General ⇒ Errors with algorithm package: Undefined control sequence

[danylo]

Hi,
this has been driving me nuts on how come its not working:

Code: Select all

\usepackage{algorithm}
\usepackage{algpseudocode}

...

\begin{algorithm}{}
\caption{$QT\_Clust(S,d)$}
\label{alg:qt}
\begin{algorithmic}[1]
\IF{$N$ is even}
\STATE $X \Leftarrow X \times X$
\STATE $N \Leftarrow N / 2$
\ELSE{$N$ is odd}
\STATE $y \Leftarrow y \times X$
\STATE $N \Leftarrow N - 1$
\ENDIF
    \State return ...
\end{algorithmic}
\end{algorithm}

for some reason I every line I get an error: Undefined control sequence: \IF (or \STATE or \ELSE). What is the problem with my syntax? Also if I want to reference this algorithm later on how would I do it?
Thanks.

[gmedina]

Beware of small caps. Try this:

Code: Select all

\begin{algorithm}{}\label{alg:qt}
 \caption{$QT\_Clust(S,d)$}
 \begin{algorithmic}
  \If {$N$ is even}
  \State $X \Leftarrow X \times X$
  \State $N \Leftarrow N / 2$
  \Else {$N$ is odd}
  \State $y \Leftarrow y \times X$
  \State $N \Leftarrow N - 1$
  \EndIf
  \State return ...
 \end{algorithmic}
\end{algorithm}

Remark: the line \Else {$N$ is odd} seems odd to me.
Since you have already defined a \label for your algorithm, you can reference it like this:

Code: Select all

As algorithm \ref{alg:qt} shows ...

[danylo]

Hi,
thanks for the \ref comment, that works now.
I removed the \Else line but I still get the same error for the other lines.
For some reason though if I change the inside of \begin{algorithmic} to this:

Code: Select all

    \For{all ...}
        \State do ...
    \EndFor
    \State return ...

I get no errors.

[gmedina]

I removed the \Else line...

My remark addressed the "meaning" of the command inside the whole algorithm (you don't have to remove it, just review your algorithm).

For some reason though if I change the inside of \begin{algorithmic} to this:
\For{all ...}
\State do ...
\EndFor
\State return

As I said in my reply, beware of capitalization; LaTeX commands are sensible to capitalization. The code I posted works perfectly (in a "formal" sense; i.e., it's syntactically correct, but the semantics is up to you).

[danylo]

It works now. I didn't realize that the commands were case sensitive. Thanks. One of other thing I'm curious about is that in the \Comment command is there a way to change the way the comment is produced? Right now a little arrow is used to show the start of the comment. I would prefer something like // instead.
One more thing - how do you force a line break in the middle of a command?
Thanks.

[danylo]

Sorry, I have one more question...
I start off my pseudo code with:
\begin{algorithm}{}
\label{alg:qt}
\caption{$QT\_Clust(S,d)$}

Then later in my text I do \ref{alg:qt} later in my text but it returns the section number the pseudo code is in and no information about the actual algorithm. How do I change this?
Thanks.

[gmedina]

I'm curious about is that in the \Comment command is there a way to change the way the comment is produced? Right now a little arrow is used to show the start of the comment. I would prefer something like // instead.

Add this in the preamble:

Code: Select all

\renewcommand{\algorithmiccomment}[1]{// #1}

One more thing - how do you force a line break in the middle of a command?

Please, be more explicit.

[gmedina]

Then later in my text I do \ref{alg:qt} later in my text but it returns the section number the pseudo code is in and no information about the actual algorithm. How do I change this?

Try this:

Code: Select all

\begin{algorithm}
\caption{QT\_Clust(S,d)}\label{alg:qt}
\begin{algorithmic}
\If {$N$ is even}
\State $X \Leftarrow X \times X$ 
\State $N \Leftarrow N / 2$
\Else {$N$ is odd}
\State $y \Leftarrow y \times X$
\State $N \Leftarrow N - 1$
\EndIf
\State return ...
\end{algorithmic}
\end{algorithm}

[Sebi]

You can even label each line of the algorithm, in order to reference to it.
Very handy if the algorithm is changed later on!