# Cross-referencing

## The \label and \ref mechanism

When you are writing a document of any length, you’ll want to refer to numbered items such as figures, tables or equations. Luckily, LaTeX can automatically add the right numbers; we just have to set things up. To have LaTeX remember a spot in your document you have to label it, and then in other places, you refer to it.

\documentclass{article}

\begin{document}
Hey world!

This is a first document.

\section{Title of the first section}

Text of material for the first section.

\subsection{Subsection of the first section}
\label{subsec:labelone}

Text of material for the first subsection.
$$e^{i\pi}+1 = 0 \label{eq:labeltwo}$$

In subsection~\ref{subsec:labelone} is equation~\ref{eq:labeltwo}.
\end{document}


There are two \label{...} commands, one after the subsection and one inside the equation environment. They are associated with the last sentence’s \ref{...} commands. When you run LaTeX, it saves information about the labels to an auxiliary file. For \label{subsec:labelone}, LaTeX knows that it is now in a subsection and so it saves the subsection’s number. For \label{eq:labeltwo}, LaTeX knows that the most recent environment of interest is an equation so it saves the information for that equation. When you ask for the reference, LaTeX gets it from the auxiliary file.

The subsec: and eq: aren’t used by LaTeX; rather, it just keeps track of what it has most recently processed. But when you are writing these help you remember what the label is about.

You may see references that show in an output PDF as boldface double question marks, ??. The explanation is that because of this auxiliary file work, the first time that you compile a document the label has not yet been saved. Run LaTeX one more time and you’ll be all set. (Usually while writing you will run LaTeX several times anyway, so in practice this is not a bother.)

Notice the tie (~) characters before the references. You don’t want a line break between subsection and its number, or between equation and its number. Putting in a tie means LaTeX won’t break the line there.

## Where to put \label

The \label command always refers to the previous numbered entity: a section, an equation, a float, etc. That means that \label always has to come after the thing you want to refer to. In particular, when you create floats, the \label has to come after (or better, in), the \caption command, but within the float environment.

## Exercises

Try adding new numbered parts (sections, subsections, enumerated lists) to the test document and finding out how many runs are needed to make \label commands work.

Add some floats and see what happens when you put \label before the \caption instead of after; can you predict the result?

What happens if you put a \label for an equation after the ?

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