For R Markdown, How do I display a matrix from R variable

Question

I have a rmd document where I have the following

```{r code_block, echo=FALSE}
A = matrix(c(1,3,0,1),2,2)
B = matrix(c(5,3,1,4),2,2)
```

$$
\begin{bmatrix} 
1  & 0 \\ 
3 & 1 \\ 
\end{bmatrix}
*
\begin{bmatrix} 
5 & 1 \\ 
3 & 4 \\ 
\end{bmatrix}
$$

Now I would like to instead of hard coding the LaTeX part manually, I could use the matrix from the variables A and B instead. How could this be done?

Thanks.

Answer 1

Straightforwardly, you can write latex line. writeLines() or cat() would be helpful.

You can use apply(A, 1) with two step paste().

1. paste(collpase = "&"): collapse each row
2. paste(, "\\\\"): collapse every column with \\

Then we can get the latex formula of matrix.

write_matex <- function(x) {
  begin <- "$$\\begin{bmatrix}"
  end <- "\\end{bmatrix}$$"
  X <-
    apply(x, 1, function(x) {
      paste(
        paste(x, collapse = "&"),
        "\\\\"
      )
    })
  writeLines(c(begin, X, end))
}

If you conduct this function, write_matex(A) gives

$$\begin{bmatrix}
1&0 \\
3&1 \\
\end{bmatrix}$$

When you use chunk option {r, results = 'asis'}, you might see the matrix in both pdf and html.

---

Latex block

Based on this function, you might freely use matrix in latex block. For this, $$ should be removed in the function. Instead of writeLines(), paste(collapse = "") can be used.

write_matex2 <- function(x) {
  begin <- "\\begin{bmatrix}"
  end <- "\\end{bmatrix}"
  X <-
    apply(x, 1, function(x) {
      paste(
        paste(x, collapse = "&"),
        "\\\\"
      )
    })
  paste(c(begin, X, end), collapse = "")
}

In the text part, you can implement this function as

$$
`r write_matex2(A)` \times `r write_matex2(B)`
$$

In r markdown, this r with back quotation can attach your r function and variable. So you can get

As you can see, this is reproducible.

(C <- matrix(1:10, nrow = 2))
#>      [,1] [,2] [,3] [,4] [,5]
#> [1,]    1    3    5    7    9
#> [2,]    2    4    6    8   10

Similarly,

$$`r write_matex2(C)` + `r write_matex2(C)`$$