[Tex/LaTex] I get double question marks when referring to labels inside figures

cross-referencing

I have some charts embedded in figures, and I want to be able to refer to them easily using the "Figure x" notation. I'm doing it like this (in this order):

\documentclass[12pt,letterpaper]{article}

...

\section*{Results}
...we performed a linear regression to obtain a standard curve 
(Figure~\ref{0:stdcurve})....

\pagebreak
\section*{Results (Graphs)}
\subsection*{Construction of Standard Graph for PNP}
\begin{figure}[h]   % Figure 1
    \centering
    \includegraphics[scale=1]{kinematics-res/graph0-1}
    \caption{The standard curve for PNP.}
    \label{0:stdcurve}
\end{figure}

However, instead of getting the output "…to obtain a standard curve (Figure 1)…", I just get "…to obtain a standard curve (Figure ??)…"

I also noticed that the PDFLaTeX compiler is generating warnings saying that it couldn't find the reference of the name 0:stdcurve. Why am I getting these warnings?

Best Answer

You need to run LaTeX twice. The first time it figures out the reference numbers. On the second pass it fills in the reference numbers.

The following generates ...standard curve (Figure {??}) upon the first pass, but generates ...standard curve (Figure 1)... upon the second pass:

\documentclass[12pt,letterpaper]{article}
\usepackage[demo]{graphicx} 

\begin{document}
\section*{Results}
...we performed a linear regression to obtain a standard curve 
(Figure~\ref{0:stdcurve})....

\pagebreak
\section*{Results (Graphs)}
\subsection*{Construction of Standard Graph for PNP}
\begin{figure}[h]   % Figure 1
    \centering
    \includegraphics[scale=1]{kinematics-res/graph0-1}
    \caption{The standard curve for PNP.}
    \label{0:stdcurve}
\end{figure}
\end{document}

Related Solutions

[Tex/LaTex] Referring an equation inside a theorem

Generating correct labels for referencing forms part of a two-step process:

A counter is increased using \refstepcounter (a "referencable" form of counter incrementation). This typically happens when you call a macro (like \section) or environment. Here's a snapshot of \refstepcounter from latex.ltx:
```
\def\refstepcounter#1{\stepcounter{#1}%
    \protected@edef\@currentlabel
       {\csname p@#1\endcsname\csname the#1\endcsname}%
}
```
See that \refstepcounter is a "modified \stepcounter", since it also updates \@currentlabel.
When inserting a \label, the default behaviour of LaTeX is to write the macro \@currentlabel and \thepage to the .aux file as arguments to \newlabel. Here's a snapshot of \label from latex.ltx:
```
\def\label#1{\@bsphack
  \protected@write\@auxout{}%
         {\string\newlabel{#1}{{\@currentlabel}{\thepage}}}%
  \@esphack}
```
Generally speaking, \newlabel here is just a mechanism built in to check whether a label already exists/not and warn you thereof (in the form of "There were multiply-defined labels" or "Label `...' multiply defined").

So, whenever a \refstepcounter is performed, \@currentlabel is updated and therefore would change any reference created with a \label. The most commonly identified problem of this incorrect labelling is when users perform (say)

\begin{figure}
  % Image stuff here
  \label{mylabel}
  \caption{My caption}
\end{figure}

and receive an incorrect reference to their figure. This is because \caption executes \refstepcounter and not the figure environment itself, resulting in an incorrect \@currentlabel to be written to the .aux file (whatever that may be). A practical example:

enter image description here

\documentclass{article}
\begin{document}
\section{A section}
See Figure~\ref{fig:ref} in section~\ref{sec:ref}.
\section{A section} Here is some text. \label{sec:ref}
\begin{figure}[bh!]
  \centering\rule{.8\textwidth}{.2\textheight}
  \label{fig:ref}\caption{A figure caption}
\end{figure}
\end{document}

Note how the figure reference \ref{fig:ref} returns 2 instead of 1. That's because an issue of \label{fig:ref} write \@currentlabel which is still set to 2 by the preceding \section. Switching around \label and \caption would yield the desired result (however simplistic).

Bottom line is that there's an appropriate placement of \label with respect to counters, otherwise references might be incorrect. The emphasis here denotes the fact that some environments gather content a post-process them, possibly correcting for inappropriate placements of things like \label.

[Tex/LaTex] Question marks in pdf file when referring to table

You missed a \caption in your table. LaTeX uses the \caption macro to number your tables thus without a caption you won't get a number and thus cannot reference the table.

Try this:

\documentclass{report} 
\usepackage{float}
\begin{document}

we refer to table \ref{tab:ex}

\begin{table}[h]
\caption{Example of a simple table.}
\label{tab:ex}
\centering
\begin{tabular}{|l|l|l|l|l|}
\hline
9  & 12 & 8 & 10 & 9           \\ \hline
10 & 7  & 8 & 13 & \textbf{50} \\ \hline
\end{tabular}
\end{table}

\end{document}

A few things to remember:

Typically table-captions go above the table, figure captions go below the figure (at least in european typography)
The \label has to appear after the \caption, because \caption creates the number the \label can then refer to.
If you do not want a caption you won't get a number. But it's wise to always provide a short caption at least.
There are means to get Captions outside of table or figure environments with KOMA-Script or the caption package

Best Answer

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[Tex/LaTex] Referring an equation inside a theorem

[Tex/LaTex] Question marks in pdf file when referring to table

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