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\documentclass{article}
\usepackage{algorithm}

\begin{document}
\setcounter{section}{54}
\section{A section with no label}
\begin{algorithm}
\label{myalg}
\caption{An algorithm with the label `myalg'.}
\end{algorithm}
Reference to myalg: \ref{myalg}.
\end{document}

With this code, the reference givens 55 (the section number); if I move the label after the caption command, I get the correct reference (1).

Why should this be the case? I find it confusing: when thinking about an algorithm as an environment, you expect to be able to label it right after you began it. At the very least I would expect some kind of error or "??" to appear, rather than the previous environment or reference-able entity being used.

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2 Answers 2

up vote 45 down vote accepted

This is in common with all captions: it's the \caption command that generates the number, so a \label before it can't know that number.

This could have been designed differently, of course, but you can think to a figure environment where two figures with distinct captions are placed, side by side in order to save space, for example.

So, always place the \label after the \caption.

Since \label is a "general purpose" command, it will always refer to the last previously generated number (be it a chapter, section, figure caption number or directly from the user with a \refstepcounter command) and to the page where it eventually falls. Actually it's not really the last generated number, but the one lastly generated at the same or higher grouping level.

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This example you give about figures is even more unintuitive: A single figure environment really has multiple figures. So a figure is not a figure, and you reference the caption instead of referencing the figure... also, in an algorithm environment you don't have that; I mean, maybe I can somehow have two algorithmics in there, but I would only reference the entire algorithm... –  einpoklum Oct 22 '11 at 8:57
3  
@EyalRozenberg The example was just to explain the rationale for the programmer's choice. An algorithm environment doesn't generate a number; it's the \caption that does it: maybe you won't, but someone else might want to keep together two algorithmic environments (each one with its caption), to show the differences between them, for instance. –  egreg Oct 22 '11 at 9:03
2  
When someone asks me why the label has to be after something and not before I always answer: "Well, the label of a bottle is also on it's outside and not on it's inside to make sense." –  Foo Bar Mar 8 '13 at 19:19

The \caption macro increases the used counter and sets the current label text which is used by \label. If you use \label before it the old label text is used instead, which leads to a wrong number. Always use \label after \caption and not before or in it.

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1  
Why not use \label in \caption? Laslie Lamport writes: "A label can appear in the argument of a sectioning or \caption command, but in no other moving arguments." –  Axel Sommerfeldt Oct 22 '11 at 10:06
1  
Yes, it must be removed internally, but it will be removed internally, because Laslie Lamport has implemented it this way. And yes, one can put \label inside \section etc., and it is documented (by Laslie Lamport) that you are allowed to do it. So I still see no problem here. (BTW: \label after \piccaption will not work, for example, you have to place it inside the argument of \piccaption. Same for \subfloat etc.) –  Axel Sommerfeldt Oct 22 '11 at 10:25
1  
The rule of thumb is to put it behind it. Feel free to do it otherwise. I'm not the caption police. Special stuff like \subfloat work anyway different. I just have the feeling that beginners get explicitly told to put it inside, and there is no need for that. –  Martin Scharrer Oct 22 '11 at 10:39
2  
"The rule of thumb is to put it behind it." Do you have any reference for that? Sorry for being so insistent, the problem I have is that I'm actually the caption police for many people. Currently I tell beginners to put the \label inside the \caption, and if this is wrong in some way I really would like to know. (Yes, there is no need for putting \label inside \caption, but there is IMHO no need for putting \label after \caption, either.) –  Axel Sommerfeldt Oct 22 '11 at 11:19
2  
One can easily reverse that: If you can't put \label always after the to-be-labeled macro then you shouldn't do it in any case. IMHO both are wrong, because you simply can't always do it the same way, because macros are implemented in a non-consistent manner regarding labeling. (Another example: supertabular) \refstepcounter will increase the counter globally, but unfortunately defining the labeling stuff only locally. I wonder why Lamport has implemented in this way. Hmm, maybe I should rewrite \captionbox and \subcaptionbox so a \label after them would give a valid result, too? –  Axel Sommerfeldt Oct 22 '11 at 11:59

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