround? I really need to display a comment on that line.

Best Answer

The answer is given in the manual. Use an optional argument.

\FORALL[loop over all puppies]{$p \in P$}

Related Solutions

[Tex/LaTex] Why does the placement of a \COMMENT in the algorithmic enviroment matter

You need to "fool" the algorithmic environment into posting just a comment on a line via \STATE \COMMENT{Blah}. The reason for this (in answer to your first question) is because the algorithmic environment handles it's contents like a list (similar to \begin{enumerate}...\end{enumerate} or \begin{itemize}...\end{itemize}). And, in a similar context, it requires a command like \item to move to the next item, or even start the list. Such a command is \STATE. That, with the following minimal example should answer both questions:

\documentclass[a4paper,10pt]{article}
\usepackage{algorithmic}
\begin{document}
\begin{algorithmic}
\FORALL{$a \in B$}
    \STATE \COMMENT{Blah.} % single line comment
    \STATE X
    \COMMENT{Blah.}
\ENDFOR
\end{algorithmic}
\end{document}

Algorithmic with line comment

EDIT: The above only works if you do not use line numbers. If you do use line numbers (via \begin{algorithmic}[1]), then you can define a new macro \LCOMMENT{<comment>} (short for "Line COMMENT") that does the trick:

\documentclass[a4paper,10pt]{article}
\usepackage{algorithmic}
\makeatletter
\newcounter{ALC@tempcntr}% Temporary counter for storage
\newcommand{\LCOMMENT}[1]{%
    \setcounter{ALC@tempcntr}{\arabic{ALC@rem}}% Store old counter
    \setcounter{ALC@rem}{1}% To avoid printing line number
    \item \{#1\}% Display comment + does not increment list item counter
    \setcounter{ALC@rem}{\arabic{ALC@tempcntr}}% Restore old counter
}%
\makeatother
\begin{document}
\begin{algorithmic}[1]
\FORALL{ $a \in B$}
    \LCOMMENT{Blah.} % single line comment
    \STATE X
    \COMMENT{Blah.}
\ENDFOR
\end{algorithmic}
\end{document}

Algorithmic with line comment without line number

There may be other/better/more elegant solutions as well.

Best Answer

Related Solutions

[Tex/LaTex] Why does the placement of a \COMMENT in the algorithmic enviroment matter

[Tex/LaTex] Algorithmic package for loop and comment at the same line

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