How to change the format of labels

By default, when a label is created, it takes on the appearance of the counter labelled, so the label appears as \the<counter> — what would be used if you asked to typeset the counter in your text. This isn’t always what you need: for example, if you have nested enumerated lists with the outer numbered and the inner labelled with letters, one might expect to want to refer to items in the inner list as “2(c)”. (Remember, you can change the structure of list items.) The change is of course possible by explicit labelling of the parent and using that label to construct the typeset result — something like

\ref{parent-item}(\ref{child-item})

which would be both tedious and error-prone. What’s more, it would be undesirable, since you would be constructing a visual representation which is inflexible (you couldn’t change all the references to elements of a list at one fell swoop).

LaTeX in fact has a label-formatting command built into every label definition; by default it’s null, but it’s available for the user to program. For any label ‹counter› there’s a LaTeX internal command \p@‹counter›; for example, a label definition on an inner list item is supposedly done using the command \p@enumii{\theenumii}. Unfortunately, the internal workings of this aren’t quite right, and you need to patch the \refstepcounter command:

\renewcommand*\refstepcounter[1]{\stepcounter{#1}%
  \protected@edef\@currentlabel{%
    \csname p@#1\expandafter\endcsname
      \csname the#1\endcsname
  }%
}

With the patch in place you can now, for example, change the labels on all inner lists by adding the following code in your preamble:

\makeatletter
\renewcommand{\p@enumii}[1]{\theenumi(#1)}
\makeatother

This would make the labels for second-level enumerated lists appear as “1(a)” (and so on). The analogous change works for any counter that gets used in a \label command.

In fact, the fncylab package does all the above (including the patch to LaTeX itself). With the package, the code above is (actually quite efficiently) rendered by the command:

\labelformat{enumii}{\theenumi(#1)}

In fact, the above example, which we can do in several different ways, has been rendered obsolete by the appearance of the enumitem package, which is discussed in the answer about decorating enumeration lists.

FAQ ID: Q-labelformat
Tags: latex–macros