Knitr：Markdown文档中LaTeX环境中的R代码

Question

I have a document in Markdown, which incorporates R code via Knitr . For rendering equations I use LaTeX, simply writing its commands in the text. Say I have the following LaTeX code:

\begin{displaymath}
\mathbf{X} = \begin{bmatrix}
2 & 12\\ 
3 & 17\\ 
5 & 10\\ 
7 & 18\\ 
9 & 13\\
\end{bmatrix}
\end{displaymath}

Which renders me a nice mathematical matrix representation (in square brackets), when I convert everything to PDF (the full workflow is then: RMD -> knitr -> MD -> pandoc -> TeX -> pandoc -> PDF).

Now, suppose we want the said matrix to be generated on-the-fly from an R object, some R matrix x . In this post we establish how to generate the required LaTeX code (the matrix2latex() function is defined there). The question now is how to get Knitr to evaluate it. I tried the following code:

\begin{displaymath}
\mathbf{X} = `r matrix2latex(x)`
\end{displaymath} 

but it simply produces blank space (NULL actually) where the R output should be. I am surprised this hasn't been asked already (at least my own search yielded nothing). Any ideas how to make this work?

EDIT:

As suggested, rewrote the function without cat() . Here is the working version of the function for future reference:

m2l <- function(matr) {

    printmrow <- function(x) {

        ret <- paste(paste(x,collapse = " & "),"\\\\")
        sprintf(ret)
    }

    out <- apply(matr,1,printmrow)
    out2 <- paste("\\begin{bmatrix}",paste(out,collapse=' '),"\\end{bmatrix}")
    return(out2)
}

Answer 1

This is because your matrix2latex function uses cat and sends its output to the standard output stream, which isn't where knitr is trying to put the output.

Two solutions: one is to rewrite your function to construct the output as a string using paste and sprintf or other string formatting functions, or as a quick hack just wrap it in capture.output thus:

m2l = function(matr){capture.output(matrix2latex(matr))}

Then in your .Rmd file:

\begin{displaymath}
\mathbf{X} = `r m2l(x)`
\end{displaymath}

becomes

\mathbf{X} = \begin{bmatrix} , 0.06099 & 0.768 \\ , 0.6112 & 0.004696 \\ , 0.02729 & 0.6198 \\ , 0.8498 & 0.3308 \\ , 0.6869 & 0.103 \\ , \end{bmatrix}

which although isn't quite perfect does illustrate the principle. The code inserted by the inline expression is the value of it, not what it prints or cats.