使用knitr和xtable在LaTex中交叉引用的问题

Question

我目前正与R Studio合作，在LaTex中使用R / knitr生成PDF文档。在这些文档中，我想在表格中提供我想要在文本中引用的部分结果。我使用R中的xtable包生成这些表，它工作得很好，给了我正确的表。 到目前为止一直很好但是当它引用时我遇到了一些麻烦。此时，生成的PDF输出指的是表所在的部分。我搜索了一些LaTex帮助文件，并指出当浮动对象的标签放在标题之前时会发生这种情况。但是，当我检查xtable的输出时，它被放置在标题之后。 有人有解决方案吗？ 这是一个类似于我使用的示例代码，它在PDF中给出了相同类型的结果（我留下了LaTex和R中使用的所有包，以确保它不是由于某些干扰而引起的）。

\documentclass[preprint,authoryear]{elsarticle}
\usepackage[a4paper]{geometry}
\usepackage{natbib}
\usepackage{mathpazo}
\usepackage{booktabs}
\usepackage{dcolumn}
\usepackage[breaklinks]{hyperref}
\usepackage{hyperref,url}
\usepackage{lscape}
\usepackage[usenames,dvipsnames,svgnames,table]{xcolor}
\usepackage{amssymb}
\usepackage{gensymb}
\usepackage[allow-number-unit-breaks=true]{siunitx}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[version=3]{mhchem}
\usepackage[section]{placeins}
\usepackage[colorinlistoftodos]{todonotes}
\usepackage{float}
\usepackage{lineno}
\begin{document}
\section{Test}

This is a test. You can see the result in Tab. \ref{tab:Test}.
\section{Table}
<<Ini,eval=TRUE,include=FALSE,echo=FALSE,warning=FALSE,error=FALSE,cache=FALSE>>=
#set chunk options
opts_chunk$set(echo=FALSE,warning=FALSE,message=FALSE,dev.args=list(pointsize=12),fig.pos="!h",dpi=500)

require(stringr)
require(reshape2)
require(lubridate)
library(plyr)
library(lattice)  #Needed for multi-panel graphs
library(mgcv)     #Needed for smoothing curves in scatterplots
library(ggplot2)
library(ggmap)
require(gridExtra)
library(scales)
library(xtable)
require(nlme)
library(zoo)
@

<<results='asis',echo=FALSE>>=
Test<-data.frame(Test=c(1:3),a=c(4:6))
print(xtable(Test,caption="Test table",label="tab:Test",
             digits=2),
      include.rownames=FALSE,
      caption.placement="top",
      latex.environments = "left")

@



\end{document}

xtable代码的结果如下所示：

% latex table generated in R 3.0.3 by xtable 1.7-4 package
% Tue Jun 02 09:27:38 2015
\begin{table}[ht]
\begin{left}
\caption{Test table} 
\label{tab:Test}
\begin{tabular}{rr}
  \hline
Test & a \\ 
  \hline
  1 &   4 \\ 
    2 &   5 \\ 
    3 &   6 \\ 
   \hline
\end{tabular}
\end{left}
\end{table}

当我尝试运行此操作时，我收到4条警告消息，这些消息看起来很奇怪，但仍然有PDF输出。所有4个警告都围绕xtable生成的代码。这里是R Studio中Log的输出：

! Missing $ inserted.
<inserted text> 
                $
l.81 \begin{left}

I've inserted a begin-math/end-math symbol since I think
you left one out. Proceed, with fingers crossed.

! Missing delimiter (. inserted).
<to be read again> 
                   \global 
l.82 \caption
             {Test table}
I was expecting to see something like `(' or `\{' or
`\}' here. If you typed, e.g., `{' instead of `\{', you
should probably delete the `{' by typing `1' now, so that
braces don't get unbalanced. Otherwise just proceed.
Acceptable delimiters are characters whose \delcode is
nonnegative, or you can use `\delimiter <delimiter code>'.

! Missing $ inserted.
<inserted text> 
                $
l.82 \caption{Test table}

I've inserted a begin-math/end-math symbol since I think
you left one out. Proceed, with fingers crossed.

! Missing \right. inserted.
<inserted text> 
                \right .
l.82 \caption{Test table}

I've inserted something that you may have forgotten.
(See the <inserted text> above.)
With luck, this will get me unwedged. But if you
really didn't forget anything, try typing `2' now; then
my insertion and my current dilemma will both disappear.

我希望有人能够帮助我！

PS：请注意，我无法发布PDF输出的屏幕截图，因为我没有足够的声誉。

Answer 1

left环境似乎搞砸了引用。我试过了

% latex table generated in R 3.2.0 by xtable 1.7-4 package
% Tue Jun 02 09:53:12 2015
\begin{table}[ht]
\caption{Test table}
\label{tab:Test}
\begin{left}
\begin{tabular}{rr}
  \hline
Test & a \\ 
  \hline
  1 &   4 \\ 
    2 &   5 \\ 
    3 &   6 \\ 
   \hline
\end{tabular}
\end{left}
\end{table}

并产生正确的参考。